On Tue, Mar 22, 2016 at 10:57 AM, Peter Geoghegan <p...@heroku.com> wrote:
> That's right - I have a small number of feedback items to work
> through. I also determined myself that there could be a very low
> probability race condition when checking the key space across sibling
> pages, and will work to address that.
I attach a V2 revision.
Behavioral changes:
* Fixes very low probability race condition when verifying ordering
across sibling pages with bt_index_check()/AccessShareLock.
Notably, the fix does not involve using the high key from the next
page instead of the first item, despite my recent suggestion that it
would; I changed my mind. In this revision,
bt_right_page_check_scankey() continues to return the first item on
the next/right page (as a scankey), just as in the first version. The
locking considerations here are more complicated than before, and
hopefully correct. I abandoned the high key idea when I realized that
a concurrent page split could happen in the right sibling, making the
high key generally no more safe in the face of concurrent target page
deletion than continuing to use the first data item. Using the first
data item from the right page is better than using the high key item
from the right page verification-wise anyway, so that's good. I found
a new way of locking down and recovering from any race conditions (the
low probability bug in V1 for bt_index_check()/AccessShareLock calls
to the C function bt_right_page_check_scankey()). I won't go into the
gory details here, but I will mention that my new approach is inspired
by how backwards scans work already.
* Adds DEBUG1 traces that indicate the level and page currently being checked.
Advanced users will probably find these traces useful on occasion.
It's nice to be able to see how amcheck walks the B-Tree in practice.
Non-behavioral changes:
* Explains why an ExclusiveLock is needed on the target index by
bt_index_parent_check() (and a ShareLock on its heap relation).
This new explanation talks about why an ExclusiveLock *is* necessary
when checking downlinks against child pages, per Tomas Vondra's
request. (Note that nothing changes about child/downlink verification
itself). Before now, I focused on why they were not necessary, but
Tomas was right to point out that that isn't enough information.
* No longer uses the word "comport" in an error message, per Amit
Langote. Similarly, minor typos fixed, per Tomas.
* Adds a variety of pg_regress-based tests for external sorting. (Via
various CREATE INDEX statements involving a variety of datatypes. Data
is generated pseudo-randomly.)
Notably, the regression tests *never* test external sorting today.
ISTM that amcheck is a convenient, reliable, and portable way of
testing the sorting of collatable types like text, so this is the best
way of testing external sorting. External sorting uses abbreviated
keys for runs, but discards them as runs are written out, and so the
merging of runs does not use abbreviated keys. Abbreviated comparisons
and authoritative comparisons are therefore reliably used within the
same sort. One particular goal here is that something like the recent
strxfrm() debacle might be caught by this testing on some platforms
(if we ever get back to trusting strxfrm() at all, that is). The style
of these new tests is unorthodox, because a variety of collations are
tested, which varies from system to system; we should discuss all
this. The tests can take a minute or two to run on my laptop, but that
could vary significantly. And so, whether or not that needs to be
targeted by something other than "check" and "installcheck" is a
discussion that needs to happen. I imagine a lot of buildfarm animals
have slow storage, but on the other hand I don't think hackers run the
contrib regression tests so often. I prefer to not follow the example
of test_decoding if possible; test_decoding's Makefile only runs tests
when wal_level=logical is expected (that's how the relevant Makefile
targets are scoped), but it's a complicated special case.
Review
------
As already noted after the first bullet point, there are some tricky
considerations on fixing the race when the user calls
bt_index_check(). Reviewers should pay close attention to this; could
I be wrong about that being race-safe even now? This seems to me to be
by far the best place for reviewers to look for problems in this
patch, as it's the only bt_index_check()/AccessShareLock case that
compares anything *across* pages. The measures taken to prevent false
positives described within bt_target_page_check() require expert
review, especially because this is the one case where a faulty
rationale may result in under-protection from false positives, and not
just harmless over-protection.
The rationale for why we can reliably recover from a race described
within the C function bt_right_page_check_scankey() is *complicated*.
I may have failed to make it as simple as possible despite significant
effort. It's hard to describe these things in an accessible way. Maybe
someone can try and understand what I've said there, and let me know
how I might have fallen short. I have used a new term, "cousin page"
in this explanation (cf. sibling page). This seems like useful
terminology that makes these difficult explanations a bit more
accessible.
--
Peter Geoghegan
From 06085b27dc36c1e05e0049d90374b9c687e5ea9a Mon Sep 17 00:00:00 2001
From: Peter Geoghegan <p...@bowt.ie>
Date: Tue, 10 Jun 2014 22:20:40 -0700
Subject: [PATCH] Add amcheck extension to contrib
The new extension makes available two SQL-callable functions, each of
which accept a single argument (a regclass/nbtree index relation
argument) and return void.
The SQL-callable function bt_index_check() checks invariants of the
target index by walking the tree, performing various checks of the
sanity of the page space using insertion scankeys to compare items. The
SQL-callable function bt_index_parent_check() performs a superset of the
checks performed by bt_index_check(): the same checks, as well as
another check verifying that each downlink is actually respected as a
lower bound on its child page.
bt_index_check() requires only an AccessShareLock on the target.
bt_index_parent_check() requires an ExclusiveLock on the target,
and ShareLock on its heap relation.
To test amcheck, add regression tests that feature a variety of CREATE
INDEX statements (these are always external sort cases), and verify the
indexes afterwards in each case. Aside from testing amcheck itself,
this approach also fills a gap in the test coverage of the standard
regression tests, since external sorts were previously completely
untested. This also makes it possible to test OS collations without any
specific expectation about an "expected" pg_regress output, or even
specific advanced knowledge of which collations are to be tested.
---
contrib/Makefile | 1 +
contrib/amcheck/Makefile | 22 +
contrib/amcheck/amcheck--1.0.sql | 20 +
contrib/amcheck/amcheck.c | 1265 ++++++++++++++++++++
contrib/amcheck/amcheck.control | 5 +
contrib/amcheck/expected/extern_sort_bytea.out | 44 +
.../amcheck/expected/extern_sort_collations.out | 42 +
contrib/amcheck/expected/extern_sort_numeric.out | 44 +
contrib/amcheck/expected/install_amcheck.out | 1 +
contrib/amcheck/sql/extern_sort_bytea.sql | 23 +
contrib/amcheck/sql/extern_sort_collations.sql | 42 +
contrib/amcheck/sql/extern_sort_numeric.sql | 29 +
contrib/amcheck/sql/install_amcheck.sql | 1 +
doc/src/sgml/amcheck.sgml | 247 ++++
doc/src/sgml/contrib.sgml | 1 +
doc/src/sgml/filelist.sgml | 1 +
16 files changed, 1788 insertions(+)
create mode 100644 contrib/amcheck/Makefile
create mode 100644 contrib/amcheck/amcheck--1.0.sql
create mode 100644 contrib/amcheck/amcheck.c
create mode 100644 contrib/amcheck/amcheck.control
create mode 100644 contrib/amcheck/expected/extern_sort_bytea.out
create mode 100644 contrib/amcheck/expected/extern_sort_collations.out
create mode 100644 contrib/amcheck/expected/extern_sort_numeric.out
create mode 100644 contrib/amcheck/expected/install_amcheck.out
create mode 100644 contrib/amcheck/sql/extern_sort_bytea.sql
create mode 100644 contrib/amcheck/sql/extern_sort_collations.sql
create mode 100644 contrib/amcheck/sql/extern_sort_numeric.sql
create mode 100644 contrib/amcheck/sql/install_amcheck.sql
create mode 100644 doc/src/sgml/amcheck.sgml
diff --git a/contrib/Makefile b/contrib/Makefile
index d12dd63..cecfb6d 100644
--- a/contrib/Makefile
+++ b/contrib/Makefile
@@ -6,6 +6,7 @@ include $(top_builddir)/src/Makefile.global
SUBDIRS = \
adminpack \
+ amcheck \
auth_delay \
auto_explain \
btree_gin \
diff --git a/contrib/amcheck/Makefile b/contrib/amcheck/Makefile
new file mode 100644
index 0000000..26b7b9b
--- /dev/null
+++ b/contrib/amcheck/Makefile
@@ -0,0 +1,22 @@
+# contrib/amcheck/Makefile
+
+MODULE_big = amcheck
+OBJS = amcheck.o $(WIN32RES)
+
+EXTENSION = amcheck
+DATA = amcheck--1.0.sql
+PGFILEDESC = "amcheck - functions to verify access method invariants"
+
+REGRESS = install_amcheck extern_sort_bytea \
+ extern_sort_collations extern_sort_numeric
+
+ifdef USE_PGXS
+PG_CONFIG = pg_config
+PGXS := $(shell $(PG_CONFIG) --pgxs)
+include $(PGXS)
+else
+subdir = contrib/amcheck
+top_builddir = ../..
+include $(top_builddir)/src/Makefile.global
+include $(top_srcdir)/contrib/contrib-global.mk
+endif
diff --git a/contrib/amcheck/amcheck--1.0.sql b/contrib/amcheck/amcheck--1.0.sql
new file mode 100644
index 0000000..ebbd6ac
--- /dev/null
+++ b/contrib/amcheck/amcheck--1.0.sql
@@ -0,0 +1,20 @@
+/* contrib/amcheck/amcheck--1.0.sql */
+
+-- complain if script is sourced in psql, rather than via CREATE EXTENSION
+\echo Use "CREATE EXTENSION amcheck" to load this file. \quit
+
+--
+-- bt_index_check()
+--
+CREATE FUNCTION bt_index_check(index regclass)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_check'
+LANGUAGE C STRICT;
+
+--
+-- bt_index_parent_check()
+--
+CREATE FUNCTION bt_index_parent_check(index regclass)
+RETURNS VOID
+AS 'MODULE_PATHNAME', 'bt_index_parent_check'
+LANGUAGE C STRICT;
diff --git a/contrib/amcheck/amcheck.c b/contrib/amcheck/amcheck.c
new file mode 100644
index 0000000..66d1f4f
--- /dev/null
+++ b/contrib/amcheck/amcheck.c
@@ -0,0 +1,1265 @@
+/*-------------------------------------------------------------------------
+ *
+ * amcheck.c
+ * Verifies the integrity of access methods based on invariants.
