On Thu, Mar 18, 2021 at 1:11 PM Bharath Rupireddy
<bharath.rupireddyforpostg...@gmail.com> wrote:
>
> On Thu, Mar 18, 2021 at 12:46 PM Fujii Masao
> <masao.fu...@oss.nttdata.com> wrote:
> > On 2021/03/17 11:58, Kyotaro Horiguchi wrote:
> > > The first suggested signature for pg_terminate_backend() with timeout
> > > was pg_terminate_backend(pid, timeout). The current signature (pid,
> > > wait?, timeout) looks redundant. Maybe the reason for rejecting 0
> > > astimeout is pg_terminate_backend(pid, true, 0) looks odd but it we
> > > can wait forever in that case (as other features does).
> >
> > I'm afraid that "waiting forever" can cause something like deadlock
> > situation,
> > as follows. We have no mechanism to detect this for now.
> >
> > 1. backend 1 took the lock on the relation A.
> > 2. backend 2 took the lock on the relation B.
> > 3. backend 1 tries to take the lock on the relation B and is waiting for
> > the lock to be released.
> > 4. backend 2 accidentally executes pg_wait_for_backend_termination() with
> > the pid of backend 1, and then is waiting for backend 1 to be
> > terminated.
>
> Yeah this can happen.
>
> So, as stated upthread, how about a timeout 0 (which is default)
> telling "don't wait", erroring out on negative value and when
> specified a positive milliseconds value, then wait for that amount of
> time. With this semantics, we can remove the wait flag for
> pg_terminate_backend(pid, 0). Thoughts?
>
> And for pg_wait_for_backend_termination timeout 0 or negative, we
> error out. Thoughts?
Attaching v11 patch that removed the wait boolean flag in the
pg_terminate_backend and timeout 0 indicates "no wait", negative value
"errors out", positive value "waits for those many milliseconds". Also
addressed other review comments that I received upthread. Please
review v11 further.
With Regards,
Bharath Rupireddy.
EnterpriseDB: http://www.enterprisedb.com
From 96c74184bdf378cb6105ae7d24bfc7a6888b955a Mon Sep 17 00:00:00 2001
From: Bharath Rupireddy <bharath.rupireddy@enterprisedb.com>
Date: Fri, 19 Mar 2021 11:19:36 +0530
Subject: [PATCH v11] pg_terminate_backend with wait and timeout
This patch adds extra default argument timeout (with default
value 0 meaning don't wait), to pg_terminate_backend which
will be used to terminate and wait timeout milliseconds for
the backend's termination.
This patch also adds a new function
pg_wait_for_backend_termination(pid, timeout) which checks
for the existence of the backend with given PID and waits
timeout milliseconds for its termination.
---
doc/src/sgml/func.sgml | 32 ++++++-
doc/src/sgml/monitoring.sgml | 4 +
src/backend/catalog/system_views.sql | 10 ++
src/backend/postmaster/pgstat.c | 3 +
src/backend/storage/ipc/signalfuncs.c | 132 +++++++++++++++++++++++++-
src/include/catalog/pg_proc.dat | 9 +-
src/include/pgstat.h | 3 +-
7 files changed, 184 insertions(+), 9 deletions(-)
diff --git a/doc/src/sgml/func.sgml b/doc/src/sgml/func.sgml
index 9492a3c6b9..523e6dd72a 100644
--- a/doc/src/sgml/func.sgml
+++ b/doc/src/sgml/func.sgml
@@ -24821,7 +24821,7 @@ SELECT collation for ('foo' COLLATE "de_DE");
<indexterm>
<primary>pg_terminate_backend</primary>
</indexterm>
- <function>pg_terminate_backend</function> ( <parameter>pid</parameter> <type>integer</type> )
+ <function>pg_terminate_backend</function> ( <parameter>pid</parameter> <type>integer</type>, <parameter>timeout</parameter> <type>bigint</type> <literal>DEFAULT</literal> <literal>0</literal> )
<returnvalue>boolean</returnvalue>
</para>
<para>
@@ -24829,7 +24829,35 @@ SELECT collation for ('foo' COLLATE "de_DE");
specified process ID. This is also allowed if the calling role
is a member of the role whose backend is being terminated or the
calling role has been granted <literal>pg_signal_backend</literal>,
- however only superusers can terminate superuser backends.
