Re: Race condition between PREPARE TRANSACTION and COMMIT PREPARED (was Re: [HACKERS] Problem with txid_snapshot_in/out() functionality)

[Heikki Linnakangas]

On 05/06/2014 02:44 PM, Andres Freund wrote:

On 2014-05-05 13:41:00 +0300, Heikki Linnakangas wrote:

+/*
+ * Exit hook to unlock the global transaction entry we're working on.
+ */
+static void
+AtProcExit_Twophase(int code, Datum arg)
+{
+   /* same logic as abort */
+   AtAbort_Twophase();
+}
+
+/*
+ * Abort hook to unlock the global transaction entry we're working on.
+ */
+void
+AtAbort_Twophase(void)
+{
+   if (MyLockedGxact == NULL)
+   return;
+
+   /*
+* If we were in process of preparing the transaction, but haven't
+* written the WAL record yet, remove the global transaction entry.
+* Same if we are in the process of finishing an already-prepared
+* transaction, and fail after having already written the WAL 2nd
+* phase commit or rollback record.
+*
+* After that it's too late to abort, so just unlock the 
GlobalTransaction
+* entry.  We might not have transfered all locks and other state to the
+* prepared transaction yet, so this is a bit bogus, but it's the best 
we
+* can do.
+*/
+   if (!MyLockedGxact-valid)
+   {
+   RemoveGXact(MyLockedGxact);
+   }
+   else
+   {
+   LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);
+
+   MyLockedGxact-locking_backend = InvalidBackendId;
+
+   LWLockRelease(TwoPhaseStateLock);
+   }
+   MyLockedGxact = NULL;
+}


Is it guaranteed that all paths have called LWLockReleaseAll()
before calling the proc exit hooks? Otherwise we might end up waiting
for ourselves...


Hmm. AbortTransaction() will release locks before we get here, but the 
before_shmem_exit() callpath will not. So an elog(FATAL), while holding 
TwoPhaseStateLock would cause us to deadlock with ourself. But there are 
no such elogs.


I copied this design from async.c, which is quite similar, so if there's 
a problem that ought to be fixed too. And there are other more 
complicated before_shmem callbacks that worry me more, like 
createdb_failure_callback(). But I think they're all all right.



  /*
   * MarkAsPreparing
@@ -261,29 +329,15 @@ MarkAsPreparing(TransactionId xid, const char *gid,
 errmsg(prepared transactions are disabled),
  errhint(Set max_prepared_transactions to a nonzero 
value.)));

-   LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);
-
-   /*
-* First, find and recycle any gxacts that failed during prepare. We do
-* this partly to ensure we don't mistakenly say their GIDs are still
-* reserved, and partly so we don't fail on out-of-slots unnecessarily.
-*/
-   for (i = 0; i  TwoPhaseState-numPrepXacts; i++)
+   /* on first call, register the exit hook */
+   if (!twophaseExitRegistered)
{
-   gxact = TwoPhaseState-prepXacts[i];
-   if (!gxact-valid  !TransactionIdIsActive(gxact-locking_xid))
-   {
-   /* It's dead Jim ... remove from the active array */
-   TwoPhaseState-numPrepXacts--;
-   TwoPhaseState-prepXacts[i] = 
TwoPhaseState-prepXacts[TwoPhaseState-numPrepXacts];
-   /* and put it back in the freelist */
-   gxact-next = TwoPhaseState-freeGXacts;
-   TwoPhaseState-freeGXacts = gxact;
-   /* Back up index count too, so we don't miss scanning 
one */
-   i--;
-   }
+   before_shmem_exit(AtProcExit_Twophase, 0);
+   twophaseExitRegistered = true;
}


It's not particularly nice to register shmem exit hooks in the middle of
normal processing because it makes it impossible to use
cancel_before_shmem_exit() previously registered hooks. I think this
should be registered at startup, if max_prepared_xacts  0.


shrug. async.c and namespace.c does the same, and it hasn't been a 
problem.


I committed this now, but please let me know if you see a concrete 
problem with the locks.