+ *
+ * Provides SQL-callable functions for verifying that various logical
+ * invariants in the structure of index access methods are respected. This
+ * includes, for example, the invariant that each page in the target B-Tree
+ * index has "real" items in logical order as reported by an insertion scankey
+ * (the insertion scankey sort-wise NULL semantics are useful for
+ * verification).
+ *
+ *
+ * Copyright (c) 2016, PostgreSQL Global Development Group
+ *
+ * IDENTIFICATION
+ * contrib/amcheck/amcheck.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "access/nbtree.h"
+#include "access/transam.h"
+#include "catalog/index.h"
+#include "catalog/pg_am.h"
+#include "commands/tablecmds.h"
+#include "miscadmin.h"
+#include "storage/lmgr.h"
+#include "utils/memutils.h"
+#include "utils/snapmgr.h"
+
+
+PG_MODULE_MAGIC;
+
+#define CHECK_SUPERUSER() { \
+ if (!superuser()) \
+ ereport(ERROR, \
+ (errcode(ERRCODE_INSUFFICIENT_PRIVILEGE), \
+ (errmsg("must be superuser to use verification functions")))); }
+
+/*
+* As noted in comments above _bt_compare(), there is special handling of the
+* first data item (that is, the first item with a valid downlink -- not the
+* high key item) on a non-leaf (internal) page. There is clearly no point in
+* having amcheck functions make any comparison of or against these "minus
+* infinity" items, because they contain no actual information other than the
+* downlink.
+*/
+#define OFFSET_IS_MINUS_INFINITY(opaque, offset) \
+ (!P_ISLEAF(opaque) && offset == P_FIRSTDATAKEY(opaque))
+
+/*
+ * Callers to verification functions should never receive a false positive
+ * indication of corruption. Therefore, when using amcheck functions for
+ * stress testing, it may be useful to temporally change the CORRUPTION elevel
+ * to PANIC, to immediately halt the server in the event of detecting an
+ * invariant condition violation. This may preserve more information about the
+ * nature of the underlying problem. Note that modifying the CORRUPTION
+ * constant to be an elevel < ERROR is not well tested.
+ */
+#ifdef NOT_USED
+#define CORRUPTION PANIC
+#define CONCERN WARNING
+#else
+#define CORRUPTION ERROR
+#define CONCERN INFO
+#endif
+
+/*
+ * A B-Tree cannot possibly have this many levels, since there must be one
+ * block per level, which is bound by the range of BlockNumber:
+ */
+#define InvalidBtreeLevel ((uint32) InvalidBlockNumber)
+
+/*
+ * State associated with verifying a B-Tree index
+ */
+typedef struct BtreeCheckState
+{
+ /*
+ * Unchanging state, established at start of verification:
+ *
+ * rel: B-Tree Index Relation
+ * exclusivelock: ExclusiveLock held on rel; else AccessShareLock
+ * checkstrategy: Buffer access strategy
+ * targetcontext: Target page memory context
+ */
+ Relation rel;
+ bool exclusivelock;
+ BufferAccessStrategy checkstrategy;
+ MemoryContext targetcontext;
+
+ /*
+ * Mutable state, for verification of particular page:
+ *
+ * target: Main target page
+ * targetblock: Main target page's block number
+ * targetlsn: Main target page's LSN
+ *
+ * target is the point of reference for a verification operation.
+ *
+ * Note: targetlsn is always from "target". It is stashed here out of
+ * convenience.
+ *
+ * Other B-Tree pages may be allocated, but those are always auxiliary
+ * (e.g. they are target's child pages). Conceptually, only the target
+ * page is checked. Each page found by verification's left/right,
+ * top/bottom scan becomes the target once.
+ *
+ * Memory is managed by resetting targetcontext after verification of some
+ * target page finishes (possibly including target verification that
+ * depends on non-target page state).
+ */
+ Page target;
+ BlockNumber targetblock;
+ XLogRecPtr targetlsn;
+
+} BtreeCheckState;
+
+/*
+ * Starting point for verifying an entire B-Tree index level
+ */
+typedef struct BtreeLevel
+{
+ /* Level number (0 is leaf page level). */
+ uint32 level;
+
+ /* Left most block on level. Scan of level begins here. */
+ BlockNumber leftmost;
+
+ /* Is this level reported as "true" root level by meta page? */
+ bool istruerootlevel;
+} BtreeLevel;
+
+PG_FUNCTION_INFO_V1(bt_index_check);
+PG_FUNCTION_INFO_V1(bt_index_parent_check);
+
+static void btree_index_checkable(Relation rel);
+static void bt_check_every_level(Relation rel, bool exclusivelock);
+static BtreeLevel bt_check_level_from_leftmost(BtreeCheckState *state,
+ BtreeLevel level);
+static void bt_target_page_check(BtreeCheckState *state);
+static ScanKey bt_right_page_check_scankey(BtreeCheckState *state);
+static void bt_downlink_check(BtreeCheckState *state, BlockNumber childblock,
+ ScanKey targetkey);
+static bool invariant_key_less_than_equal_offset(BtreeCheckState *state,
+ ScanKey key,
+ OffsetNumber upperbound);
+static bool invariant_key_greater_than_equal_offset(BtreeCheckState *state,
+ ScanKey key,
+ OffsetNumber lowerbound);
+static bool invariant_key_less_than_equal_nontarget_offset(BtreeCheckState *state,
+ Page other,
+ ScanKey key,
+ OffsetNumber upperbound);
+static Page palloc_btree_page(BtreeCheckState *state, BlockNumber blocknum);
+
+/*
+ * bt_index_check(index regclass)
+ *
+ * Verify integrity of B-Tree index.
+ *
+ * Only acquires AccessShareLock on index relation. Does not consider
+ * invariants that exist between parent/child pages.
+ */
+Datum
+bt_index_check(PG_FUNCTION_ARGS)
+{
+ Oid relid = PG_GETARG_OID(0);
+ Relation indrel;
+
+ CHECK_SUPERUSER();
+
+ indrel = relation_open(relid, AccessShareLock);
+
+ /* Relation suitable for checking as B-Tree? */
+ btree_index_checkable(indrel);
+
+ /* Check index */
+ bt_check_every_level(indrel, false);
+
+ relation_close(indrel, AccessShareLock);
+
+ PG_RETURN_VOID();
+}
+
+/*
+ * bt_index_parent_check(index regclass)
+ *
+ * Verify integrity of B-Tree index.
+ *
+ * Acquires ExclusiveLock on index relation, and ShareLock on the associated
+ * heap relation, a bit like REINDEX. Verifies that downlinks in parent pages
+ * are valid lower bounds on child pages.
+ */
+Datum
+bt_index_parent_check(PG_FUNCTION_ARGS)
+{
+ Oid indrelid = PG_GETARG_OID(0);
+ Oid heapid;
+ Relation indrel;
+ Relation heaprel;
+
+ CHECK_SUPERUSER();
+
+ /*
+ * We must lock table before index to avoid deadlocks. However, if the
+ * passed indrelid isn't an index then IndexGetRelation() will fail.
+ * Rather than emitting a not-very-helpful error message, postpone
+ * complaining, expecting that the is-it-an-index test below will fail.
+ */
+ heapid = IndexGetRelation(indrelid, true);
+ if (OidIsValid(heapid))
+ heaprel = heap_open(heapid, ShareLock);
+ else
+ heaprel = NULL;
+
+ /*
+ * Open the target index relations separately (like relation_openrv(), but
+ * with heap relation locked first to prevent deadlocking). In hot standby
+ * mode this will raise an error.
+ */
+ indrel = index_open(indrelid, ExclusiveLock);
+
+ /* Check for active uses of the index in the current transaction */
+ CheckTableNotInUse(indrel, "bt_index_parent_check");
+
+ /* Relation suitable for checking as B-Tree? */
+ btree_index_checkable(indrel);
+
+ /* Check index */
+ bt_check_every_level(indrel, true);
+
+ index_close(indrel, ExclusiveLock);
+ if (heaprel)
+ heap_close(heaprel, ShareLock);
+
+ PG_RETURN_VOID();
+}
+
+/*
+ * btree_index_checkable()
+ *
+ * Basic checks about the suitability of a relation for checking as a B-Tree
+ * index.
+ */
+static void
+btree_index_checkable(Relation rel)
+{
+ if (rel->rd_rel->relkind != RELKIND_INDEX ||
+ rel->rd_rel->relam != BTREE_AM_OID)
+ ereport(ERROR,
+ (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+ errmsg("only nbtree access method indexes are supported"),
+ errdetail("index \"%s\" does not using the nbtree access method.",
+ RelationGetRelationName(rel))));
+
+ if (RELATION_IS_OTHER_TEMP(rel))
+ ereport(ERROR,
+ (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+ errmsg("cannot access temporary tables of other sessions"),
+ errdetail("index \"%s\" is associated with temporary relation.",
+ RelationGetRelationName(rel))));
+
+ if (!rel->rd_index->indisready)
+ ereport(ERROR,
+ (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+ errmsg("cannot check index \"%s\"",
+ RelationGetRelationName(rel)),
+ errdetail("index is not yet ready for insertions")));
+
+ if (!rel->rd_index->indisvalid)
+ ereport(ERROR,
+ (errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+ errmsg("cannot check index \"%s\"",
+ RelationGetRelationName(rel)),
+ errdetail("index is not valid")));
+}
+
+/*
+ * bt_check_every_level()
+ *
+ * Main entry point for B-Tree SQL-callable functions. Walks the B-Tree in
+ * logical order, verifying invariants as it goes.
+ *
+ * It is the caller's responsibility to acquire appropriate heavyweight lock on
+ * the index relation, and advise us if extra checks are safe when an
+ * ExclusiveLock is held. An ExclusiveLock is generally assumed to prevent any
+ * kind of physical modification to the index structure, including
+ * modifications that VACUUM may make.
+ */
+static void
+bt_check_every_level(Relation rel, bool exclusivelock)
+{
+ BtreeCheckState *state;
+ Page metapage;
+ BTMetaPageData *metad;
+ uint32 previouslevel;
+ BtreeLevel current;
+
+ /*
+ * RecentGlobalXmin assertion matches index_getnext_tid(). See note on
+ * RecentGlobalXmin/B-Tree page deletion.
