Fix up plpgsql's "simple expression" evaluation mechanism so that it behaves

safely in the presence of subtransactions.  To ensure that any ExprContext
shutdown callbacks are called at the right times, we have to have a separate
EState for each level of subtransaction.  Per "TupleDesc reference leak" bug
report from Stefan Kaltenbrunner.

Although I'm convinced the code is wrong as far back as 8.0, it doesn't seem
that there are any ways for the problem to really manifest before 8.2: AFAICS,
8.0 and 8.1 only use the ExprContextCallback mechanism to handle set-returning
functions, which cannot usefully be executed in a "simple expression" anyway.
Hence, no backpatch before 8.2 --- the risk of unforeseen breakage seems
to outweigh the chance of fixing something.

src/pl/plpgsql/src/pl_handler.c

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/pl/plpgsql/src/pl_handler.c,v 1.34 2007/01/05 22:20:02 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/pl/plpgsql/src/pl_handler.c,v 1.35 2007/01/28 16:15:49 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -46,6 +46,7 @@ _PG_init(void)
 
 	plpgsql_HashTableInit();
 	RegisterXactCallback(plpgsql_xact_cb, NULL);
+	RegisterSubXactCallback(plpgsql_subxact_cb, NULL);
 
 	/* Set up a rendezvous point with optional instrumentation plugin */
 	plugin_ptr = (PLpgSQL_plugin **) find_rendezvous_variable("PLpgSQL_plugin");

src/pl/plpgsql/src/plpgsql.h

@@ -8,7 +8,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/pl/plpgsql/src/plpgsql.h,v 1.82 2007/01/05 22:20:02 momjian Exp $
+ *	  $PostgreSQL: pgsql/src/pl/plpgsql/src/plpgsql.h,v 1.83 2007/01/28 16:15:49 tgl Exp $
  *
  *-------------------------------------------------------------------------
  */
@@ -180,11 +180,13 @@ typedef struct PLpgSQL_expr
 	Oid			expr_simple_type;
 
 	/*
-	 * if expr is simple AND in use in current xact, expr_simple_state is
-	 * valid.  Test validity by seeing if expr_simple_xid matches current XID.
+	 * if expr is simple AND prepared in current eval_estate,
+	 * expr_simple_state is valid.  Test validity by seeing if expr_simple_id
+	 * matches eval_estate_simple_id.
 	 */
 	ExprState  *expr_simple_state;
-	TransactionId expr_simple_xid;
+	long int	expr_simple_id;
+
 	/* params to pass to expr */
 	int			nparams;
 	int			params[1];		/* VARIABLE SIZE ARRAY ... must be last */
@@ -612,7 +614,9 @@ typedef struct
 	SPITupleTable *eval_tuptable;
 	uint32		eval_processed;
 	Oid			eval_lastoid;
-	ExprContext *eval_econtext;
+	ExprContext *eval_econtext;	/* for executing simple expressions */
+	EState	   *eval_estate;	/* EState containing eval_econtext */
+	long int	eval_estate_simple_id;		/* ID for eval_estate */
 
 	/* status information for error context reporting */
 	PLpgSQL_function *err_func; /* current func */
@@ -738,6 +742,8 @@ extern Datum plpgsql_exec_function(PLpgSQL_function *func,
 extern HeapTuple plpgsql_exec_trigger(PLpgSQL_function *func,
 					 TriggerData *trigdata);
 extern void plpgsql_xact_cb(XactEvent event, void *arg);
+extern void plpgsql_subxact_cb(SubXactEvent event, SubTransactionId mySubid,
+							   SubTransactionId parentSubid, void *arg);
 
 /* ----------
  * Functions for the dynamic string handling in pl_funcs.c