- Heikki


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Re: Race condition between PREPARE TRANSACTION and COMMIT PREPARED (was Re: [HACKERS] Problem with txid_snapshot_in/out() functionality)

[Andres Freund]

On 2014-05-15 17:21:28 +0300, Heikki Linnakangas wrote:
 Is it guaranteed that all paths have called LWLockReleaseAll()
 before calling the proc exit hooks? Otherwise we might end up waiting
 for ourselves...
 
 Hmm. AbortTransaction() will release locks before we get here, but the
 before_shmem_exit() callpath will not. So an elog(FATAL), while holding
 TwoPhaseStateLock would cause us to deadlock with ourself. But there are no
 such elogs.

 I copied this design from async.c, which is quite similar, so if there's a
 problem that ought to be fixed too. And there are other more complicated
 before_shmem callbacks that worry me more, like createdb_failure_callback().
 But I think they're all all right.

Perhaps we should enforce that LWLockReleaseAll() is called first?
E.g. in shmem_exit()? It'll happen in ProcKill() atm, but that's
normally pretty much at the bottom of the stack.

 It's not particularly nice to register shmem exit hooks in the middle of
 normal processing because it makes it impossible to use
 cancel_before_shmem_exit() previously registered hooks. I think this
 should be registered at startup, if max_prepared_xacts  0.
 
 shrug. async.c and namespace.c does the same, and it hasn't been a
 problem.

Well, it doesn't seem unreasonable to have C code using
PG_ENSURE_ERROR_CLEANUP/PG_END_ENSURE_ERROR_CLEANUP around a 2pc commit
to me. That'll break with this.
Perhaps we should just finally make cancel_before_shmem_exit search the
stack of callbacks.

Greetings,

Andres Freund

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Re: Race condition between PREPARE TRANSACTION and COMMIT PREPARED (was Re: [HACKERS] Problem with txid_snapshot_in/out() functionality)

[Robert Haas]

On Thu, May 15, 2014 at 10:38 AM, Andres Freund and...@2ndquadrant.com wrote:
 shrug. async.c and namespace.c does the same, and it hasn't been a
 problem.

 Well, it doesn't seem unreasonable to have C code using
 PG_ENSURE_ERROR_CLEANUP/PG_END_ENSURE_ERROR_CLEANUP around a 2pc commit
 to me. That'll break with this.
 Perhaps we should just finally make cancel_before_shmem_exit search the
 stack of callbacks.

Yes, please.  And while we're at it, perhaps we should make it Trap()
or fail an Assert() if it doesn't find the callback it was told to
remove.

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EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company


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Re: Race condition between PREPARE TRANSACTION and COMMIT PREPARED (was Re: [HACKERS] Problem with txid_snapshot_in/out() functionality)

[Andres Freund]

Hi,

On 2014-05-05 13:41:00 +0300, Heikki Linnakangas wrote:
 I came up with the attached fix for this. Currently, the entry is implicitly
 considered dead or unlocked if the locking_xid transaction is no longer
 active, but this patch essentially turns locking_xid into a simple boolean,
 and makes it the backend's responsibility to clear it on abort. (it's not
 actually a boolean, it's a BackendId, but that's just for debugging purposes
 to track who's keeping the entry locked). This requires a process exit hook,
 and an abort hook, to make sure the entry is always released, but that's not
 too difficult. It allows the backend to release the entry at exactly the
 right time, instead of having it implicitly released by

 I considered Andres' idea of using a new heavy-weight lock, but didn't like
 it much. It would be a larger patch, which is not nice for back-patching.
 One issue would be that if you run out of lock memory, you could not roll
 back any prepared transactions, which is not nice because it could be a
 prepared transaction that's hoarding the lock memory.

I am not convinced by the latter reasoning but you're right that any
such change would hardly be backpatchable.