+ */
+ Assert(TransactionIdIsValid(RecentGlobalXmin));
+
+ /*
+ * Initialize state for entire verification operation
+ */
+ state = palloc(sizeof(BtreeCheckState));
+ state->rel = rel;
+ state->exclusivelock = exclusivelock;
+ state->checkstrategy = GetAccessStrategy(BAS_BULKREAD);
+ /* Create context for page */
+ state->targetcontext = AllocSetContextCreate(CurrentMemoryContext,
+ "amcheck page data",
+ ALLOCSET_DEFAULT_MINSIZE,
+ ALLOCSET_DEFAULT_INITSIZE,
+ ALLOCSET_DEFAULT_MAXSIZE);
+
+ /* Get true root block from meta-page */
+ metapage = palloc_btree_page(state, BTREE_METAPAGE);
+ metad = BTPageGetMeta(metapage);
+
+ /*
+ * Certain deletion patterns can result in "skinny" B-Tree indexes, where
+ * the fast root and true root differ.
+ *
+ * Start from the true root, not the fast root, unlike conventional index
+ * scans. This approach is more thorough, and removes the risk of
+ * following a stale fast root from the meta page.
+ */
+ if (metad->btm_fastroot != metad->btm_root)
+ ereport(CONCERN,
+ (errcode(ERRCODE_DUPLICATE_OBJECT),
+ errmsg("fast root mismatch in index %s",
+ RelationGetRelationName(rel)),
+ errdetail_internal("Fast block %u (level %u) "
+ "differs from true root "
+ "block %u (level %u).",
+ metad->btm_fastroot, metad->btm_fastlevel,
+ metad->btm_root, metad->btm_level)));
+
+ /*
+ * Starting at the root, verify every level. Move left to right, top to
+ * bottom. Note that there may be no pages other than the meta page (meta
+ * page can indicate that root is P_NONE when the index is totally empty).
+ */
+ previouslevel = InvalidBtreeLevel;
+ current.level = metad->btm_level;
+ current.leftmost = metad->btm_root;
+ current.istruerootlevel = true;
+ while (current.leftmost != P_NONE)
+ {
+ /*
+ * Verify this level, and get left most page for next level down, if
+ * not at leaf level
+ */
+ current = bt_check_level_from_leftmost(state, current);
+
+ if (current.leftmost == InvalidBlockNumber)
+ ereport(CORRUPTION,
+ (errcode(ERRCODE_INDEX_CORRUPTED),
+ errmsg("index \"%s\" has no valid pages on level below %u or first level",
+ RelationGetRelationName(rel), previouslevel)));
+
+ previouslevel = current.level;
+ }
+
+ /* Be tidy: */
+ MemoryContextDelete(state->targetcontext);
+}
+
+/*
+ * bt_check_level_from_leftmost()
+ *
+ * Given a left-most block at some level, move right, verifying each page
+ * individually (with more verification across pages for "exclusivelock"
+ * callers). Caller should pass the true root page as the leftmost initially,
+ * working their way down by passing what is returned for the last call here
+ * until level 0 (leaf page level) was reached.
+ *
+ * Returns state for next call, if any. This includes left-most block number
+ * one level lower that should be checked on next call here, or P_NONE when
+ * leaf level is to be checked. The nbtree level number convention is that
+ * higher levels have higher numbers, because new levels are added only due to
+ * a root page split. Prior to the first root page split, when there are only
+ * two pages (the root page and the meta page), the root page is classified as
+ * a leaf page, not an internal page.
+ *
+ * Note on memory management: State's per-page context is reset here, between
+ * each call to bt_target_page_check().
+ */
+static BtreeLevel
+bt_check_level_from_leftmost(BtreeCheckState *state, BtreeLevel level)
+{
+ /* State to establish early, concerning entire level */
+ BTPageOpaque opaque;
+ MemoryContext oldcontext;
+ BtreeLevel nextleveldown;
+
+ /* Variables for iterating across level using right links */
+ BlockNumber leftcurrent = P_NONE;
+ BlockNumber current = level.leftmost;
+
+ /* Initialize return state */
+ nextleveldown.leftmost = InvalidBlockNumber;
+ nextleveldown.level = InvalidBtreeLevel;
+ nextleveldown.istruerootlevel = false;
+
+ /* Use page-level context for duration of this call */
+ oldcontext = MemoryContextSwitchTo(state->targetcontext);
+
+ elog(DEBUG1, "verifying level %u%s", level.level,
+ level.istruerootlevel?
+ " (true root level)" : level.level == 0 ? " (leaf level)" : "");
+
+ do
+ {
+ /* Don't rely on CHECK_FOR_INTERRUPTS() calls at lower level */
+ CHECK_FOR_INTERRUPTS();
+
+ /* Initialize state for this iteration */
+ state->targetblock = current;
+ state->target = palloc_btree_page(state, state->targetblock);
+ state->targetlsn = PageGetLSN(state->target);
+
+ opaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+
+ if (P_IGNORE(opaque))
+ {
+ if (P_RIGHTMOST(opaque))
+ ereport(CORRUPTION,
+ (errcode(ERRCODE_INDEX_CORRUPTED),
+ errmsg("block %u fell off the end of index \"%s\"",
+ current, RelationGetRelationName(state->rel))));
+ else
+ ereport(CONCERN,
+ (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+ errmsg("block %u of index \"%s\" ignored",
+ current, RelationGetRelationName(state->rel))));
+ goto nextpage;
+ }
+ else if (nextleveldown.leftmost == InvalidBlockNumber)
+ {
+ /*
+ * A concurrent page split could make the caller supplied leftmost
+ * block no longer contain the leftmost page, or no longer be the
+ * true root, but where that isn't possible due to heavyweight
+ * locking, check that the first valid page meets caller's
+ * expectations.
+ */
+ if (state->exclusivelock)
+ {
+ if (!P_LEFTMOST(opaque))
+ ereport(CORRUPTION,
+ (errcode(ERRCODE_INDEX_CORRUPTED),
+ errmsg("block %u is not leftmost in index \"%s\"",
+ current, RelationGetRelationName(state->rel))));
+
+ if (level.istruerootlevel && !P_ISROOT(opaque))
+ ereport(CORRUPTION,
+ (errcode(ERRCODE_INDEX_CORRUPTED),
+ errmsg("block %u is not true root in index \"%s\"",
+ current, RelationGetRelationName(state->rel))));
+ }
+
+ /*
+ * Before beginning any non-trivial examination of level, establish
+ * next level down's leftmost block number, which next call here
+ * will pass as its leftmost (iff this isn't leaf level).
+ *
+ * There should be at least one non-ignorable page per level.
+ */
+ if (!P_ISLEAF(opaque))
+ {
+ IndexTuple itup;
+ ItemId itemid;
+
+ /* Internal page -- downlink gets leftmost on next level */
+ itemid = PageGetItemId(state->target, P_FIRSTDATAKEY(opaque));
+ itup = (IndexTuple) PageGetItem(state->target, itemid);
+ nextleveldown.leftmost = ItemPointerGetBlockNumber(&(itup->t_tid));
+ nextleveldown.level = opaque->btpo.level - 1;
+ }
+ else
+ {
+ /*
+ * Leaf page -- final level caller must process.
+ *
+ * Note that this could also be the root page, if there has
+ * been no root page split yet.
+ */
+ nextleveldown.leftmost = P_NONE;
+ nextleveldown.level = InvalidBtreeLevel;
+ }
+
+ /*
+ * Finished setting up state for this call/level. Control will
+ * never end up back here in any future loop iteration for this
+ * level.
+ */
+ }
+
+ if (state->exclusivelock && opaque->btpo_prev != leftcurrent)
+ ereport(CORRUPTION,
+ (errcode(ERRCODE_INDEX_CORRUPTED),
+ errmsg("right link/left link pair in index \"%s\" not in mutual agreement",
+ RelationGetRelationName(state->rel)),
+ errdetail_internal("Deleted block=%u left block=%u link reported block=%u.",
+ current, leftcurrent, opaque->btpo_prev)));
+
+ /* Verify invariants for page -- all important checks occur here */
+ bt_target_page_check(state);
+
+nextpage:
+
+ /* Try to detect circular links */
+ if (current == leftcurrent || current == opaque->btpo_prev)
+ ereport(CORRUPTION,
+ (errcode(ERRCODE_INDEX_CORRUPTED),
+ errmsg("circular link chain found in block %u of index \"%s\"",
+ current, RelationGetRelationName(state->rel))));
+
+ leftcurrent = current;
+ current = opaque->btpo_next;
+
+ /* Free page and associated memory for this iteration */
+ MemoryContextReset(state->targetcontext);
+ }
+ while (current != P_NONE);
+
+ /* Don't change context for caller */
+ MemoryContextSwitchTo(oldcontext);
+
+ return nextleveldown;
+}
+
+/*
+ * bt_target_page_check()
+ *
+ * Function performs the following checks on target page, or pages ancillary to
+ * target page:
+ *
+ * - That every "real" data item is less than or equal to the high key, which
+ * is an upper bound on the items on the pages (where there is a high key at
+ * all -- pages that are rightmost lack one).
+ *
+ * - That within the page, every "real" item is less than or equal to the item
+ * immediately to its right, if any (i.e., that the items are in order within
+ * the page, so that the binary searches performed by index scans are sane).
+ *
+ * - That the last item stored on the page is less than or equal to the first
+ * "real" data item on the page to the right (if such a first item is
+ * available from next page).
+ *
+ * Furthermore, when state passed shows ExclusiveLock held, function also
+ * checks:
+ *
+ * - That all child pages respect downlinks lower bound (internal pages only).
+ *
+ * Note: This routine is not especially proactive in freeing memory. High
+ * watermark memory consumption is bound to some small fixed multiple of
+ * BLCKSZ, though. Caller should reset the current context between calls here.