+ however only superusers can terminate superuser backends. When
+ <parameter>timeout</parameter> is not specified, this function sends
+ SIGTERM to the backend with the given process ID without waiting for
+ its actual termination and returns <literal>true</literal>. Sometimes
+ the backend process may still exist. With <parameter>timeout</parameter>
+ specified in milliseconds, the function ensures that the backend
+ process is actually goes away from the system processes. It keeps
+ checking for the existence of the backend process until the given
+ <parameter>timeout</parameter> occurs and returns <literal>true</literal>
+ if the process doesn't exit. On timeout a warning is emitted and
+ <literal>false</literal> is returned.
+ </para></entry>
+ </row>
+
+ <row>
+ <entry role="func_table_entry"><para role="func_signature">
+ <indexterm>
+ <primary>pg_wait_for_backend_termination</primary>
+ </indexterm>
+ <function>pg_wait_for_backend_termination</function> ( <parameter>pid</parameter> <type>integer</type>, <parameter>timeout</parameter> <type>bigint</type> <literal>DEFAULT</literal> <literal>5000</literal> )
+ <returnvalue>boolean</returnvalue>
+ </para>
+ <para>
+ Checks the existence of backend process with specified process ID. It
+ waits until the given <parameter>timeout</parameter> or the default
+ 5000 milliseconds or 5 seconds. <literal>true</literal> is returned
+ when the backend is found to be not existing within the
+ <parameter>timeout</parameter> milliseconds. On timeout, a warning is
+ emitted and <literal>false</literal> is returned.
</para></entry>
</row>
</tbody>
diff --git a/doc/src/sgml/monitoring.sgml b/doc/src/sgml/monitoring.sgml
index db4b4e460c..f362184d5e 100644
--- a/doc/src/sgml/monitoring.sgml
+++ b/doc/src/sgml/monitoring.sgml
@@ -1569,6 +1569,10 @@ postgres 27093 0.0 0.0 30096 2752 ? Ss 11:34 0:00 postgres: ser
</thead>
<tbody>
+ <row>
+ <entry><literal>BackendTermination</literal></entry>
+ <entry>Waiting for the termination of a backend.</entry>
+ </row>
<row>
<entry><literal>BackupWaitWalArchive</literal></entry>
<entry>Waiting for WAL files required for a backup to be successfully
diff --git a/src/backend/catalog/system_views.sql b/src/backend/catalog/system_views.sql
index 0dca65dc7b..192c0ae2bd 100644
--- a/src/backend/catalog/system_views.sql
+++ b/src/backend/catalog/system_views.sql
@@ -1277,6 +1277,16 @@ CREATE OR REPLACE FUNCTION
RETURNS boolean STRICT VOLATILE LANGUAGE INTERNAL AS 'pg_promote'
PARALLEL SAFE;
+CREATE OR REPLACE FUNCTION
+ pg_terminate_backend(pid integer, timeout int8 DEFAULT 0)
+ RETURNS boolean STRICT VOLATILE LANGUAGE INTERNAL AS 'pg_terminate_backend'
+ PARALLEL SAFE;
+
+CREATE OR REPLACE FUNCTION
+ pg_wait_for_backend_termination(pid integer, timeout int8 DEFAULT 5000)
+ RETURNS boolean STRICT VOLATILE LANGUAGE INTERNAL AS 'pg_wait_for_backend_termination'
+ PARALLEL SAFE;
+
-- legacy definition for compatibility with 9.3
CREATE OR REPLACE FUNCTION
json_populate_record(base anyelement, from_json json, use_json_as_text boolean DEFAULT false)
diff --git a/src/backend/postmaster/pgstat.c b/src/backend/postmaster/pgstat.c
index 208a33692f..78729f1b92 100644
--- a/src/backend/postmaster/pgstat.c
+++ b/src/backend/postmaster/pgstat.c
@@ -3998,6 +3998,9 @@ pgstat_get_wait_ipc(WaitEventIPC w)
switch (w)
{
+ case WAIT_EVENT_BACKEND_TERMINATION:
+ event_name = "BackendTermination";
+ break;
case WAIT_EVENT_BACKUP_WAIT_WAL_ARCHIVE:
event_name = "BackupWaitWalArchive";
break;
diff --git a/src/backend/storage/ipc/signalfuncs.c b/src/backend/storage/ipc/signalfuncs.c
index 69fe23a256..1e141fba7c 100644
--- a/src/backend/storage/ipc/signalfuncs.c
+++ b/src/backend/storage/ipc/signalfuncs.c
@@ -18,6 +18,7 @@
#include "catalog/pg_authid.h"
#include "miscadmin.h"
+#include "pgstat.h"
#include "postmaster/syslogger.h"
#include "storage/pmsignal.h"
#include "storage/proc.h"
@@ -126,15 +127,96 @@ pg_cancel_backend(PG_FUNCTION_ARGS)
}
/*
- * Signal to terminate a backend process. This is allowed if you are a member
- * of the role whose process is being terminated.