 +/*
 + * Exit hook to unlock the global transaction entry we're working on.
 + */
 +static void
 +AtProcExit_Twophase(int code, Datum arg)
 +{
 + /* same logic as abort */
 + AtAbort_Twophase();
 +}
 +
 +/*
 + * Abort hook to unlock the global transaction entry we're working on.
 + */
 +void
 +AtAbort_Twophase(void)
 +{
 + if (MyLockedGxact == NULL)
 + return;
 +
 + /*
 +  * If we were in process of preparing the transaction, but haven't
 +  * written the WAL record yet, remove the global transaction entry.
 +  * Same if we are in the process of finishing an already-prepared
 +  * transaction, and fail after having already written the WAL 2nd
 +  * phase commit or rollback record.
 +  *
 +  * After that it's too late to abort, so just unlock the 
 GlobalTransaction
 +  * entry.  We might not have transfered all locks and other state to the
 +  * prepared transaction yet, so this is a bit bogus, but it's the best 
 we
 +  * can do.
 +  */
 + if (!MyLockedGxact-valid)
 + {
 + RemoveGXact(MyLockedGxact);
 + }
 + else
 + {
 + LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);
 +
 + MyLockedGxact-locking_backend = InvalidBackendId;
 +
 + LWLockRelease(TwoPhaseStateLock);
 + }
 + MyLockedGxact = NULL;
 +}

Is it guaranteed that all paths have called LWLockReleaseAll()
before calling the proc exit hooks? Otherwise we might end up waiting
for ourselves...

  /*
   * MarkAsPreparing
 @@ -261,29 +329,15 @@ MarkAsPreparing(TransactionId xid, const char *gid,
errmsg(prepared transactions are disabled),
 errhint(Set max_prepared_transactions to a nonzero 
 value.)));
  
 - LWLockAcquire(TwoPhaseStateLock, LW_EXCLUSIVE);
 -
 - /*
 -  * First, find and recycle any gxacts that failed during prepare. We do
 -  * this partly to ensure we don't mistakenly say their GIDs are still
 -  * reserved, and partly so we don't fail on out-of-slots unnecessarily.
 -  */
 - for (i = 0; i  TwoPhaseState-numPrepXacts; i++)
 + /* on first call, register the exit hook */
 + if (!twophaseExitRegistered)
   {
 - gxact = TwoPhaseState-prepXacts[i];
 - if (!gxact-valid  !TransactionIdIsActive(gxact-locking_xid))
 - {
 - /* It's dead Jim ... remove from the active array */
 - TwoPhaseState-numPrepXacts--;
 - TwoPhaseState-prepXacts[i] = 
 TwoPhaseState-prepXacts[TwoPhaseState-numPrepXacts];
 - /* and put it back in the freelist */
 - gxact-next = TwoPhaseState-freeGXacts;
 - TwoPhaseState-freeGXacts = gxact;
 - /* Back up index count too, so we don't miss scanning 
 one */
 - i--;
 - }
 + before_shmem_exit(AtProcExit_Twophase, 0);
 + twophaseExitRegistered = true;
   }

It's not particularly nice to register shmem exit hooks in the middle of
normal processing because it makes it impossible to use
cancel_before_shmem_exit() previously registered hooks. I think this
should be registered at startup, if max_prepared_xacts  0.

Greetings,

Andres Freund

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Re: Race condition between PREPARE TRANSACTION and COMMIT PREPARED (was Re: [HACKERS] Problem with txid_snapshot_in/out() functionality)

[Heikki Linnakangas]

On 04/14/2014 09:48 PM, Heikki Linnakangas wrote:

On 04/14/2014 07:51 PM, Tom Lane wrote:

I'd prefer to leave the prepare sequence alone and instead find a way
to reject COMMIT PREPARED until after the source transaction is safely
clear of the race conditions.  The upthread idea of looking at vxid
instead of xid might help, except that I see we clear both of them
in ProcArrayClearTransaction.  We'd need some state in PGPROC that
isn't cleared till later than that.


Hmm. What if one of the post-cleanup action fails? We can't bail out of
the prepare sequence until we have transfered the locks to the new
PGPROC. Otherwise the locks are lost. In essence, there should be a
critical section from the EndPrepare call until all the critical cleanup
actions like PostPrepare_Locks have been done, and I don't think we want
that. We might be able to guarantee that the built-in post-cleanup
operations are safe enough for that, but there's also CallXactCallbacks
in there.