+ */
+static void
+bt_target_page_check(BtreeCheckState *state)
+{
+ OffsetNumber offset;
+ OffsetNumber max;
+ BTPageOpaque topaque;
+
+ topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+ max = PageGetMaxOffsetNumber(state->target);
+
+ elog(DEBUG1, "verifying %u items on %s block %u", max,
+ P_ISLEAF(topaque) ? "leaf":"internal", state->targetblock);
+
+ /*
+ * Loop over page items, but don't start from P_HIKEY (don't have iteration
+ * directly considering high key item, if any). That's something that is
+ * used as part of verifying all other items, but doesn't get its own
+ * iteration.
+ */
+ for (offset = P_FIRSTDATAKEY(topaque);
+ offset <= max;
+ offset = OffsetNumberNext(offset))
+ {
+ ItemId itemid;
+ IndexTuple itup;
+ ScanKey skey;
+
+ CHECK_FOR_INTERRUPTS();
+
+ /* Don't try to generate scankey using "minus infinity" garbage data */
+ if (OFFSET_IS_MINUS_INFINITY(topaque, offset))
+ continue;
+
+ /* Build insertion scankey for current page offset */
+ itemid = PageGetItemId(state->target, offset);
+ itup = (IndexTuple) PageGetItem(state->target, itemid);
+ skey = _bt_mkscankey(state->rel, itup);
+
+ /*
+ * ********************
+ * * High key check *
+ * ********************
+ *
+ * If there is a high key, which there must be for a non-rightmost
+ * page, check that it actually is upper bound on all page items.
+ *
+ * We prefer to check all items, rather than checking just the first
+ * and trusting that the operator class obeys the transitive law (which
+ * implies that all subsequent items also respected the high key
+ * invariant if they pass the page order check).
+ *
+ * Ideally, we'd compare every item in the index against every other
+ * item in the index, and not trust opclass obedience of the transitive
+ * law to bridge the gap between children and their grandparents (as
+ * well as great-grandparents, and so on). We don't go to those
+ * lengths because that would be prohibitively expensive, and probably
+ * not markedly more effective in practice.
+ */
+ if (!P_RIGHTMOST(topaque) &&
+ !invariant_key_less_than_equal_offset(state, skey, P_HIKEY))
+ {
+ char *itid, *htid;
+
+ itid = psprintf("(%u,%u)", state->targetblock, offset);
+ htid = psprintf("(%u,%u)",
+ ItemPointerGetBlockNumber(&(itup->t_tid)),
+ ItemPointerGetOffsetNumber(&(itup->t_tid)));
+
+ ereport(CORRUPTION,
+ (errcode(ERRCODE_INDEX_CORRUPTED),
+ errmsg("high key invariant violated for index \"%s\"",
+ RelationGetRelationName(state->rel)),
+ errdetail_internal("Index tid=%s points to %s tid=%s "
+ "page lsn=%X/%X.",
+ itid,
+ P_ISLEAF(topaque) ? "heap":"index",
+ htid,
+ (uint32) (state->targetlsn >> 32),
+ (uint32) state->targetlsn)));
+ }
+
+ /*
+ * ********************
+ * * Page order check *
+ * ********************
+ *
+ * Check that items are stored on page in logical order, by checking
+ * current item is less than or equal to next item (if any).
+ */
+ if (OffsetNumberNext(offset) <= max &&
+ !invariant_key_less_than_equal_offset(state, skey,
+ OffsetNumberNext(offset)))
+ {
+ char *itid, *htid,
+ *nitid, *nhtid;
+
+ itid = psprintf("(%u,%u)", state->targetblock, offset);
+ htid = psprintf("(%u,%u)",
+ ItemPointerGetBlockNumber(&(itup->t_tid)),
+ ItemPointerGetOffsetNumber(&(itup->t_tid)));
+ nitid = psprintf("(%u,%u)", state->targetblock,
+ OffsetNumberNext(offset));
+
+ /* Reuse itup to get pointed-to heap location of second item */
+ itemid = PageGetItemId(state->target, OffsetNumberNext(offset));
+ itup = (IndexTuple) PageGetItem(state->target, itemid);
+ nhtid = psprintf("(%u,%u)",
+ ItemPointerGetBlockNumber(&(itup->t_tid)),
+ ItemPointerGetOffsetNumber(&(itup->t_tid)));
+
+ ereport(CORRUPTION,
+ (errcode(ERRCODE_INDEX_CORRUPTED),
+ errmsg("page order invariant violated for index \"%s\"",
+ RelationGetRelationName(state->rel)),
+ errdetail_internal("Lower index tid=%s (points to %s tid=%s) "
+ "higher index tid=%s (points to %s tid=%s) "
+ "page lsn=%X/%X.",
+ itid,
+ P_ISLEAF(topaque) ? "heap":"index",
+ htid,
+ nitid,
+ P_ISLEAF(topaque) ? "heap":"index",
+ nhtid,
+ (uint32) (state->targetlsn >> 32),
+ (uint32) state->targetlsn)));
+ }
+ /*
+ * ********************
+ * * Last item check *
+ * ********************
+ *
+ * Check last item against next/right page's first data item's when
+ * last item on page is reached.
+ *
+ * The general idea here is that checking the ordering of items on the
+ * page should still perform some check on the last item on the page,
+ * if at all possible. In other words, this is roughly the same
+ * process as the page order check that has already been performed for
+ * every other "real" item on target page by now; we just need to reach
+ * into the next page to get a scankey to compare against lower bound
+ * of max.
+ */
+ else if (offset == max)
+ {
+ ScanKey rightkey;
+
+ /* Get item in next/right page */
+ rightkey = bt_right_page_check_scankey(state);
+
+ if (rightkey &&
+ !invariant_key_greater_than_equal_offset(state, rightkey, max))
+ {
+ /*
+ * As discussed in bt_right_page_check_scankey(),
+ * non-ExclusiveLock case might have had target page deleted,
+ * in which case no error is raised
+ */
+ if (!state->exclusivelock)
+ {
+ /* Get fresh copy of target page */
+ state->target = palloc_btree_page(state, state->targetblock);
+ topaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+
+ /*
+ * Because of RecentGlobalXmin interlock against VACUUM's
+ * recycling of blocks, we can safely assume that this is
+ * substantively the same target page as before.
+ *
+ * Just return, because all !exclusivelock checks already
+ * performed against target.
+ */
+ if (P_IGNORE(topaque))
+ return;
+ }
+ ereport(CORRUPTION,
+ (errcode(ERRCODE_INDEX_CORRUPTED),
+ errmsg("cross page order invariant violated for index \"%s\"",
+ RelationGetRelationName(state->rel)),
+ errdetail_internal("Last item on page tid=(%u,%u) "
+ "right page block=%u "
+ "page lsn=%X/%X.",
+ state->targetblock, offset,
+ topaque->btpo_next,
+ (uint32) (state->targetlsn >> 32),
+ (uint32) state->targetlsn)));
+ }
+ }
+
+ /*
+ * ********************
+ * * Downlink check *
+ * ********************
+ *
+ * Additional check of child items against target page (their parent),
+ * iff this is internal page and caller holds ExclusiveLock on index
+ * relation. This involves a pass over each child page at the end of
+ * each iteration (excluding the minus infinity iteration which
+ * internal pages always have, which is immediately skipped).
+ */
+ if (!P_ISLEAF(topaque) && state->exclusivelock)
+ {
+ BlockNumber childblock = ItemPointerGetBlockNumber(&(itup->t_tid));
+
+ bt_downlink_check(state, childblock, skey);
+ }
+ }
+}
+
+/*
+ * bt_right_page_check_scankey()
+ *
+ * Return a scankey for an item on page to right of current target (or the
+ * first non-ignorable page), sufficient to check ordering invariant on last
+ * item in current target page. Returned scankey relies on local memory
+ * allocated for the child page, which caller cannot pfree(). Caller's memory
+ * context should be reset between calls here.
+ *
+ * This is the first data item, and so all adjacent items are checked against
+ * their immediate sibling item (which may be on the same page, or may be the
+ * cases this function helps with: comparisons of items across sibling pages,
+ * or even comparisons of items across "cousin" pages).
+ *
+ * If no first data item is available on right page, just return NULL. Note
+ * that !exclusivelock callers must reverify that target page has not been
+ * concurrently deleted. There is a complex explanation for why this is below.
+ */
+static ScanKey
+bt_right_page_check_scankey(BtreeCheckState *state)
+{
+ BTPageOpaque opaque;
+ ItemId rightitem;
+ BlockNumber targetnext;
+ Page rightpage;
+ OffsetNumber nline;
+
+ /* Determine target's next block number */
+ opaque = (BTPageOpaque) PageGetSpecialPointer(state->target);
+
+ /* If target is already rightmost, no right sibling; nothing to do here */
+ if (P_RIGHTMOST(opaque))
+ return NULL;
+
+ /*
+ * General notes on concurrent page splits and page deletion:
+ *
+ * Concurrent page splits are not a problem for ordinary index scans, since
+ * the key space always moves in a way that lets index scans not miss
+ * things: they might have to move right, but they never have to move left
+ * (leaving aside index scans backwards, a special case). A concurrent
+ * page split could occur here, but just as with index scans we're
+ * following the stale right link, which will reliably get us further along
+ * in the key space, which is all we really need to get an item further
+ * along in key space to check invariant in target page.
+ *
+ * A deleted page won't actually be recycled by VACUUM early enough for us
+ * to fail to be able follow its right link (or left link, or downlink),
+ * because it doesn't do so until it knows that no possible index scan
+ * could land on the page with the expectation of at least being able to
+ * move right and eventually find a non-ignorable page. (see page
+ * recycling/RecentGlobalXmin notes in nbtree README.)
+ *
+ * It's okay if we follow a rightlink and find a half-dead or dead
+ * (ignorable) page. Either way, there must be a sane further right link
+ * to follow for these ignorable pages, because page deletion refuses to
+ * merge the key space between adjacent pages that do not share a common
+ * parent (that is, merging of the key space has to be among true sibling
+ * pages, never cousin pages). We should succeed in finding a page to the
+ * right that isn't ignorable before too long.
