Allow a partdesc-omitting-partitions to be cached

Makes partition descriptor acquisition faster during the transient
period in which a partition is in the process of being detached.

This also adds the restriction that only one partition can be in
pending-detach state for a partitioned table.

While at it, return find_inheritance_children() API to what it was
before 71f4c8c, and create a separate
find_inheritance_children_extended() that returns detailed info about
detached partitions.

(This incidentally fixes a bug in 8aba932 whereby a memory context
holding a transient partdesc is reparented to a NULL PortalContext,
leading to permanent leak of that memory.  The fix is to no longer rely
on reparenting contexts to PortalContext.   Reported by Amit Langote.)

Per gripe from Amit Langote
Discussion: https://postgr.es/m/CA+HiwqFgpP1LxJZOBYGt9rpvTjXXkg5qG2+Xch2Z1Q7KrqZR1A@mail.gmail.com

Author: alvherre

doc/src/sgml/ref/alter_table.sgml

@@ -986,6 +986,8 @@ WITH ( MODULUS <replaceable class="parameter">numeric_literal</replaceable>, REM
       If <literal>FINALIZE</literal> is specified, a previous
       <literal>DETACH CONCURRENTLY</literal> invocation that was cancelled or
       interrupted is completed.
+      At most one partition in a partitioned table can be pending detach at
+      a time.
      </para>
     </listitem>
    </varlistentry>

src/backend/catalog/pg_inherits.c

@@ -52,6 +52,22 @@ typedef struct SeenRelsEntry
  * then no locks are acquired, but caller must beware of race conditions
  * against possible DROPs of child relations.
  *
+ * Partitions marked as being detached are omitted; see
+ * find_inheritance_children_extended for details.
+ */
+List *
+find_inheritance_children(Oid parentrelId, LOCKMODE lockmode)
+{
+	return find_inheritance_children_extended(parentrelId, true, lockmode,
+											  NULL, NULL);
+}
+
+/*
+ * find_inheritance_children_extended
+ *
+ * As find_inheritance_children, with more options regarding detached
+ * partitions.
+ *
  * If a partition's pg_inherits row is marked "detach pending",
  * *detached_exist (if not null) is set true.
  *
@@ -60,11 +76,13 @@ typedef struct SeenRelsEntry
  * marked "detach pending" is visible to that snapshot, then that partition is
  * omitted from the output list.  This makes partitions invisible depending on
  * whether the transaction that marked those partitions as detached appears
- * committed to the active snapshot.
+ * committed to the active snapshot.  In addition, *detached_xmin (if not null)
+ * is set to the xmin of the row of the detached partition.
  */
 List *
-find_inheritance_children(Oid parentrelId, bool omit_detached,
-						  LOCKMODE lockmode, bool *detached_exist)
+find_inheritance_children_extended(Oid parentrelId, bool omit_detached,
+								   LOCKMODE lockmode, bool *detached_exist,
+								   TransactionId *detached_xmin)
 {
 	List	   *list = NIL;
 	Relation	relation;
@@ -132,7 +150,32 @@ find_inheritance_children(Oid parentrelId, bool omit_detached,
 				snap = GetActiveSnapshot();
 
 				if (!XidInMVCCSnapshot(xmin, snap))
+				{
+					if (detached_xmin)
+					{
+						/*
+						 * Two detached partitions should not occur (see
+						 * checks in MarkInheritDetached), but if they do,
+						 * track the newer of the two.  Make sure to warn the
+						 * user, so that they can clean up.  Since this is
+						 * just a cross-check against potentially corrupt
+						 * catalogs, we don't make it a full-fledged error
+						 * message.
+						 */
+						if (*detached_xmin != InvalidTransactionId)
+						{
+							elog(WARNING, "more than one partition pending detach found for table with OID %u",
+								 parentrelId);
+							if (TransactionIdFollows(xmin, *detached_xmin))
+								*detached_xmin = xmin;
+						}
+						else
+							*detached_xmin = xmin;
+					}
+
+					/* Don't add the partition to the output list */
 					continue;
+				}
 			}
 		}
 
@@ -247,8 +290,7 @@ find_all_inheritors(Oid parentrelId, LOCKMODE lockmode, List **numparents)
 		ListCell   *lc;
 