+ * Wait until there is no backend process with the given PID and return true.
+ * On timeout, a warning is emitted and false is returned.
+ */
+static bool
+pg_wait_until_termination(int pid, int64 timeout)
+{
+ /*
+ * Wait in steps of waittime milliseconds until this function exits or
+ * timeout.
+ */
+ int64 waittime = 100;
+ /*
+ * Initially remaining time is the entire timeout specified by the user.
+ */
+ int64 remainingtime = timeout;
+
+ /*
+ * Check existence of the backend. If it doesn't exist, then check for the
+ * errno ESRCH that confirms it and return true. If the backend still
+ * exists, then wait for waittime milliseconds, again check for the
+ * existence. Repeat this until timeout or an error occurs or a pending
+ * interrupt such as query cancel gets processed.
+ */
+ do
+ {
+ if (remainingtime < waittime)
+ waittime = remainingtime;
+
+ if (kill(pid, 0) == -1)
+ {
+ if (errno == ESRCH)
+ return true;
+ else
+ ereport(ERROR,
+ (errcode(ERRCODE_INTERNAL_ERROR),
+ errmsg("could not check the existence of the backend with PID %d: %m",
+ pid)));
+ }
+
+ /* Process interrupts, if any, before going for wait. */
+ CHECK_FOR_INTERRUPTS();
+
+ (void) WaitLatch(MyLatch,
+ WL_LATCH_SET | WL_TIMEOUT | WL_EXIT_ON_PM_DEATH,
+ waittime,
+ WAIT_EVENT_BACKEND_TERMINATION);
+
+ ResetLatch(MyLatch);
+
+ remainingtime -= waittime;
+ } while (remainingtime > 0);
+
+ ereport(WARNING,
+ (errmsg("backend with PID %d did not terminate within %lld milliseconds",
+ pid, (long long int) timeout)));
+
+ return false;
+}
+
+/*
+ * Signal to terminate a backend process. This is allowed if you are a member
+ * of the role whose process is being terminated. If timeout input argument is
+ * 0 (which is default), then this function just signals the backend and
+ * doesn't wait. Otherwise it waits until given the timeout milliseconds or no
+ * process has the given PID and returns true. On timeout, a warning is emitted
+ * and false is returned.
*
* Note that only superusers can signal superuser-owned processes.