+ */
+ targetnext = opaque->btpo_next;
+ for (;;)
+ {
+ CHECK_FOR_INTERRUPTS();
+
+ rightpage = palloc_btree_page(state, targetnext);
+ opaque = (BTPageOpaque) PageGetSpecialPointer(rightpage);
+
+ if (!P_IGNORE(opaque) || P_RIGHTMOST(opaque))
+ break;
+
+ /* We landed on a deleted page, so step right to find a live page */
+ targetnext = opaque->btpo_next;
+ ereport(CONCERN,
+ (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+ errmsg("level %u leftmost page of index \"%s\" was found deleted or half dead",
+ opaque->btpo.level, RelationGetRelationName(state->rel)),
+ errdetail_internal("Deleted page found when building scankey from right sibling.")));
+
+ /* Be slightly more pro-active in freeing this memory, just in case */
+ pfree(rightpage);
+ }
+
+ /*
+ * No ExclusiveLock held case -- why it's safe to proceed.
+ *
+ * Problem:
+ *
+ * We must avoid false positive reports of corruption when caller treats
+ * item returned here as an upper bound on target's last item. In general,
+ * false positives are disallowed. Ensuring they don't happen in
+ * !exclusivelock case is subtle.
+ *
+ * A concurrent page deletion by VACUUM of the target page can result in
+ * the insertion of items on to this right sibling page that would
+ * previously have been inserted on our target page. There might have been
+ * insertions that followed target's downlink after it was made to point to
+ * right sibling instead of target by page deletion's first phase. The
+ * inserters insert items that would belong on target page. This race is
+ * very tight, but it's possible. This is our only problem.
+ *
+ * Non-problems:
+ *
+ * We are not hindered by a concurrent page split of the target; we'll
+ * never land on the second half of the page anyway. A concurrent split of
+ * the right page will also not matter, because the first data item remains
+ * the same within the left half, which we'll reliably land on. If we had
+ * to skip over ignorable/deleted pages, it cannot matter because their key
+ * space has already been atomically merged with the first non-ignorable
+ * page we eventually find (doesn't matter whether the page we eventually
+ * find is a true sibling or a cousin of target, which we go into below).
+ *
+ * Solution:
+ *
+ * Caller knows that it should reverify that target is not ignorable
+ * (half-dead or deleted) when cross-page sibling item comparison appears
+ * to indicate corruption (invariant fails). This detects the single race
+ * condition that exists for caller. This is correct because the continued
+ * existence of target block as non-ignorable (not half-dead or deleted)
+ * implies that target page was not merged into from the right by deletion;
+ * the key space at or after target never moved left. Target's parent
+ * either has the same downlink to target as before, or a <= downlink due
+ * to deletion at the left of target. Target either has the same high key
+ * as before, or a high key <= before when there is a page split. (The
+ * rightmost concurrently-split-from-target-page page will still have the
+ * same high key as target was originally found to have, which for our
+ * purposes is equivalent to target's high key itself never changing, since
+ * we reliably skip over concurrently-split-from-target-page pages.)
+ *
+ * In simpler terms, we allow that the key space of the target may expand
+ * left (the key space can move left on the left side of target only), but
+ * the target key space cannot expand right and get ahead of us without our
+ * detecting it. The key space of the target cannot shrink, unless it
+ * shrinks to zero due to the deletion of the original page, our canary
+ * condition. (To be very precise, we're a bit stricter than that because
+ * it might just have been that the target page split and only the original
+ * target page was deleted. We can be more strict, just not more lax.)
+ *
+ * Top level tree walk caller moves on to next page (makes it the new
+ * target) following recovery from this race. (cf. The rationale for
+ * child/downlink verification needing an ExclusiveLock within
+ * bt_downlink_check(), where page deletion is also the main source of
+ * trouble.)
+ *
+ * Note that it doesn't matter if right sibling page here is actually a
+ * cousin page, because in order for the key space to be readjusted in a
+ * way that causes us issues in next level up (guiding problematic
+ * concurrent insertions to the cousin from the grandparent rather than to
+ * the sibling from the parent), there'd have to be page deletion of
+ * target's parent page (affecting target's parent's downlink in target's
+ * grandparent page). Internal page deletion only occurs when there are no
+ * child pages (they were all fully deleted), and caller is checking that
+ * the target's parent has at least one non-deleted (so non-ignorable)
+ * child: the target page. (Note that the first phase of deletion
+ * atomically marks the page to be deleted half-dead/ignorable at the same
+ * time downlink in its parent is removed, so we'll definitely be able to
+ * detect that this might have happened just from the target page.)
+ *
+ * This trick is inspired by the method backward scans use for dealing with
+ * concurrent page splits; concurrent page deletion is a problem that
+ * similarly receives special consideration sometimes (it's possible that
+ * the backwards scan will re-read its "original" block after failing to
+ * find a right-link to it, having already moved in the opposite direction
+ * (right/"forwards") a few times to try to locate one). Just like us,
+ * that happens only to determine if there was a concurrent page deletion
+ * of a reference page, and just like us if there was a page deletion of
+ * that reference page it means we can move on from caring about the
+ * reference page. See the nbtree README for a full description of how
+ * that works.
+ */
+ nline = PageGetMaxOffsetNumber(rightpage);
+
+ /*
+ * Get first data item.
+ *
+ * Importantly, this allows the verification of page order across target
+ * and rightmost page when rightmost page is the target's right sibling.
+ * Moreover, the only way logical inconsistencies can really be missed
+ * across a given level is if the transitive law is broken by an opclass,
+ * because contiguous pairs are always compared, even across page
+ * boundaries. (Assuming we have a "stable snapshot" of structure, as in
+ * exclusivelock case. Otherwise, strictly speaking, we might just miss
+ * corruption that would have been caught in exclusivelock case.)
+ */
+ if (P_ISLEAF(opaque) && nline >= P_FIRSTDATAKEY(opaque))
+ {
+ /*
+ * For leaf page, return first data item (if any), which cannot be a
+ * "minus infinity" item
+ */
+ rightitem = PageGetItemId(rightpage, P_FIRSTDATAKEY(opaque));
+ }
+ else if (!P_ISLEAF(opaque) &&
+ nline >= OffsetNumberNext(P_FIRSTDATAKEY(opaque)))
+ {
+ /*
+ * Return first item after the internal page's undefined "minus
+ * infinity" item. Also skip the high key when page isn't rightmost on
+ * level.
+ */
+ rightitem = PageGetItemId(rightpage,
+ OffsetNumberNext(P_FIRSTDATAKEY(opaque)));
+ }
+ else
+ {
+ /*
+ * No first item. Page is probably empty leaf page, but it's also
+ * possible that it's an internal page with only a minus infinity item,
+ * because caller moves left to right.
+ */
+ ereport(CONCERN,
+ (errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+ errmsg("%s block %u of index \"%s\" has no first data item",
+ P_ISLEAF(opaque) ? "leaf":"internal", targetnext,
+ RelationGetRelationName(state->rel))));
+ return NULL;
+ }
+
+ /*
+ * Return first real item scankey. Note that this relies on right page
+ * memory remaining allocated.
+ */
+ return _bt_mkscankey(state->rel,
+ (IndexTuple) PageGetItem(rightpage, rightitem));
+}
+
+/*
+ * bt_downlink_check()
+ *
+ * Checks one of target's downlink against its child page.
+ *
+ * Conceptually, the target page continues to be what is checked here. The
+ * target block is still blamed in the event of finding an invariant violation.
+ * The downlink insertion into the target is probably where any problem raised
+ * here arises, and there is no such thing as a parent link, so doing the
+ * verification this way around is much more practical.
+ */
+static void
+bt_downlink_check(BtreeCheckState *state, BlockNumber childblock,
+ ScanKey targetkey)
+{
+ OffsetNumber offset;
+ OffsetNumber maxoffset;
+ Page child;
+ BTPageOpaque copaque;
+
+ /*
+ * Caller must have ExclusiveLock on target relation, because of
+ * considerations around page deletion by VACUUM.
+ *
+ * N.B.: In general, page deletion deletes the right sibling's downlink,
+ * not the downlink of the page being deleted; the deleted page's downlink
+ * is reused for its sibling. The key space is thereby consolidated
+ * between the deleted page and its right sibling. (We cannot delete a
+ * parent page's rightmost page unless it is the last child page, and we
+ * intend to delete the parent itself.)
+ *
+ * If this verification happened without an ExclusiveLock, the following
+ * race condition could cause false positives (which are generally
+ * disallowed):
+ *
+ * Not having an ExclusiveLock would allow concurrent page deletion,
+ * including deletion of the left sibling of the child page that is
+ * examined here. If such a page deletion occurred, and was then closely
+ * followed by an insertion into the newly expanded key space of the child,
+ * a false positive may result: our stale parent/target downlink would
+ * legitimately not be a lower bound on all items in the page anymore,
+ * because the key space was concurrently expanded "left" (insertion
+ * followed the "new" downlink for the child, not our now-stale downlink,
+ * which was concurrently physically removed in target/parent as part of
+ * deletion's first phase).
+ *
+ * Note that while the cross-page-same-level check uses a trick that allows
+ * it to perform verification for !exclusivelock callers, an analogous
+ * trick seems very difficult here. The trick that that other check uses
+ * is, in essence, to lock down race conditions to those that occur due to
+ * concurrent page deletion of the target; that's a race that can be
+ * reliably detected before actually reporting corruption. On the other
+ * hand, we'd need to lock down race conditions involving deletion of
+ * child's left page, at least for long enough to read the child page into
+ * memory. Any more granular locking schemes all seem to involve multiple
+ * concurrently held buffer locks. That's unacceptable for amcheck on
+ * general principle, though; amcheck functions never hold more than one
+ * buffer lock at a time.
+ */
+ Assert(state->exclusivelock);
+
+ /*
+ * Verify child page has the downlink key from target page (its parent) as
+ * a lower bound.
+ *
+ * We prefer to check all items, rather than checking just the first and
+ * trusting that the operator class obeys the transitive law (which implies
+ * that all subsequent items also respected the downlink-as-lower-bound
+ * invariant if they pass the page order check when the child later becomes
+ * our target for verification).
+ */
+ child = palloc_btree_page(state, childblock);
+ copaque = (BTPageOpaque) PageGetSpecialPointer(child);
+ maxoffset = PageGetMaxOffsetNumber(child);
+
+ for (offset = P_FIRSTDATAKEY(copaque);
+ offset <= maxoffset;
+ offset = OffsetNumberNext(offset))
+ {
+ /*
+ * Skip comparison of target page key against "minus infinity" item, if
+ * any. Checking it would indicate that its not an upper bound, but
+ * that's only because of the hard-coding within _bt_compare().
