Fix visibility check when XID is committed in CLOG but not in procarray.

TransactionIdIsInProgress had a fast path to return 'false' if the
single-item CLOG cache said that the transaction was known to be
committed. However, that was wrong, because a transaction is first
marked as committed in the CLOG but doesn't become visible to others
until it has removed its XID from the proc array. That could lead to an
error:

    ERROR:  t_xmin is uncommitted in tuple to be updated

or for an UPDATE to go ahead without blocking, before the previous
UPDATE on the same row was made visible.

The window is usually very short, but synchronous replication makes it
much wider, because the wait for synchronous replica happens in that
window.

Another thing that makes it hard to hit is that it's hard to get such
a commit-in-progress transaction into the single item CLOG cache.
Normally, if you call TransactionIdIsInProgress on such a transaction,
it determines that the XID is in progress without checking the CLOG
and without populating the cache. One way to prime the cache is to
explicitly call pg_xact_status() on the XID. Another way is to use a
lot of subtransactions, so that the subxid cache in the proc array is
overflown, making TransactionIdIsInProgress rely on pg_subtrans and
CLOG checks.

This has been broken ever since it was introduced in 2008, but the race
condition is very hard to hit, especially without synchronous
replication. There were a couple of reports of the error starting from
summer 2021, but no one was able to find the root cause then.

TransactionIdIsKnownCompleted() is now unused. In 'master', remove it,
but I left it in place in backbranches in case it's used by extensions.

Also change pg_xact_status() to check TransactionIdIsInProgress().
Previously, it only checked the CLOG, and returned "committed" before
the transaction was actually made visible to other queries. Note that
this also means that you cannot use pg_xact_status() to reproduce the
bug anymore, even if the code wasn't fixed.

Report and analysis by Konstantin Knizhnik. Patch by Simon Riggs, with
the pg_xact_status() change added by me.

Author: Simon Riggs
Reviewed-by: Andres Freund
Discussion: https://www.postgresql.org/message-id/flat/4da7913d-398c-e2ad-d777-f752cf7f0bbb%40garret.ru

src/backend/access/transam/transam.c

@@ -219,33 +219,6 @@ TransactionIdDidAbort(TransactionId transactionId)
 	return false;
 }
 
-/*
- * TransactionIdIsKnownCompleted
- *		True iff transaction associated with the identifier is currently
- *		known to have either committed or aborted.
- *
- * This does NOT look into pg_xact but merely probes our local cache
- * (and so it's not named TransactionIdDidComplete, which would be the
- * appropriate name for a function that worked that way).  The intended
- * use is just to short-circuit TransactionIdIsInProgress calls when doing
- * repeated heapam_visibility.c checks for the same XID.  If this isn't
- * extremely fast then it will be counterproductive.
- *
- * Note:
- *		Assumes transaction identifier is valid.
- */
-bool
-TransactionIdIsKnownCompleted(TransactionId transactionId)
-{
-	if (TransactionIdEquals(transactionId, cachedFetchXid))
-	{
-		/* If it's in the cache at all, it must be completed. */
-		return true;
-	}
-
-	return false;
-}
-
 /*
  * TransactionIdCommitTree
  *		Marks the given transaction and children as committed

src/backend/storage/ipc/procarray.c

@@ -261,6 +261,11 @@ static ProcArrayStruct *procArray;
 
 static PGPROC *allProcs;
 
+/*
+ * Cache to reduce overhead of repeated calls to TransactionIdIsInProgress()
+ */
+static TransactionId cachedXidIsNotInProgress = InvalidTransactionId;
+
 /*
  * Bookkeeping for tracking emulated transactions in recovery
  */
@@ -1396,7 +1401,7 @@ TransactionIdIsInProgress(TransactionId xid)
 	 * already known to be completed, we can fall out without any access to
 	 * shared memory.
 	 */
-	if (TransactionIdIsKnownCompleted(xid))
+	if (TransactionIdEquals(cachedXidIsNotInProgress, xid))
 	{
 		xc_by_known_xact_inc();
 		return false;
@@ -1554,6 +1559,7 @@ TransactionIdIsInProgress(TransactionId xid)
 	if (nxids == 0)
 	{
 		xc_no_overflow_inc();
+		cachedXidIsNotInProgress = xid;
 		return false;
 	}
 
@@ -1568,7 +1574,10 @@ TransactionIdIsInProgress(TransactionId xid)
 	xc_slow_answer_inc();
 
 	if (TransactionIdDidAbort(xid))
+	{
+		cachedXidIsNotInProgress = xid;
 		return false;
+	}
 
 	/*
 	 * It isn't aborted, so check whether the transaction tree it belongs to
@@ -1586,6 +1595,7 @@ TransactionIdIsInProgress(TransactionId xid)
 		}
 	}
 
+	cachedXidIsNotInProgress = xid;
 	return false;
 }
 

src/backend/utils/adt/xid8funcs.c

@@ -36,6 +36,7 @@
 #include "miscadmin.h"
 #include "postmaster/postmaster.h"
 #include "storage/lwlock.h"
+#include "storage/procarray.h"
 #include "utils/builtins.h"
 #include "utils/memutils.h"
 #include "utils/snapmgr.h"
@@ -676,29 +677,22 @@ pg_xact_status(PG_FUNCTION_ARGS)
 	{
 		Assert(TransactionIdIsValid(xid));
 
-		if (TransactionIdIsCurrentTransactionId(xid))
+		/*
+		 * Like when doing visiblity checks on a row, check whether the
+		 * transaction is still in progress before looking into the CLOG.
+		 * Otherwise we would incorrectly return "committed" for a transaction
+		 * that is committing and has already updated the CLOG, but hasn't
+		 * removed its XID from the proc array yet. (See comment on that race
+		 * condition at the top of heapam_visibility.c)
+		 */
+		if (TransactionIdIsInProgress(xid))
 			status = "in progress";
 		else if (TransactionIdDidCommit(xid))
 			status = "committed";
-		else if (TransactionIdDidAbort(xid))
-			status = "aborted";
 		else
 		{
-			/*
-			 * The xact is not marked as either committed or aborted in clog.
-			 *
-			 * It could be a transaction that ended without updating clog or
-			 * writing an abort record due to a crash. We can safely assume
-			 * it's aborted if it isn't committed and is older than our
-			 * snapshot xmin.
-			 *
-			 * Otherwise it must be in-progress (or have been at the time we
-			 * checked commit/abort status).
-			 */
-			if (TransactionIdPrecedes(xid, GetActiveSnapshot()->xmin))
-				status = "aborted";
-			else
-				status = "in progress";
+			/* it must have aborted or crashed */
+			status = "aborted";
 		}
 	}
 	else

src/include/access/transam.h

@@ -273,7 +273,6 @@ extern PGDLLIMPORT VariableCache ShmemVariableCache;
  */
 extern bool TransactionIdDidCommit(TransactionId transactionId);
 extern bool TransactionIdDidAbort(TransactionId transactionId);
-extern bool TransactionIdIsKnownCompleted(TransactionId transactionId);
 extern void TransactionIdCommitTree(TransactionId xid, int nxids, TransactionId *xids);
 extern void TransactionIdAsyncCommitTree(TransactionId xid, int nxids, TransactionId *xids, XLogRecPtr lsn);
 extern void TransactionIdAbortTree(TransactionId xid, int nxids, TransactionId *xids);