Check that partitions are not in use when dropping constraints

If the user creates a deferred constraint in a partition, and in a
transaction they cause the constraint's trigger execution to be deferred
until commit time *and* drop the constraint, then when commit time comes
the queued trigger will fail to run because the trigger object will have
been dropped.

This is explained because when a constraint gets dropped in a
partitioned table, the recursion to drop the ones in partitions is done
by the dependency mechanism, not by ALTER TABLE traversing the recursion
tree as in all other cases.  In the non-partitioned case, this problem
is avoided by checking that the table is not "in use" by alter-table;
other alter-table subcommands that recurse to partitions do that check
for each partition.  But the dependency mechanism doesn't have a way to
do that.  Fix the problem by applying the same check to all partitions
during ALTER TABLE's "prep" phase, which correctly raises the necessary
error.

Reported-by: Rajkumar Raghuwanshi <rajkumar.raghuwanshi@enterprisedb.com>
Discussion: https://postgr.es/m/CAKcux6nZiO9-eEpr1ZD84bT1mBoVmeZkfont8iSpcmYrjhGWgA@mail.gmail.com

1 files changed, 45 insertions, 11 deletions

diff --git a/src/backend/commands/tablecmds.c b/src/backend/commands/tablecmds.c
index 9265f0ef55c..fb2be10794b 100644
--- a/src/backend/commands/tablecmds.c
+++ b/src/backend/commands/tablecmds.c
@@ -354,6 +354,7 @@ static void ATSimplePermissions(Relation rel, int allowed_targets);
 static void ATWrongRelkindError(Relation rel, int allowed_targets);
 static void ATSimpleRecursion(List **wqueue, Relation rel,
 							  AlterTableCmd *cmd, bool recurse, LOCKMODE lockmode);
+static void ATCheckPartitionsNotInUse(Relation rel, LOCKMODE lockmode);
 static void ATTypedTableRecursion(List **wqueue, Relation rel, AlterTableCmd *cmd,
 								  LOCKMODE lockmode);
 static List *find_typed_table_dependencies(Oid typeOid, const char *typeName,
@@ -3421,8 +3422,7 @@ CheckTableNotInUse(Relation rel, const char *stmt)
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_IN_USE),
 		/* translator: first %s is a SQL command, eg ALTER TABLE */
-				 errmsg("cannot %s \"%s\" because "
-						"it is being used by active queries in this session",
+				 errmsg("cannot %s \"%s\" because it is being used by active queries in this session",
 						stmt, RelationGetRelationName(rel))));
 
 	if (rel->rd_rel->relkind != RELKIND_INDEX &&
@@ -3431,8 +3431,7 @@ CheckTableNotInUse(Relation rel, const char *stmt)
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_IN_USE),
 		/* translator: first %s is a SQL command, eg ALTER TABLE */
-				 errmsg("cannot %s \"%s\" because "
-						"it has pending trigger events",
+				 errmsg("cannot %s \"%s\" because it has pending trigger events",
 						stmt, RelationGetRelationName(rel))));
 }
 
@@ -3910,16 +3909,19 @@ ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
 			break;
 		case AT_AddIdentity:
 			ATSimplePermissions(rel, ATT_TABLE | ATT_VIEW | ATT_FOREIGN_TABLE);
+			/* This command never recurses */
 			pass = AT_PASS_ADD_CONSTR;
 			break;
-		case AT_DropIdentity:
-			ATSimplePermissions(rel, ATT_TABLE | ATT_VIEW | ATT_FOREIGN_TABLE);
-			pass = AT_PASS_DROP;
-			break;
 		case AT_SetIdentity:
 			ATSimplePermissions(rel, ATT_TABLE | ATT_VIEW | ATT_FOREIGN_TABLE);
+			/* This command never recurses */
 			pass = AT_PASS_COL_ATTRS;
 			break;
+		case AT_DropIdentity:
+			ATSimplePermissions(rel, ATT_TABLE | ATT_VIEW | ATT_FOREIGN_TABLE);
+			/* This command never recurses */
+			pass = AT_PASS_DROP;
+			break;
 		case AT_DropNotNull:	/* ALTER COLUMN DROP NOT NULL */
 			ATSimplePermissions(rel, ATT_TABLE | ATT_FOREIGN_TABLE);
 			ATPrepDropNotNull(rel, recurse, recursing);
@@ -3985,7 +3987,8 @@ ATPrepCmd(List **wqueue, Relation rel, AlterTableCmd *cmd,
 			break;
 		case AT_DropConstraint: /* DROP CONSTRAINT */
 			ATSimplePermissions(rel, ATT_TABLE | ATT_FOREIGN_TABLE);
-			/* Recursion occurs during execution phase */
+			ATCheckPartitionsNotInUse(rel, lockmode);
+			/* Other recursion occurs during execution phase */
 			/* No command-specific prep needed except saving recurse flag */
 			if (recurse)
 				cmd->subtype = AT_DropConstraintRecurse;
@@ -5224,8 +5227,9 @@ ATSimpleRecursion(List **wqueue, Relation rel,
 				  AlterTableCmd *cmd, bool recurse, LOCKMODE lockmode)
 {
 	/*
-	 * Propagate to children if desired.  Only plain tables and foreign tables
-	 * have children, so no need to search for other relkinds.
+	 * Propagate to children if desired.  Only plain tables, foreign tables
+	 * and partitioned tables have children, so no need to search for other
+	 * relkinds.
 	 */
 	if (recurse &&
 		(rel->rd_rel->relkind == RELKIND_RELATION ||
@@ -5260,6 +5264,36 @@ ATSimpleRecursion(List **wqueue, Relation rel,
 }
 
 /*
+ * Obtain list of partitions of the given table, locking them all at the given
+ * lockmode and ensuring that they all pass CheckTableNotInUse.
+ *
+ * This function is a no-op if the given relation is not a partitioned table;
+ * in particular, nothing is done if it's a legacy inheritance parent.
+ */
+static void
+ATCheckPartitionsNotInUse(Relation rel, LOCKMODE lockmode)
+{
+	if (rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE)
+	{
+		List	   *inh;
+		ListCell   *cell;
+
+		inh = find_all_inheritors(RelationGetRelid(rel), lockmode, NULL);
+		/* first element is the parent rel; must ignore it */
+		for_each_cell(cell, inh, list_second_cell(inh))
+		{
+			Relation	childrel;
+
+			/* find_all_inheritors already got lock */
+			childrel = table_open(lfirst_oid(cell), NoLock);
+			CheckTableNotInUse(childrel, "ALTER TABLE");
+			table_close(childrel, NoLock);
+		}
+		list_free(inh);
+	}
+}
+
+/*
  * ATTypedTableRecursion
  *
  * Propagate ALTER TYPE operations to the typed tables of that type.