+ */
+ if (OFFSET_IS_MINUS_INFINITY(copaque, offset))
+ continue;
+
+ if (!invariant_key_less_than_equal_nontarget_offset(state, child,
+ targetkey, offset))
+ ereport(CORRUPTION,
+ (errcode(ERRCODE_INDEX_CORRUPTED),
+ errmsg("down-link lower bound invariant violated for index \"%s\"",
+ RelationGetRelationName(state->rel)),
+ errdetail_internal("Parent block=%u child index tid=(%u,%u) "
+ "parent page lsn=%X/%X.",
+ state->targetblock, childblock, offset,
+ (uint32) (state->targetlsn >> 32),
+ (uint32) state->targetlsn)));
+ }
+
+ pfree(child);
+}
+
+/*
+ * invariant_key_less_than_equal_offset()
+ *
+ * Does the invariant hold that the key is less than or equal to a given upper
+ * bound offset item?
+ *
+ * If this function returns false, convention is that caller throws error due
+ * to corruption.
+ */
+static bool
+invariant_key_less_than_equal_offset(BtreeCheckState *state, ScanKey key,
+ OffsetNumber upperbound)
+{
+ int16 natts = state->rel->rd_rel->relnatts;
+ int32 cmp;
+
+ cmp = _bt_compare(state->rel, natts, key, state->target, upperbound);
+
+ return cmp <= 0;
+}
+
+/*
+ * invariant_key_less_than_equal_offset()
+ *
+ * Does the invariant hold that the key is greater than or equal to a given
+ * lower bound offset item?
+ *
+ * If this function returns false, convention is that caller throws error due
+ * to corruption.
+ */
+static bool
+invariant_key_greater_than_equal_offset(BtreeCheckState *state, ScanKey key,
+ OffsetNumber lowerbound)
+{
+ int16 natts = state->rel->rd_rel->relnatts;
+ int32 cmp;
+
+ cmp = _bt_compare(state->rel, natts, key, state->target, lowerbound);
+
+ return cmp >= 0;
+}
+
+/*
+ * invariant_key_less_than_equal_nontarget_offset()
+ *
+ * Does the invariant hold that the key is less than or equal to a given upper
+ * bound offset item, with the offset relating to a caller-supplied page that
+ * is not the current target page? Caller's non-target page is typically a
+ * child page of the target, checked as part of checking a property of the
+ * target page (i.e. the key comes from the target).
+ *
+ * If this function returns false, convention is that caller throws error due
+ * to corruption.
+ */
+static bool
+invariant_key_less_than_equal_nontarget_offset(BtreeCheckState *state,
+ Page nontarget, ScanKey key,
+ OffsetNumber upperbound)
+{
+ int16 natts = state->rel->rd_rel->relnatts;
+ int32 cmp;
+
+ cmp = _bt_compare(state->rel, natts, key, nontarget, upperbound);
+
+ return cmp <= 0;
+}
+
+/*
+ * palloc_btree_page()
+ *
+ * Given a block number of a B-Tree page, return page in palloc()'d memory.
+ * While at it, perform some basic checks of the page.
+ *
+ * There is never an attempt to get a consistent view of multiple pages using
+ * multiple concurrent buffer locks; in general, we prefer to have only one pin
+ * and buffer lock at a time, which is often all that the nbtree code requires.
+ *
+ * Operating on a copy of the page is useful because it prevents control
+ * getting stuck in an uninterruptible state when an underlying operator class
+ * misbehaves.
+ */
+static Page
+palloc_btree_page(BtreeCheckState *state, BlockNumber blocknum)
+{
+ Buffer buffer;
+ Page page;
+ BTPageOpaque opaque;
+
+ page = palloc(BLCKSZ);
+
+ /*
+ * We copy the page into local storage to avoid holding pin on the buffer
+ * longer than we must.
+ */
+ buffer = ReadBufferExtended(state->rel, MAIN_FORKNUM, blocknum, RBM_NORMAL,
+ state->checkstrategy);
+ LockBuffer(buffer, BT_READ);
+
+ /*
+ * Perform the same basic sanity checking that nbtree itself performs for
+ * every page:
+ */
+ _bt_checkpage(state-> rel, buffer);
+
+ /* Only use copy of page in palloc()'d memory */
+ memcpy(page, BufferGetPage(buffer), BLCKSZ);
+ UnlockReleaseBuffer(buffer);
+
+ opaque = (BTPageOpaque) PageGetSpecialPointer(page);
+
+ if (opaque->btpo_flags & BTP_META && blocknum != BTREE_METAPAGE)
+ ereport(CORRUPTION,
+ (errcode(ERRCODE_INDEX_CORRUPTED),
+ errmsg("invalid meta page found at block %u in index \"%s\"",
+ blocknum, RelationGetRelationName(state->rel))));
+
+ /* Check page from block that ought to be meta page */
+ if (blocknum == BTREE_METAPAGE)
+ {
+ BTMetaPageData *metad = BTPageGetMeta(page);
+
+ if (!(opaque->btpo_flags & BTP_META) ||
+ metad->btm_magic != BTREE_MAGIC)
+ ereport(CORRUPTION,
+ (errcode(ERRCODE_INDEX_CORRUPTED),
+ errmsg("index \"%s\" meta page is corrupt",
+ RelationGetRelationName(state->rel))));
+
+ if (metad->btm_version != BTREE_VERSION)
+ ereport(CORRUPTION,
+ (errcode(ERRCODE_INDEX_CORRUPTED),
+ errmsg("version mismatch in index \"%s\": file version %d, code version %d",
+ RelationGetRelationName(state->rel),
+ metad->btm_version, BTREE_VERSION)));
+ }
+
+ /*
+ * Deleted pages have no sane "level" field, so can only check non-deleted
+ * page level
+ */
+ if (P_ISLEAF(opaque) && !P_ISDELETED(opaque) && opaque->btpo.level != 0)
+ ereport(CORRUPTION,
+ (errcode(ERRCODE_INDEX_CORRUPTED),
+ errmsg("invalid leaf page level %u for block %u in index \"%s\"",
+ opaque->btpo.level, blocknum, RelationGetRelationName(state->rel))));
+
+ if (blocknum != BTREE_METAPAGE && !P_ISLEAF(opaque) &&
+ !P_ISDELETED(opaque) && opaque->btpo.level == 0)
+ ereport(CORRUPTION,
+ (errcode(ERRCODE_INDEX_CORRUPTED),
+ errmsg("invalid internal page level 0 for block %u in index \"%s\"",
+ opaque->btpo.level, RelationGetRelationName(state->rel))));
+
+ if (!P_ISLEAF(opaque) && P_HAS_GARBAGE(opaque))
+ ereport(CORRUPTION,
+ (errcode(ERRCODE_INDEX_CORRUPTED),
+ errmsg("internal page block %u in index \"%s\" has garbage items",
+ blocknum, RelationGetRelationName(state->rel))));
+
+ return page;
+}
diff --git a/contrib/amcheck/amcheck.control b/contrib/amcheck/amcheck.control
new file mode 100644
index 0000000..180f150
--- /dev/null
+++ b/contrib/amcheck/amcheck.control
@@ -0,0 +1,5 @@
+# amcheck extension
+comment = 'verify access method invariants'
+default_version = '1.0'
+module_pathname = '$libdir/amcheck'
+relocatable = true
diff --git a/contrib/amcheck/expected/extern_sort_bytea.out b/contrib/amcheck/expected/extern_sort_bytea.out
new file mode 100644
index 0000000..54a8f3b
--- /dev/null
+++ b/contrib/amcheck/expected/extern_sort_bytea.out
@@ -0,0 +1,44 @@
+CREATE TEMP TABLE bytea_test (i bytea);
+-- Seed random number generator to make outcome deterministic.
+SELECT setseed(0.5);
+ setseed
+---------
+
+(1 row)
+
+INSERT INTO bytea_test SELECT decode(repeat(md5(random()::text), (random() * 100)::int4), 'hex')
+ FROM generate_series(0, 100000);
+SET maintenance_work_mem = '1MB';
+CREATE INDEX idx_bytea ON bytea_test(i);
+SELECT bt_index_parent_check('idx_bytea');
+ bt_index_parent_check
+-----------------------
+
+(1 row)
+
+SET maintenance_work_mem = '3MB';
+REINDEX INDEX idx_bytea;
+SELECT bt_index_parent_check('idx_bytea');
+ bt_index_parent_check
+-----------------------
+
+(1 row)
+
+SET maintenance_work_mem = '5MB';
+REINDEX INDEX idx_bytea;
+SELECT bt_index_parent_check('idx_bytea');
+ bt_index_parent_check
+-----------------------
+
+(1 row)
+
+SET maintenance_work_mem = '20MB';
+REINDEX INDEX idx_bytea;
+SELECT bt_index_parent_check('idx_bytea');
+ bt_index_parent_check
+-----------------------
+
+(1 row)
+
+-- drop the temporary table
+DROP TABLE bytea_test;
diff --git a/contrib/amcheck/expected/extern_sort_collations.out b/contrib/amcheck/expected/extern_sort_collations.out
new file mode 100644
index 0000000..63ee4d0
--- /dev/null
+++ b/contrib/amcheck/expected/extern_sort_collations.out
@@ -0,0 +1,42 @@
+-- Perform tests of external sorting collated text. Only test at most 10
+-- available collations, to make sure that the tests complete in a reasonable
+-- amount of time.
+DO $x$
+DECLARE
+ r record;
+ index_build_mem int4;
+BEGIN
+ FOR r IN SELECT oid, collname FROM pg_collation ORDER BY oid LIMIT 10 LOOP
+
+ EXECUTE format($y$CREATE TEMP TABLE text_test_collation (i text COLLATE "%s");$y$, r.collname);
+
+ -- Seed random number generator so that each collation that happens to be
+ -- available on the system consistently sorts the same strings, using the
+ -- same maintenance_work_mem setting. In theory, varying these makes the
+ -- tests more likely to catch problems.