Given the lack of reports of that happening, though, perhaps that's not
an issue.


I came up with the attached fix for this. Currently, the entry is 
implicitly considered dead or unlocked if the locking_xid transaction is 
no longer active, but this patch essentially turns locking_xid into a 
simple boolean, and makes it the backend's responsibility to clear it on 
abort. (it's not actually a boolean, it's a BackendId, but that's just 
for debugging purposes to track who's keeping the entry locked). This 
requires a process exit hook, and an abort hook, to make sure the entry 
is always released, but that's not too difficult. It allows the backend 
to release the entry at exactly the right time, instead of having it 
implicitly released by ProcArrayClearTransaction.


If we error during prepare, after having written the prepare WAL record 
but before the locks have been transfered to the dummy PGPROC, the locks 
are simply released. This is wrong, but it's always been like that and 
we haven't heard any complaints of that from the field, so I'm inclined 
to leave it as it is. We could use a critical section to force a panic, 
but that cure could be a worse than the disease.


I considered Andres' idea of using a new heavy-weight lock, but didn't 
like it much. It would be a larger patch, which is not nice for 
back-patching. One issue would be that if you run out of lock memory, 
you could not roll back any prepared transactions, which is not nice 
because it could be a prepared transaction that's hoarding the lock memory.


- Heikki

diff --git a/src/backend/access/transam/twophase.c b/src/backend/access/transam/twophase.c
index 66dbf58..995f51f 100644
--- a/src/backend/access/transam/twophase.c
+++ b/src/backend/access/transam/twophase.c
@@ -58,6 +58,7 @@
 #include replication/walsender.h
 #include replication/syncrep.h
 #include storage/fd.h
+#include storage/ipc.h
 #include storage/predicate.h
 #include storage/proc.h
 #include storage/procarray.h
@@ -82,25 +83,25 @@ int			max_prepared_xacts = 0;
  *
  * The lifecycle of a global transaction is:
  *
- * 1. After checking that the requested GID is not in use, set up an
- * entry in the TwoPhaseState-prepXacts array with the correct XID and GID,
- * with locking_xid = my own XID and valid = false.
+ * 1. After checking that the requested GID is not in use, set up an entry in
+ * the TwoPhaseState-prepXacts array with the correct GID and valid = false,
+ * and mark it as locked by my backend.
  *
  * 2. After successfully completing prepare, set valid = true and enter the
  * referenced PGPROC into the global ProcArray.
  *
- * 3. To begin COMMIT PREPARED or ROLLBACK PREPARED, check that the entry
- * is valid and its locking_xid is no longer active, then store my current
- * XID into locking_xid.  This prevents concurrent attempts to commit or
- * rollback the same prepared xact.
+ * 3. To begin COMMIT PREPARED or ROLLBACK PREPARED, check that the entry is
+ * valid and not locked, then mark the entry as locked by storing my current
+ * backend ID into locking_backend.  This prevents concurrent attempts to
+ * commit or rollback the same prepared xact.
  *
  * 4. On completion of COMMIT PREPARED or ROLLBACK PREPARED, remove the entry
  * from the ProcArray and the TwoPhaseState-prepXacts array and return it to
  * the freelist.
  *
  * Note that if the preparing transaction fails between steps 1 and 2, the
- * entry will remain in prepXacts until recycled.  We can detect recyclable
- * entries by checking for valid = false and locking_xid no longer active.
+ * entry must be removed so that the GID and the GlobalTransaction struct
+ * can be reused.  See AtAbort_Twophase().
  *
  * typedef struct GlobalTransactionData *GlobalTransaction appears in
  * twophase.h
@@ -115,8 +116,8 @@ typedef struct GlobalTransactionData
 	TimestampTz prepared_at;	/* time of preparation */
 	XLogRecPtr	prepare_lsn;	/* XLOG offset of prepare record */
 	Oid			owner;			/* ID of user that executed the xact */
-	

