Throw error if expiring tuple is again updated or deleted.

This prevents surprising behavior when a FOR EACH ROW trigger
BEFORE UPDATE or BEFORE DELETE directly or indirectly updates or
deletes the the old row.  Prior to this patch the requested action
on the row could be silently ignored while all triggered actions
based on the occurence of the requested action could be committed.
One example of how this could happen is if the BEFORE DELETE
trigger for a "parent" row deleted "children" which had trigger
functions to update summary or status data on the parent.

This also prevents similar surprising problems if the query has a
volatile function which updates a target row while it is already
being updated.

There are related issues present in FOR UPDATE cursors and READ
COMMITTED queries which are not handled by this patch.  These
issues need further evalution to determine what change, if any, is
needed.

Where the new error messages are generated, in most cases the best
fix will be to move code from the BEFORE trigger to an AFTER
trigger.  Where this is not feasible, the trigger can avoid the
error by re-issuing the triggering statement and returning NULL.

Documentation changes will be submitted in a separate patch.

Kevin Grittner and Tom Lane with input from Florian Pflug and
Robert Haas, based on problems encountered during conversion of
Wisconsin Circuit Court trigger logic to plpgsql triggers.

1 files changed, 22 insertions, 9 deletions

diff --git a/src/backend/commands/trigger.c b/src/backend/commands/trigger.c
index 4d3ed9cb62c..98b82074b5f 100644
--- a/src/backend/commands/trigger.c
+++ b/src/backend/commands/trigger.c
@@ -2571,8 +2571,7 @@ GetTupleForTrigger(EState *estate,
 	if (newSlot != NULL)
 	{
 		HTSU_Result test;
-		ItemPointerData update_ctid;
-		TransactionId update_xmax;
+		HeapUpdateFailureData hufd;
 
 		*newSlot = NULL;
 
@@ -2584,13 +2583,27 @@ GetTupleForTrigger(EState *estate,
 		 */
 ltrmark:;
 		tuple.t_self = *tid;
-		test = heap_lock_tuple(relation, &tuple, &buffer,
-							   &update_ctid, &update_xmax,
+		test = heap_lock_tuple(relation, &tuple,
 							   estate->es_output_cid,
-							   LockTupleExclusive, false);
+							   LockTupleExclusive, false /* wait */,
+							   &buffer, &hufd);
 		switch (test)
 		{
 			case HeapTupleSelfUpdated:
+				/*
+				 * The target tuple was already updated or deleted by the
+				 * current command, or by a later command in the current
+				 * transaction.  We ignore the tuple in the former case, and
+				 * throw error in the latter case, for the same reasons
+				 * enumerated in ExecUpdate and ExecDelete in
+				 * nodeModifyTable.c.
+				 */
+				if (hufd.cmax != estate->es_output_cid)
+					ereport(ERROR,
+							(errcode(ERRCODE_TRIGGERED_DATA_CHANGE_VIOLATION),
+							 errmsg("tuple to be updated was already modified by an operation triggered by the current command"),
+							 errhint("Consider using an AFTER trigger instead of a BEFORE trigger to propagate changes to other rows.")));
+
 				/* treat it as deleted; do not process */
 				ReleaseBuffer(buffer);
 				return NULL;
@@ -2604,7 +2617,7 @@ ltrmark:;
 					ereport(ERROR,
 							(errcode(ERRCODE_T_R_SERIALIZATION_FAILURE),
 							 errmsg("could not serialize access due to concurrent update")));
-				if (!ItemPointerEquals(&update_ctid, &tuple.t_self))
+				if (!ItemPointerEquals(&hufd.ctid, &tuple.t_self))
 				{
 					/* it was updated, so look at the updated version */
 					TupleTableSlot *epqslot;
@@ -2613,11 +2626,11 @@ ltrmark:;
 										   epqstate,
 										   relation,
 										   relinfo->ri_RangeTableIndex,
-										   &update_ctid,
-										   update_xmax);
+										   &hufd.ctid,
+										   hufd.xmax);
 					if (!TupIsNull(epqslot))
 					{
-						*tid = update_ctid;
+						*tid = hufd.ctid;
 						*newSlot = epqslot;
 
 						/*