+ PERFORM setseed(get_byte(decode(md5(r.collname), 'hex'), 5) / 256.0);
+ -- Don't actually output collation names
+ -- RAISE NOTICE '%', r.collname;
+
+ -- Make sure MD5 strings contain some space characters, and a doublequote, so
+ -- multiple "weight levels" are represented. Typically, OS strcoll()
+ -- implementations follow the conventions of the standard Unicode collation
+ -- algorithm, and primarily weight alphabetical ordering when comparing
+ -- strings.
+ INSERT INTO text_test_collation
+ SELECT replace(replace(md5(random()::text), '4', ' '), '9', '"')
+ FROM generate_series(0, 100000);
+
+ -- Set maintenance_work_mem to at least mimimum value, 1MB, and at most 10MB
+ -- to test external sorting:
+ index_build_mem := (SELECT greatest((random() * 1024 * 10)::int4, 1024));
+ -- Don't actually output pseudo-random maintenance_work_mem:
+ -- RAISE NOTICE '%', index_build_mem;
+ EXECUTE 'SET maintenance_work_mem = ' || index_build_mem;
+ CREATE INDEX idx_text ON text_test_collation(i);
+ PERFORM bt_index_parent_check('idx_text');
+ -- drop the temporary table
+ DROP TABLE text_test_collation;
+ END LOOP;
+END;
+$x$;
diff --git a/contrib/amcheck/expected/extern_sort_numeric.out b/contrib/amcheck/expected/extern_sort_numeric.out
new file mode 100644
index 0000000..253f9b3
--- /dev/null
+++ b/contrib/amcheck/expected/extern_sort_numeric.out
@@ -0,0 +1,44 @@
+CREATE TEMP TABLE numeric_test (i numeric);
+-- Seed random number generator to make outcome deterministic.
+SELECT setseed(0.5);
+ setseed
+---------
+
+(1 row)
+
+INSERT INTO numeric_test SELECT random() * 100000000 FROM generate_series(0, 5000000);
+-- special cases
+INSERT INTO numeric_test SELECT exp(0.0);
+INSERT INTO numeric_test SELECT exp(1.0);
+INSERT INTO numeric_test SELECT exp(1.0::numeric(71,70));
+INSERT INTO numeric_test SELECT 0.0 ^ 0.0;
+INSERT INTO numeric_test SELECT (-12.34) ^ 0.0;
+INSERT INTO numeric_test SELECT 12.34 ^ 0.0;
+INSERT INTO numeric_test SELECT 0.0 ^ 12.34;
+SET maintenance_work_mem = '1MB';
+CREATE INDEX idx_numeric ON numeric_test(i);
+SELECT bt_index_parent_check('idx_numeric');
+ bt_index_parent_check
+-----------------------
+
+(1 row)
+
+SET maintenance_work_mem = '5MB';
+REINDEX INDEX idx_numeric;
+SELECT bt_index_parent_check('idx_numeric');
+ bt_index_parent_check
+-----------------------
+
+(1 row)
+
+-- Only this final REINDEX requires just one merge pass:
+SET maintenance_work_mem = '20MB';
+REINDEX INDEX idx_numeric;
+SELECT bt_index_parent_check('idx_numeric');
+ bt_index_parent_check
+-----------------------
+
+(1 row)
+
+-- drop the temporary table
+DROP TABLE numeric_test;
diff --git a/contrib/amcheck/expected/install_amcheck.out b/contrib/amcheck/expected/install_amcheck.out
new file mode 100644
index 0000000..5b3e6d5
--- /dev/null
+++ b/contrib/amcheck/expected/install_amcheck.out
@@ -0,0 +1 @@
+CREATE EXTENSION amcheck;
diff --git a/contrib/amcheck/sql/extern_sort_bytea.sql b/contrib/amcheck/sql/extern_sort_bytea.sql
new file mode 100644
index 0000000..a302c55e
--- /dev/null
+++ b/contrib/amcheck/sql/extern_sort_bytea.sql
@@ -0,0 +1,23 @@
+CREATE TEMP TABLE bytea_test (i bytea);
+
+-- Seed random number generator to make outcome deterministic.
+SELECT setseed(0.5);
+
+INSERT INTO bytea_test SELECT decode(repeat(md5(random()::text), (random() * 100)::int4), 'hex')
+ FROM generate_series(0, 100000);
+
+SET maintenance_work_mem = '1MB';
+CREATE INDEX idx_bytea ON bytea_test(i);
+SELECT bt_index_parent_check('idx_bytea');
+SET maintenance_work_mem = '3MB';
+REINDEX INDEX idx_bytea;
+SELECT bt_index_parent_check('idx_bytea');
+SET maintenance_work_mem = '5MB';
+REINDEX INDEX idx_bytea;
+SELECT bt_index_parent_check('idx_bytea');
+SET maintenance_work_mem = '20MB';
+REINDEX INDEX idx_bytea;
+SELECT bt_index_parent_check('idx_bytea');
+
+-- drop the temporary table
+DROP TABLE bytea_test;
diff --git a/contrib/amcheck/sql/extern_sort_collations.sql b/contrib/amcheck/sql/extern_sort_collations.sql
new file mode 100644
index 0000000..63ee4d0
--- /dev/null
+++ b/contrib/amcheck/sql/extern_sort_collations.sql
@@ -0,0 +1,42 @@
+-- Perform tests of external sorting collated text. Only test at most 10
+-- available collations, to make sure that the tests complete in a reasonable
+-- amount of time.
+DO $x$
+DECLARE
+ r record;
+ index_build_mem int4;
+BEGIN
+ FOR r IN SELECT oid, collname FROM pg_collation ORDER BY oid LIMIT 10 LOOP
+
+ EXECUTE format($y$CREATE TEMP TABLE text_test_collation (i text COLLATE "%s");$y$, r.collname);
+
+ -- Seed random number generator so that each collation that happens to be
+ -- available on the system consistently sorts the same strings, using the
+ -- same maintenance_work_mem setting. In theory, varying these makes the
+ -- tests more likely to catch problems.
+ PERFORM setseed(get_byte(decode(md5(r.collname), 'hex'), 5) / 256.0);
+ -- Don't actually output collation names
+ -- RAISE NOTICE '%', r.collname;
+
+ -- Make sure MD5 strings contain some space characters, and a doublequote, so
+ -- multiple "weight levels" are represented. Typically, OS strcoll()
+ -- implementations follow the conventions of the standard Unicode collation
+ -- algorithm, and primarily weight alphabetical ordering when comparing
+ -- strings.
+ INSERT INTO text_test_collation
+ SELECT replace(replace(md5(random()::text), '4', ' '), '9', '"')
+ FROM generate_series(0, 100000);
+
+ -- Set maintenance_work_mem to at least mimimum value, 1MB, and at most 10MB
+ -- to test external sorting:
+ index_build_mem := (SELECT greatest((random() * 1024 * 10)::int4, 1024));
+ -- Don't actually output pseudo-random maintenance_work_mem:
+ -- RAISE NOTICE '%', index_build_mem;
+ EXECUTE 'SET maintenance_work_mem = ' || index_build_mem;
+ CREATE INDEX idx_text ON text_test_collation(i);
+ PERFORM bt_index_parent_check('idx_text');
+ -- drop the temporary table
+ DROP TABLE text_test_collation;
+ END LOOP;
+END;
+$x$;
diff --git a/contrib/amcheck/sql/extern_sort_numeric.sql b/contrib/amcheck/sql/extern_sort_numeric.sql
new file mode 100644
index 0000000..018b87a
--- /dev/null
+++ b/contrib/amcheck/sql/extern_sort_numeric.sql
@@ -0,0 +1,29 @@
+CREATE TEMP TABLE numeric_test (i numeric);
+
+-- Seed random number generator to make outcome deterministic.
+SELECT setseed(0.5);
+
+INSERT INTO numeric_test SELECT random() * 100000000 FROM generate_series(0, 5000000);
+
+-- special cases
+INSERT INTO numeric_test SELECT exp(0.0);
+INSERT INTO numeric_test SELECT exp(1.0);
+INSERT INTO numeric_test SELECT exp(1.0::numeric(71,70));
+INSERT INTO numeric_test SELECT 0.0 ^ 0.0;
+INSERT INTO numeric_test SELECT (-12.34) ^ 0.0;
+INSERT INTO numeric_test SELECT 12.34 ^ 0.0;
+INSERT INTO numeric_test SELECT 0.0 ^ 12.34;
+
+SET maintenance_work_mem = '1MB';
+CREATE INDEX idx_numeric ON numeric_test(i);
+SELECT bt_index_parent_check('idx_numeric');
+SET maintenance_work_mem = '5MB';
+REINDEX INDEX idx_numeric;
+SELECT bt_index_parent_check('idx_numeric');
+-- Only this final REINDEX requires just one merge pass:
+SET maintenance_work_mem = '20MB';
+REINDEX INDEX idx_numeric;
+SELECT bt_index_parent_check('idx_numeric');
+
+-- drop the temporary table
+DROP TABLE numeric_test;
diff --git a/contrib/amcheck/sql/install_amcheck.sql b/contrib/amcheck/sql/install_amcheck.sql
new file mode 100644
index 0000000..5b3e6d5
--- /dev/null
+++ b/contrib/amcheck/sql/install_amcheck.sql
@@ -0,0 +1 @@
+CREATE EXTENSION amcheck;
diff --git a/doc/src/sgml/amcheck.sgml b/doc/src/sgml/amcheck.sgml
new file mode 100644
index 0000000..524f88d
--- /dev/null
+++ b/doc/src/sgml/amcheck.sgml
@@ -0,0 +1,247 @@
+<!-- doc/src/sgml/amcheck.sgml -->
+
+<sect1 id="amcheck" xreflabel="amcheck">
+ <title>amcheck</title>
+
+ <indexterm zone="amcheck">
+ <primary>amcheck</primary>
+ </indexterm>
+
+ <para>
+ The <filename>amcheck</> module provides functions that allow you to
+ verify the logical consistency of the structure of indexes. If the
+ structure appears to be valid, no error is raised.