Re: Race condition between PREPARE TRANSACTION and COMMIT PREPARED (was Re: [HACKERS] Problem with txid_snapshot_in/out() functionality)

[Heikki Linnakangas]

On 04/13/2014 11:39 PM, Heikki Linnakangas wrote:

However, I just noticed that there's a race condition between PREPARE
TRANSACTION and COMMIT/ROLLBACK PREPARED. PostPrepare_Locks runs after
the prepared transaction is already marked as fully prepared. That means
that by the time we get to PostPrepare_Locks, another backend might
already have finished and removed the prepared transaction. That leads
to a PANIC (put a breakpoint just before PostPrepare_Locks):

postgres=# commit prepared 'foo';
PANIC:  failed to re-find shared proclock object
PANIC:  failed to re-find shared proclock object
The connection to the server was lost. Attempting reset: Failed.

FinishPrepareTransaction reads the list of locks from the two-phase
state file, but PANICs when it doesn't find the corresponding locks in
the lock manager (because PostPrepare_Locks hasn't transfered them to
the dummy PGPROC yet).

I think we'll need to transfer of the locks earlier, before the
transaction is marked as fully prepared. I'll take a closer look at this
tomorrow.


Here's a patch to do that. It's very straightforward, I just moved the 
calls to transfer locks earlier, before ProcArrayClearTransaction. 
PostPrepare_MultiXact had a similar race -  it also transfer state from 
the old PGPROC entry to the new, and needs to be done before allowing 
another backend to remove the new PGPROC entry. I changed the names of 
the functions to distinguish them from the other PostPrepare_* functions 
that now happen at a different time.


The patch is simple, but it's a bit scary to change the order of things 
like this. Looking at all the calls that now happen after transferring 
the locks, I believe this is OK. The change also applies to the 
callbacks called by the RegisterXactCallback mechanism, which means that 
in theory there might be a 3rd party extension out there that's 
affected. All the callbacks in contrib and plpgsql are OK, and it's 
questionable to do anything complicated that would depend on 
heavy-weight locks to be held in those callbacks, so I think this is OK. 
Warrants a note in the release notes, though.


- Heikki

diff --git a/src/backend/access/transam/multixact.c b/src/backend/access/transam/multixact.c
index d4ad678..b505c62 100644
--- a/src/backend/access/transam/multixact.c
+++ b/src/backend/access/transam/multixact.c
@@ -1580,11 +1580,12 @@ AtPrepare_MultiXact(void)
 }
 
 /*
- * PostPrepare_MultiXact
- *		Clean up after successful PREPARE TRANSACTION
+ * TransferPrepare_MultiXact
+ *		Called after successful PREPARE TRANSACTION, before releasing our
+ *		PGPROC entry.
  */
 void
-PostPrepare_MultiXact(TransactionId xid)
+TransferPrepare_MultiXact(TransactionId xid)
 {
 	MultiXactId myOldestMember;
 
diff --git a/src/backend/access/transam/xact.c b/src/backend/access/transam/xact.c
index b20d973..c60edf1 100644
--- a/src/backend/access/transam/xact.c
+++ b/src/backend/access/transam/xact.c
@@ -2224,6 +2224,16 @@ PrepareTransaction(void)
 	XactLastRecEnd = 0;
 
 	/*
+	 * Transfer any heavy-weight locks we're holding to the dummy ProcArray.
+	 *
+	 * NB: this has to be done before clearing our own ProcArray entry.
+	 * This is different from normal commit, where locks are released after
+	 * clearing the ProcArray entry!
+	 */
+	TransferPrepare_MultiXact(xid);	  /* also transfer our multixact state */
+	TransferPrepare_Locks(xid);
+
+	/*
 	 * Let others know about no transaction in progress by me.	This has to be
 	 * done *after* the prepared transaction has been marked valid, else
 	 * someone may think it is unlocked and recyclable.
@@ -2234,6 +2244,10 @@ PrepareTransaction(void)
 	 * This is all post-transaction cleanup.  Note that if an error is raised
 	 * here, it's too late to abort the transaction.  This should be just
 	 * noncritical resource releasing.	See notes in CommitTransaction.
+	 *
+	 * NB: we already transfered the locks to the prepared ProcArray entry,
+	 * so even the cleanup before ResourceOwnerRelease(RESOURCE_RELEASE_LOCKS)
+	 * cannot rely on any heavy-weight locks being held!
 	 */
 