 		/* Get the direct children of this rel */
-		currentchildren = find_inheritance_children(currentrel, true,
-													lockmode, NULL);
+		currentchildren = find_inheritance_children(currentrel, lockmode);
 
 		/*
 		 * Add to the queue only those children not already seen. This avoids

src/backend/commands/tablecmds.c

@@ -3492,7 +3492,7 @@ renameatt_internal(Oid myrelid,
 		 * expected_parents will only be 0 if we are not already recursing.
 		 */
 		if (expected_parents == 0 &&
-			find_inheritance_children(myrelid, true, NoLock, NULL) != NIL)
+			find_inheritance_children(myrelid, NoLock) != NIL)
 			ereport(ERROR,
 					(errcode(ERRCODE_INVALID_TABLE_DEFINITION),
 					 errmsg("inherited column \"%s\" must be renamed in child tables too",
@@ -3691,7 +3691,7 @@ rename_constraint_internal(Oid myrelid,
 		else
 		{
 			if (expected_parents == 0 &&
-				find_inheritance_children(myrelid, true, NoLock, NULL) != NIL)
+				find_inheritance_children(myrelid, NoLock) != NIL)
 				ereport(ERROR,
 						(errcode(ERRCODE_INVALID_TABLE_DEFINITION),
 						 errmsg("inherited constraint \"%s\" must be renamed in child tables too",
@@ -6565,7 +6565,7 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 	 */
 	if (colDef->identity &&
 		recurse &&
-		find_inheritance_children(myrelid, true, NoLock, NULL) != NIL)
+		find_inheritance_children(myrelid, NoLock) != NIL)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_TABLE_DEFINITION),
 				 errmsg("cannot recursively add identity column to table that has child tables")));
@@ -6811,7 +6811,7 @@ ATExecAddColumn(List **wqueue, AlteredTableInfo *tab, Relation rel,
 	 * use find_all_inheritors to do it in one pass.
 	 */
 	children =
-		find_inheritance_children(RelationGetRelid(rel), true, lockmode, NULL);
+		find_inheritance_children(RelationGetRelid(rel), lockmode);
 
 	/*
 	 * If we are told not to recurse, there had better not be any child
@@ -7674,7 +7674,7 @@ ATPrepDropExpression(Relation rel, AlterTableCmd *cmd, bool recurse, bool recurs
 	 * resulting state can be properly dumped and restored.
 	 */
 	if (!recurse &&
-		find_inheritance_children(RelationGetRelid(rel), true, lockmode, NULL))
+		find_inheritance_children(RelationGetRelid(rel), lockmode))
 		ereport(ERROR,
 				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
 				 errmsg("ALTER TABLE / DROP EXPRESSION must be applied to child tables too")));
@@ -8282,7 +8282,7 @@ ATExecDropColumn(List **wqueue, Relation rel, const char *colName,
 	 * use find_all_inheritors to do it in one pass.
 	 */
 	children =
-		find_inheritance_children(RelationGetRelid(rel), true, lockmode, NULL);
+		find_inheritance_children(RelationGetRelid(rel), lockmode);
 
 	if (children)
 	{
@@ -8770,7 +8770,7 @@ ATAddCheckConstraint(List **wqueue, AlteredTableInfo *tab, Relation rel,
 	 * use find_all_inheritors to do it in one pass.
 	 */
 	children =
-		find_inheritance_children(RelationGetRelid(rel), true, lockmode, NULL);
+		find_inheritance_children(RelationGetRelid(rel), lockmode);
 
 	/*
 	 * Check if ONLY was specified with ALTER TABLE.  If so, allow the
@@ -11303,8 +11303,7 @@ ATExecDropConstraint(Relation rel, const char *constrName,
 	 * use find_all_inheritors to do it in one pass.
 	 */
 	if (!is_no_inherit_constraint)
-		children = find_inheritance_children(RelationGetRelid(rel), true,
-											 lockmode, NULL);
+		children = find_inheritance_children(RelationGetRelid(rel), lockmode);
 	else
 		children = NIL;
 
