Fix event triggers for partitioned tables

Index DDL cascading on partitioned tables introduced a way for ALTER
TABLE to be called reentrantly.  This caused an an important deficiency
in event trigger support to be exposed: on exiting the reentrant call,
the alter table state object was clobbered, causing a crash when the
outer alter table tries to finalize its processing.  Fix the crash by
creating a stack of event trigger state objects.  There are still ways
to cause things to misbehave (and probably other crashers) with more
elaborate tricks, but at least it now doesn't crash in the obvious
scenario.

Backpatch to 9.5, where DDL deparsing of event triggers was introduced.

Reported-by: Marco Slot
Authors: Michaël Paquier, Álvaro Herrera
Discussion: https://postgr.es/m/CANNhMLCpi+HQ7M36uPfGbJZEQLyTy7XvX=5EFkpR-b1bo0uJew@mail.gmail.com

1 files changed, 7 insertions, 6 deletions

diff --git a/src/backend/commands/event_trigger.c b/src/backend/commands/event_trigger.c
index eecc85d14e5..2c1dc47541c 100644
--- a/src/backend/commands/event_trigger.c
+++ b/src/backend/commands/event_trigger.c
@@ -1696,11 +1696,6 @@ EventTriggerCollectSimpleCommand(ObjectAddress address,
  * Note we don't collect the command immediately; instead we keep it in
  * currentCommand, and only when we're done processing the subcommands we will
  * add it to the command list.
- *
- * XXX -- this API isn't considering the possibility of an ALTER TABLE command
- * being called reentrantly by an event trigger function.  Do we need stackable
- * commands at this level?	Perhaps at least we should detect the condition and
- * raise an error.
  */
 void
 EventTriggerAlterTableStart(Node *parsetree)
@@ -1725,6 +1720,7 @@ EventTriggerAlterTableStart(Node *parsetree)
 	command->d.alterTable.subcmds = NIL;
 	command->parsetree = copyObject(parsetree);
 
+	command->parent = currentEventTriggerState->currentCommand;
 	currentEventTriggerState->currentCommand = command;
 
 	MemoryContextSwitchTo(oldcxt);
@@ -1765,6 +1761,7 @@ EventTriggerCollectAlterTableSubcmd(Node *subcmd, ObjectAddress address)
 		return;
 
 	Assert(IsA(subcmd, AlterTableCmd));
+	Assert(OidIsValid(currentEventTriggerState->currentCommand));
 	Assert(OidIsValid(currentEventTriggerState->currentCommand->d.alterTable.objectId));
 
 	oldcxt = MemoryContextSwitchTo(currentEventTriggerState->cxt);
@@ -1790,11 +1787,15 @@ EventTriggerCollectAlterTableSubcmd(Node *subcmd, ObjectAddress address)
 void
 EventTriggerAlterTableEnd(void)
 {
+	CollectedCommand *parent;
+
 	/* ignore if event trigger context not set, or collection disabled */
 	if (!currentEventTriggerState ||
 		currentEventTriggerState->commandCollectionInhibited)
 		return;
 
+	parent = currentEventTriggerState->currentCommand->parent;
+
 	/* If no subcommands, don't collect */
 	if (list_length(currentEventTriggerState->currentCommand->d.alterTable.subcmds) != 0)
 	{
@@ -1805,7 +1806,7 @@ EventTriggerAlterTableEnd(void)
 	else
 		pfree(currentEventTriggerState->currentCommand);
 
-	currentEventTriggerState->currentCommand = NULL;
+	currentEventTriggerState->currentCommand = parent;
 }
 
 /*