 	CallXactCallbacks(XACT_EVENT_PREPARE);
@@ -2256,11 +2270,12 @@ PrepareTransaction(void)
 
 	PostPrepare_smgr();
 
-	PostPrepare_MultiXact(xid);
-
-	PostPrepare_Locks(xid);
 	PostPrepare_PredicateLocks(xid);
 
+	/*
+	 * we're not actually holding any locks anymore, but clean up any other
+	 * resources that might need to be cleaned up at this stage.
+	 */
 	ResourceOwnerRelease(TopTransactionResourceOwner,
 		 RESOURCE_RELEASE_LOCKS,
 		 true, true);
diff --git a/src/backend/storage/lmgr/lock.c b/src/backend/storage/lmgr/lock.c
index 6825063..779f0cb 100644
--- a/src/backend/storage/lmgr/lock.c
+++ b/src/backend/storage/lmgr/lock.c
@@ -3069,8 +3069,8 @@ AtPrepare_Locks(void)
 }
 
 /*
- * PostPrepare_Locks
- *		Clean up after successful PREPARE
+ * TransferPrepare_Locks
+ *		Transfer locks to prepared transaction after successful PREPARE
  *
  * Here, we want to transfer ownership of our locks to a dummy PGPROC
  

Re: Race condition between PREPARE TRANSACTION and COMMIT PREPARED (was Re: [HACKERS] Problem with txid_snapshot_in/out() functionality)

[Tom Lane]

Heikki Linnakangas hlinnakan...@vmware.com writes:
 I think we'll need to transfer of the locks earlier, before the
 transaction is marked as fully prepared. I'll take a closer look at this
 tomorrow.

 Here's a patch to do that. It's very straightforward, I just moved the 
 calls to transfer locks earlier, before ProcArrayClearTransaction. 
 PostPrepare_MultiXact had a similar race -  it also transfer state from 
 the old PGPROC entry to the new, and needs to be done before allowing 
 another backend to remove the new PGPROC entry. I changed the names of 
 the functions to distinguish them from the other PostPrepare_* functions 
 that now happen at a different time.

Why didn't you also move up PostPrepare_PredicateLocks?  Seems like its
access to MySerializableXact is also racy.

 The patch is simple, but it's a bit scary to change the order of things 
 like this.

Yeah.  There are a lot of assumptions in there about the order of resource
release, in particular that it is safe to do certain things because we're
still holding locks.

I poked around a bit and noticed one theoretical problem sequence: if the
prepared xact drops some relation that we're still holding buffer pins on.
This shouldn't really happen (why are we still pinning some rel we think
we dropped?) but if it did, the commit would do DropRelFileNodeBuffers
which would end up busy-looping until we drop our pins (see
InvalidateBuffer, which thinks this must be an I/O wait situation).
So it would work, more or less, but it seems pretty fragile.  I'm afraid
there are more assumptions like this one.

The whole thing feels like we are solving the wrong problem, anyway.
IIUC, the complaint arises because we are allowing COMMIT PREPARED
to occur before the source transaction has reported successful prepare
to its client.  Surely that does not need to be a legal case?  No
correctly-operating 2PC xact manager would do that.