*/
Datum
pg_terminate_backend(PG_FUNCTION_ARGS)
{
- int r = pg_signal_backend(PG_GETARG_INT32(0), SIGTERM);
+ int pid;
+ int r;
+ int timeout;
+ bool wait;
+ bool result;
+
+ pid = PG_GETARG_INT32(0);
+ timeout = PG_GETARG_INT64(1);
+
+ if (timeout < 0)
+ ereport(ERROR,
+ (errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
+ errmsg("\"timeout\" must not be negative")));
+
+ /* If timeout is zero (which is default), then don't wait. */
+ wait = (timeout == 0) ? false : true;
+
+ r = pg_signal_backend(pid, SIGTERM);
if (r == SIGNAL_BACKEND_NOSUPERUSER)
ereport(ERROR,
@@ -146,7 +228,49 @@ pg_terminate_backend(PG_FUNCTION_ARGS)
(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
errmsg("must be a member of the role whose process is being terminated or member of pg_signal_backend")));
- PG_RETURN_BOOL(r == SIGNAL_BACKEND_SUCCESS);
+ result = (r == SIGNAL_BACKEND_SUCCESS);
+
+ /* Wait only if requested and the termination is successful. */
+ if (wait && result)
+ result = pg_wait_until_termination(pid, timeout);
+
+ PG_RETURN_BOOL(result);
+}
+
+/*
+ * This function waits for a backend with the given PID to be terminated or
+ * until the given timeout milliseconds occurs. Returns true if the backend
+ * doesn't exist. On Timeout a warning is emitted and false is returned.
+ */
+Datum
+pg_wait_for_backend_termination(PG_FUNCTION_ARGS)
+{
+ int pid;
+ int64 timeout;
+ PGPROC *proc = NULL;
+ bool result = false;
+
+ pid = PG_GETARG_INT32(0);
+ timeout = PG_GETARG_INT64(1);
+
+ if (timeout <= 0)
+ ereport(ERROR,
+ (errcode(ERRCODE_NUMERIC_VALUE_OUT_OF_RANGE),
+ errmsg("\"timeout\" must not be negative or zero")));
+
+ proc = BackendPidGetProc(pid);
+
+ if (proc == NULL)
+ {
+ ereport(WARNING,
+ (errmsg("PID %d is not a PostgreSQL server process", pid)));
+
+ PG_RETURN_BOOL(result);
+ }
+
+ result = pg_wait_until_termination(pid, timeout);
+
+ PG_RETURN_BOOL(result);
}
/*
diff --git a/src/include/catalog/pg_proc.dat b/src/include/catalog/pg_proc.dat
index 93393fcfd4..2f5fa24bf0 100644
--- a/src/include/catalog/pg_proc.dat
+++ b/src/include/catalog/pg_proc.dat
@@ -6143,9 +6143,14 @@
{ oid => '2171', descr => 'cancel a server process\' current query',
proname => 'pg_cancel_backend', provolatile => 'v', prorettype => 'bool',
proargtypes => 'int4', prosrc => 'pg_cancel_backend' },
-{ oid => '2096', descr => 'terminate a server process',
+{ oid => '2096', descr => 'terminate a backend process and if timeout is specified, wait for its exit or until timeout occurs',
proname => 'pg_terminate_backend', provolatile => 'v', prorettype => 'bool',
- proargtypes => 'int4', prosrc => 'pg_terminate_backend' },
+ proargtypes => 'int4 int8', proargnames => '{pid,timeout}',
+ prosrc => 'pg_terminate_backend' },
+{ oid => '16386', descr => 'wait for a backend process exit or timeout occurs',
+ proname => 'pg_wait_for_backend_termination', provolatile => 'v', prorettype => 'bool',
+ proargtypes => 'int4 int8', proargnames => '{pid,timeout}',
+ prosrc => 'pg_wait_for_backend_termination' },
{ oid => '2172', descr => 'prepare for taking an online backup',
proname => 'pg_start_backup', provolatile => 'v', proparallel => 'r',
prorettype => 'pg_lsn', proargtypes => 'text bool bool',
diff --git a/src/include/pgstat.h b/src/include/pgstat.h
index be43c04802..bbfe2f2f04 100644
--- a/src/include/pgstat.h
+++ b/src/include/pgstat.h
@@ -967,7 +967,8 @@ typedef enum
*/
typedef enum
{
- WAIT_EVENT_BACKUP_WAIT_WAL_ARCHIVE = PG_WAIT_IPC,
+ WAIT_EVENT_BACKEND_TERMINATION = PG_WAIT_IPC,
+ WAIT_EVENT_BACKUP_WAIT_WAL_ARCHIVE,
WAIT_EVENT_BGWORKER_SHUTDOWN,
WAIT_EVENT_BGWORKER_STARTUP,
WAIT_EVENT_BTREE_PAGE,
--
2.25.1