+ </para>
+
+ <para>
+ The functions verify various <emphasis>invariants</> in the
+ structure of the representation of particular indexes. The
+ correctness of the access method functions behind index scans and
+ other important operations is predicated on these invariants always
+ holding. For example, certain functions verify, among other things,
+ that all B-Tree pages have items in <quote>logical</> order; if that
+ particular invariant somehow fails to hold, we can expect binary
+ searches on any affected page to produce wrong answers. As a
+ consequence of that, index scans may subtly produce wrong answers.
+ </para>
+
+ <para>
+ Verification is performed using the same procedures as those used by
+ index scans themselves, which may be user-defined operator class
+ code. For example, B-Tree index verification relies on comparisons
+ made with one or more B-Tree support function 1 routines, much like
+ B-Tree index scans rely on the routines to guide the scan to a point
+ in the underlying table; see <xref linkend="xindex-support"> for
+ details of operator class support functions.
+ </para>
+
+ <para>
+ <filename>amcheck</> functions may be used only by superusers.
+ </para>
+
+ <sect2>
+ <title>Functions</title>
+
+ <variablelist>
+ <varlistentry>
+ <term>
+ <function>bt_index_check(index regclass) returns void</function>
+ <indexterm>
+ <primary>bt_index_check</primary>
+ </indexterm>
+ </term>
+
+ <listitem>
+ <para>
+ <function>bt_index_check</function> tests that its target, a
+ B-Tree index, respects a variety of invariants. Example usage:
+<screen>
+test=# SELECT bt_index_check(c.oid), c.relname, c.relpages
+FROM pg_index i
+JOIN pg_opclass op ON i.indclass[0] = op.oid
+JOIN pg_am am ON op.opcmethod = am.oid
+JOIN pg_class c ON i.indexrelid = c.oid
+JOIN pg_namespace n ON c.relnamespace = n.oid
+WHERE am.amname = 'btree' AND n.nspname = 'pg_catalog'
+-- Don't check pg_class (bt_index_parent_check() requires this):
+AND c.relname NOT LIKE 'pg_class%'
+-- Function may throw an error when this is omitted:
+AND i.indisready AND i.indisvalid
+ORDER BY c.relpages DESC LIMIT 10;
+ bt_index_check | relname | relpages
+----------------+---------------------------------+----------
+ | pg_depend_reference_index | 43
+ | pg_depend_depender_index | 40
+ | pg_proc_proname_args_nsp_index | 31
+ | pg_description_o_c_o_index | 21
+ | pg_attribute_relid_attnam_index | 14
+ | pg_proc_oid_index | 10
+ | pg_attribute_relid_attnum_index | 9
+ | pg_amproc_fam_proc_index | 5
+ | pg_amop_opr_fam_index | 5
+ | pg_amop_fam_strat_index | 5
+(10 rows)
+</screen>
+ This example shows a session that performs verification of every
+ catalog index in the database <quote>test</> (except those
+ associated with the <structname>pg_class</structname> catalog).
+ Details of just the 10 largest indexes verified are displayed.
+ Since no error is raised, all indexes tested appear to be
+ logically consistent. Naturally, this query could easily be
+ changed to call <function>bt_index_check</function> for every
+ index in the database where verification is supported.
+ </para>
+ <para>
+ An <literal>AccessShareLock</> is acquired on the target index
+ by <function>bt_index_check</function>. This lock mode is the
+ same lock mode acquired on relations by simple
+ <literal>SELECT</> statements.
+ <function>bt_index_check</function> does not verify invariants
+ that span child/parent relationships, nor does it verify that
+ the target index is consistent with its heap relation. When a
+ routine, lightweight test for corruption is required in a live
+ production environment, using
+ <function>bt_index_check</function> often provides the best
+ trade-off between thoroughness of verification and limiting the
+ impact on application performance and availability.
+ </para>
+ </listitem>
+ </varlistentry>
+
+ <varlistentry>
+ <term>
+ <function>bt_index_parent_check(index regclass) returns void</function>
+ <indexterm>
+ <primary>bt_index_parent_check</primary>
+ </indexterm>
+ </term>
+
+ <listitem>
+ <para>
+ <function>bt_index_parent_check</function> tests that its
+ target, a B-Tree index, respects a variety of invariants. The
+ checks performed by <function>bt_index_parent_check</function>
+ are a superset of the checks performed by
+ <function>bt_index_check</function>.
+ <function>bt_index_parent_check</function> can be thought of as
+ a more thorough variant of <function>bt_index_check</function>:
+ unlike <function>bt_index_check</function>,
+ <function>bt_index_parent_check</function> also checks
+ invariants that span parent/child relationships. However, it
+ does not verify that the target index is consistent with its
+ heap relation. <function>bt_index_parent_check</function>
+ follows the general convention of raising an error if it finds a
+ logical inconsistency or other problem.
+ </para>
+ <para>
+ An <literal>ExclusiveLock</> is required on the target index by
+ <function>bt_index_parent_check</function> (a
+ <literal>ShareLock</> is also acquired on the heap relation).
+ These locks prevent concurrent data modification from
+ <command>INSERT</>, <command>UPDATE</>, and <command>DELETE</>
+ commands. The locks also prevent the underlying relation from
+ being concurrently processed by <command>VACUUM</> and certain
+ other utility commands. Note that the function holds locks for
+ as short a duration as possible, so there is no advantage to
+ verifying each index individually in a series of transactions.
+ </para>
+ <para>
+ <function>bt_index_parent_check</function>'s additional
+ verification is more likely to detect various pathological
+ cases. These cases may involve an incorrectly implemented
+ B-Tree operator class used by the index that is checked, or,
+ hypothetically, undiscovered bugs in the underlying B-Tree index
+ access method code. Note that
+ <function>bt_index_parent_check</function> will raise an error
+ when called while Hot Standby mode is enabled, unlike
+ <function>bt_index_check</function>.
+ </para>
+ </listitem>
+ </varlistentry>
+ </variablelist>
+ </sect2>
+
+ <sect2>
+ <title>Using <filename>amcheck</> effectively</title>
+
+ <para>
+ <filename>amcheck</> can be effective at detecting various types of
+ failure modes that <link
+ linkend="app-initdb-data-checksums"><application>data page
+ checksums</></link> will always fail to catch. These include:
+
+ <itemizedlist>
+ <listitem>
+ <para>
+ Filesystem or storage subsystem faults where checksums happen to
+ simply not be enabled. Note that <filename>amcheck</> examines a
+ page as represented in some shared memory buffer at the time of
+ verification if there is only a shared buffer hit when accessing
+ the block. Consequently, <filename>amcheck</> does not
+ necessarily examine data read from the filesystem at the time of
+ verification. Note that when checksums are enabled,
+ <filename>amcheck</> may raise an error due to a checksum failure
+ when a corrupt block is read into a buffer.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Corruption caused by faulty RAM, and the broader memory subsystem
+ and operating system. <productname>PostgreSQL</> does not
+ protect against correctable memory errors and it is assumed you
+ will operate using RAM that uses industry standard Error
+ Correcting Codes (ECC) or better protection. However, ECC memory
+ is typically only immune to single-bit errors, and should not be
+ assumed to provide <emphasis>absolute</emphasis> protection
+ against failures that result in memory corruption.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Structural inconsistencies caused by incorrect operator class
+ implementations, including issues due to the comparison rules of
+ operating system collations changing the ordering implied by
+ comparisons of an underlying collatable type, such as
+ <type>text</>. Though rare, updates to operating system
+ collation rules can cause these issues. More commonly, an
+ inconsistency in the collation order between a master server and
+ a standby server is implicated, possibly because the
+ <emphasis>major</> operating system version in use is
+ inconsistent. Such inconsistencies will generally only arise on
+ standby servers, and so can generally only be detected on standby
+ servers. If a problem like this arises, it may not affect each
+ individual index that is ordered using an affected collation,
+ simply because <emphasis>indexed</> values might happen to have
+ the same absolute ordering regardless of the behavioral
+ inconsistency. See <xref linkend="locale"> and <xref
+ linkend="collation"> for further details about how
+ <productname>PostgreSQL</> uses operating system locales and
+ collations.
+ </para>
+ </listitem>
+ <listitem>
+ <para>
+ Corruption caused by hypothetical undiscovered bugs in the
+ underlying <productname>PostgreSQL</> access method code.
+ Automatic verification of the structural integrity of indexes
+ plays a role in the general testing of new or proposed
+ <productname>PostgreSQL</> features that could plausibly allow a
+ logical inconsistency to be introduced. One obvious testing
+ strategy is to call <filename>amcheck</> functions continuously
+ when running the standard regression tests. See <xref
+ linkend="regress-run"> for details on running the tests.
+ </para>
+ </listitem>
+ </itemizedlist>
+
+ No error concerning corruption raised by <filename>amcheck</> should
+ ever be a false positive. Users are strongly advised to perform a
+ <command>REINDEX</> for any index where corruption is indicated,
+ although it is also important to identify and correct the underlying
+ problem. In general, <filename>amcheck</> can only prove the
+ presence of corruption; it cannot prove its absence.
+ </para>
+
+ </sect2>
+
+</sect1>
diff --git a/doc/src/sgml/contrib.sgml b/doc/src/sgml/contrib.sgml
index 4e3f337..9f6389c 100644
--- a/doc/src/sgml/contrib.sgml
+++ b/doc/src/sgml/contrib.sgml
@@ -103,6 +103,7 @@ CREATE EXTENSION <replaceable>module_name</> FROM unpackaged;
</para>
&adminpack;
+ &amcheck;
&auth-delay;
&auto-explain;
&btree-gin;
diff --git a/doc/src/sgml/filelist.sgml b/doc/src/sgml/filelist.sgml
index 30adece..5675d6d 100644
--- a/doc/src/sgml/filelist.sgml
+++ b/doc/src/sgml/filelist.sgml
@@ -104,6 +104,7 @@
<!-- contrib information -->
<!ENTITY contrib SYSTEM "contrib.sgml">
<!ENTITY adminpack SYSTEM "adminpack.sgml">
+<!ENTITY amcheck SYSTEM "amcheck.sgml">
<!ENTITY auth-delay SYSTEM "auth-delay.sgml">
<!ENTITY auto-explain SYSTEM "auto-explain.sgml">
<!ENTITY btree-gin SYSTEM "btree-gin.sgml">
--
1.9.1
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