I'd prefer to leave the prepare sequence alone and instead find a way
to reject COMMIT PREPARED until after the source transaction is safely
clear of the race conditions.  The upthread idea of looking at vxid
instead of xid might help, except that I see we clear both of them
in ProcArrayClearTransaction.  We'd need some state in PGPROC that
isn't cleared till later than that.

regards, tom lane


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Re: Race condition between PREPARE TRANSACTION and COMMIT PREPARED (was Re: [HACKERS] Problem with txid_snapshot_in/out() functionality)

[Andres Freund]

On 2014-04-14 12:51:02 -0400, Tom Lane wrote:
 The whole thing feels like we are solving the wrong problem, anyway.
 IIUC, the complaint arises because we are allowing COMMIT PREPARED
 to occur before the source transaction has reported successful prepare
 to its client.  Surely that does not need to be a legal case?  No
 correctly-operating 2PC xact manager would do that.

I agree here. This seems somewhat risky, just to support a case that
shouldn't happen in reality - as somewhat evidenced by the fact that
there don't seem to be field reports around this.

 The upthread idea of looking at vxid
 instead of xid might help, except that I see we clear both of them
 in ProcArrayClearTransaction.  We'd need some state in PGPROC that
 isn't cleared till later than that.

I wonder if the most natural way to express this wouldn't be to have a
heavyweight lock for every 2pc xact
'slot'. ResourceOwnerRelease(RESOURCE_RELEASE_LOCKS) should be scheduled
correctly to make error handling for this work.

Greetings,

Andres Freund

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Re: Race condition between PREPARE TRANSACTION and COMMIT PREPARED (was Re: [HACKERS] Problem with txid_snapshot_in/out() functionality)

[Tom Lane]

Andres Freund and...@2ndquadrant.com writes:
 I wonder if the most natural way to express this wouldn't be to have a
 heavyweight lock for every 2pc xact
 'slot'. ResourceOwnerRelease(RESOURCE_RELEASE_LOCKS) should be scheduled
 correctly to make error handling for this work.

That seems like not a bad idea.  Could we also use the same lock to
prevent concurrent attempts to commit/rollback the same already-prepared
transaction?  I forget what we're doing to forestall such cases right now.

regards, tom lane


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Re: Race condition between PREPARE TRANSACTION and COMMIT PREPARED (was Re: [HACKERS] Problem with txid_snapshot_in/out() functionality)

[Andres Freund]

On 2014-04-14 13:47:35 -0400, Tom Lane wrote:
 Andres Freund and...@2ndquadrant.com writes:
  I wonder if the most natural way to express this wouldn't be to have a
  heavyweight lock for every 2pc xact
  'slot'. ResourceOwnerRelease(RESOURCE_RELEASE_LOCKS) should be scheduled
  correctly to make error handling for this work.
 
 That seems like not a bad idea.  Could we also use the same lock to
 prevent concurrent attempts to commit/rollback the same already-prepared
 transaction?  I forget what we're doing to forestall such cases right now.

GlobalTransaction-locking_xid is currently used. If it points to a live
transaction by another backned prepared transaction with identifier
\%s\ is busy will be thrown.
ISTM if there were using a lock for every slot, that logic couldbe
thrown away.

Greetings,

Andres Freund

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 PostgreSQL Development, 24x7 Support, Training  Services


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Re: Race condition between PREPARE TRANSACTION and COMMIT PREPARED (was Re: [HACKERS] Problem with txid_snapshot_in/out() functionality)

[Heikki Linnakangas]

On 04/14/2014 07:51 PM, Tom Lane wrote:

I'd prefer to leave the prepare sequence alone and instead find a way
to reject COMMIT PREPARED until after the source transaction is safely
clear of the race conditions.  The upthread idea of looking at vxid
instead of xid might help, except that I see we clear both of them
in ProcArrayClearTransaction.  We'd need some state in PGPROC that
isn't cleared till later than that.


Hmm. What if one of the post-cleanup action fails? We can't bail out of 
the prepare sequence until we have transfered the locks to the new 
PGPROC. Otherwise the locks are lost. In essence, there should be a 
critical section from the EndPrepare call until all the critical cleanup 
actions like PostPrepare_Locks have been done, and I don't think we want 
that. We might be able to guarantee that the built-in post-cleanup 
operations are safe enough for that, but there's also CallXactCallbacks 
in there.


Given the lack of reports of that happening, though, perhaps that's not 
an issue.


- Heikki


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