Check for pending trigger events on far end when dropping an FK constraint.

When dropping a foreign key constraint with ALTER TABLE DROP CONSTRAINT,
we refuse the drop if there are any pending trigger events on the named
table; this ensures that we won't remove the pg_trigger row that will be
consulted by those events.  But we should make the same check for the
referenced relation, else we might remove a due-to-be-referenced pg_trigger
row for that relation too, resulting in "could not find trigger NNN" or
"relation NNN has no triggers" errors at commit.  Per bug #14431 from
Benjie Gillam.  Back-patch to all supported branches.

Report: <20161124114911.6530.31200@wrigleys.postgresql.org>

1 files changed, 18 insertions, 0 deletions

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 35fc9f0548d..e70d75260bd 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -7689,6 +7689,24 @@ ATExecDropConstraint(Relation rel, const char *constrName,
 		is_no_inherit_constraint = con->connoinherit;
 
 		/*
+		 * If it's a foreign-key constraint, we'd better lock the referenced
+		 * table and check that that's not in use, just as we've already done
+		 * for the constrained table (else we might, eg, be dropping a trigger
+		 * that has unfired events).  But we can/must skip that in the
+		 * self-referential case.
+		 */
+		if (con->contype == CONSTRAINT_FOREIGN &&
+			con->confrelid != RelationGetRelid(rel))
+		{
+			Relation	frel;
+
+			/* Must match lock taken by RemoveTriggerById: */
+			frel = heap_open(con->confrelid, AccessExclusiveLock);
+			CheckTableNotInUse(frel, "ALTER TABLE");
+			heap_close(frel, NoLock);
+		}
+
+		/*
 		 * Perform the actual constraint deletion
 		 */
 		conobj.classId = ConstraintRelationId;