Fix assorted core dumps and Assert failures that could occur during

AbortTransaction or AbortSubTransaction, when trying to clean up after an
error that prevented (sub)transaction start from completing:
* access to TopTransactionResourceOwner that might not exist
* assert failure in AtEOXact_GUC, if AtStart_GUC not called yet
* assert failure or core dump in AfterTriggerEndSubXact, if
  AfterTriggerBeginSubXact not called yet

Per testing by injecting elog(ERROR) at successive steps in StartTransaction
and StartSubTransaction.  It's not clear whether all of these cases could
really occur in the field, but at least one of them is easily exposed by
simple stress testing, as per my accidental discovery yesterday.

src/backend/access/transam/xact.c

@@ -10,7 +10,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.281 2010/01/16 10:05:50 sriggs Exp $
+ *	  $PostgreSQL: pgsql/src/backend/access/transam/xact.c,v 1.282 2010/01/24 21:49:17 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -2170,37 +2170,40 @@ AbortTransaction(void)
 
 	/*
 	 * Post-abort cleanup.	See notes in CommitTransaction() concerning
-	 * ordering.
+	 * ordering.  We can skip all of it if the transaction failed before
+	 * creating a resource owner.
 	 */
+	if (TopTransactionResourceOwner != NULL)
+	{
+		CallXactCallbacks(XACT_EVENT_ABORT);
 
-	CallXactCallbacks(XACT_EVENT_ABORT);
-
-	ResourceOwnerRelease(TopTransactionResourceOwner,
-						 RESOURCE_RELEASE_BEFORE_LOCKS,
-						 false, true);
-	AtEOXact_Buffers(false);
-	AtEOXact_RelationCache(false);
-	AtEOXact_Inval(false);
-	smgrDoPendingDeletes(false);
-	AtEOXact_MultiXact();
-	ResourceOwnerRelease(TopTransactionResourceOwner,
-						 RESOURCE_RELEASE_LOCKS,
-						 false, true);
-	ResourceOwnerRelease(TopTransactionResourceOwner,
-						 RESOURCE_RELEASE_AFTER_LOCKS,
-						 false, true);
-	AtEOXact_CatCache(false);
-
-	AtEOXact_GUC(false, 1);
-	AtEOXact_SPI(false);
-	AtEOXact_on_commit_actions(false);
-	AtEOXact_Namespace(false);
-	AtEOXact_Files();
-	AtEOXact_ComboCid();
-	AtEOXact_HashTables(false);
-	AtEOXact_PgStat(false);
-	AtEOXact_Snapshot(false);
-	pgstat_report_xact_timestamp(0);
+		ResourceOwnerRelease(TopTransactionResourceOwner,
+							 RESOURCE_RELEASE_BEFORE_LOCKS,
+							 false, true);
+		AtEOXact_Buffers(false);
+		AtEOXact_RelationCache(false);
+		AtEOXact_Inval(false);
+		smgrDoPendingDeletes(false);
+		AtEOXact_MultiXact();
+		ResourceOwnerRelease(TopTransactionResourceOwner,
+							 RESOURCE_RELEASE_LOCKS,
+							 false, true);
+		ResourceOwnerRelease(TopTransactionResourceOwner,
+							 RESOURCE_RELEASE_AFTER_LOCKS,
+							 false, true);
+		AtEOXact_CatCache(false);
+
+		AtEOXact_GUC(false, 1);
+		AtEOXact_SPI(false);
+		AtEOXact_on_commit_actions(false);
+		AtEOXact_Namespace(false);
+		AtEOXact_Files();
+		AtEOXact_ComboCid();
+		AtEOXact_HashTables(false);
+		AtEOXact_PgStat(false);
+		AtEOXact_Snapshot(false);
+		pgstat_report_xact_timestamp(0);
+	}
 
 	/*
 	 * State remains TRANS_ABORT until CleanupTransaction().

src/backend/commands/trigger.c

@@ -7,7 +7,7 @@
  * Portions Copyright (c) 1994, Regents of the University of California
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/commands/trigger.c,v 1.259 2010/01/17 22:56:21 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/commands/trigger.c,v 1.260 2010/01/24 21:49:17 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -3690,10 +3690,9 @@ AfterTriggerEndSubXact(bool isCommit)
 	/*
 	 * Pop the prior state if needed.
 	 */
-	Assert(my_level < afterTriggers->maxtransdepth);
-
 	if (isCommit)
 	{
+		Assert(my_level < afterTriggers->maxtransdepth);
 		/* If we saved a prior state, we don't need it anymore */
 		state = afterTriggers->state_stack[my_level];
 		if (state != NULL)
@@ -3706,8 +3705,16 @@ AfterTriggerEndSubXact(bool isCommit)
 	else
 	{
 		/*
-		 * Aborting.  Release any event lists from queries being aborted, and
-		 * restore query_depth to its pre-subxact value.
+		 * Aborting.  It is possible subxact start failed before calling
+		 * AfterTriggerBeginSubXact, in which case we mustn't risk touching
+		 * stack levels that aren't there.
+		 */
+		if (my_level >= afterTriggers->maxtransdepth)
+			return;
+
+		/*
+		 * Release any event lists from queries being aborted, and restore
+		 * query_depth to its pre-subxact value.
 		 */
 		while (afterTriggers->query_depth > afterTriggers->depth_stack[my_level])
 		{

src/backend/utils/misc/guc.c

@@ -10,7 +10,7 @@
  * Written by Peter Eisentraut <peter_e@gmx.net>.
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.534 2010/01/23 16:37:12 sriggs Exp $
+ *	  $PostgreSQL: pgsql/src/backend/utils/misc/guc.c,v 1.535 2010/01/24 21:49:17 tgl Exp $
  *
  *--------------------------------------------------------------------
  */
@@ -3904,7 +3904,14 @@ AtEOXact_GUC(bool isCommit, int nestLevel)
 	bool		still_dirty;
 	int			i;
 
-	Assert(nestLevel > 0 && nestLevel <= GUCNestLevel);
+	/*
+	 * Note: it's possible to get here with GUCNestLevel == nestLevel-1 during
+	 * abort, if there is a failure during transaction start before
+	 * AtStart_GUC is called.
+	 */
+	Assert(nestLevel > 0 &&
+		   (nestLevel <= GUCNestLevel ||
+			(nestLevel == GUCNestLevel + 1 && !isCommit)));
 
 	/* Quick exit if nothing's changed in this transaction */
 	if (!guc_dirty)