@@ -11688,8 +11687,7 @@ ATPrepAlterColumnType(List **wqueue,
 		}
 	}
 	else if (!recursing &&
-			 find_inheritance_children(RelationGetRelid(rel), true,
-									   NoLock, NULL) != NIL)
+			 find_inheritance_children(RelationGetRelid(rel), NoLock) != NIL)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_TABLE_DEFINITION),
 				 errmsg("type of inherited column \"%s\" must be changed in child tables too",
@@ -14601,7 +14599,8 @@ ATExecDropInherit(Relation rel, RangeVar *parent, LOCKMODE lockmode)
  * MarkInheritDetached
  *
  * Set inhdetachpending for a partition, for ATExecDetachPartition
- * in concurrent mode.
+ * in concurrent mode.  While at it, verify that no other partition is
+ * already pending detach.
  */
 static void
 MarkInheritDetached(Relation child_rel, Relation parent_rel)
@@ -14617,32 +14616,45 @@ MarkInheritDetached(Relation child_rel, Relation parent_rel)
 	Assert(parent_rel->rd_rel->relkind == RELKIND_PARTITIONED_TABLE);
 
 	/*
-	 * Find pg_inherits entries by inhrelid.
+	 * Find pg_inherits entries by inhparent.  (We need to scan them all in
+	 * order to verify that no other partition is pending detach.)
 	 */
 	catalogRelation = table_open(InheritsRelationId, RowExclusiveLock);
 	ScanKeyInit(&key,
-				Anum_pg_inherits_inhrelid,
+				Anum_pg_inherits_inhparent,
 				BTEqualStrategyNumber, F_OIDEQ,
-				ObjectIdGetDatum(RelationGetRelid(child_rel)));
-	scan = systable_beginscan(catalogRelation, InheritsRelidSeqnoIndexId,
+				ObjectIdGetDatum(RelationGetRelid(parent_rel)));
+	scan = systable_beginscan(catalogRelation, InheritsParentIndexId,
 							  true, NULL, 1, &key);
 
 	while (HeapTupleIsValid(inheritsTuple = systable_getnext(scan)))
 	{
-		HeapTuple	newtup;
+		Form_pg_inherits inhForm;
 
-		if (((Form_pg_inherits) GETSTRUCT(inheritsTuple))->inhparent !=
-			RelationGetRelid(parent_rel))
-			elog(ERROR, "bad parent tuple found for partition %u",
-				 RelationGetRelid(child_rel));
+		inhForm = (Form_pg_inherits) GETSTRUCT(inheritsTuple);
+		if (inhForm->inhdetachpending)
+			ereport(ERROR,
+					errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+					errmsg("partition \"%s\" already pending detach in partitioned table \"%s.%s\"",
+						   get_rel_name(inhForm->inhrelid),
+						   get_namespace_name(parent_rel->rd_rel->relnamespace),
+						   RelationGetRelationName(parent_rel)),
+					errhint("Use ALTER TABLE ... DETACH PARTITION ... FINALIZE to complete the detach operation."));
+
+		if (inhForm->inhrelid == RelationGetRelid(child_rel))
+		{
+			HeapTuple	newtup;
 
-		newtup = heap_copytuple(inheritsTuple);
-		((Form_pg_inherits) GETSTRUCT(newtup))->inhdetachpending = true;
+			newtup = heap_copytuple(inheritsTuple);
+			((Form_pg_inherits) GETSTRUCT(newtup))->inhdetachpending = true;
 
-		CatalogTupleUpdate(catalogRelation,
-						   &inheritsTuple->t_self,
-						   newtup);
-		found = true;
+			CatalogTupleUpdate(catalogRelation,
+							   &inheritsTuple->t_self,
+							   newtup);
+			found = true;
+			heap_freetuple(newtup);
+			/* keep looking, to ensure we catch others pending detach */
+		}
 	}
 
 	/* Done */

src/backend/commands/trigger.c

@@ -1141,8 +1141,7 @@ CreateTrigger(CreateTrigStmt *stmt, const char *queryString,
 			ListCell   *l;
 			List	   *idxs = NIL;
 
-			idxs = find_inheritance_children(indexOid, true,
-											 ShareRowExclusiveLock, NULL);
+			idxs = find_inheritance_children(indexOid, ShareRowExclusiveLock);
 			foreach(l, idxs)
 				childTbls = lappend_oid(childTbls,
 										IndexGetRelation(lfirst_oid(l),