Fix crash when logical decoding is invoked from a PL function.

The logical decoding functions do BeginInternalSubTransaction and
RollbackAndReleaseCurrentSubTransaction to clean up after themselves.
It turns out that AtEOSubXact_SPI has an unrecognized assumption that
we always need to cancel the active SPI operation in the SPI context
that surrounds the subtransaction (if there is one).  That's true
when the RollbackAndReleaseCurrentSubTransaction call is coming from
the SPI-using function itself, but not when it's happening inside
some unrelated function invoked by a SPI query.  In practice the
affected callers are the various PLs.

To fix, record the current subtransaction ID when we begin a SPI
operation, and clean up only if that ID is the subtransaction being
canceled.

Also, remove AtEOSubXact_SPI's assertion that it must have cleaned
up the surrounding SPI context's active tuptable.  That's proven
wrong by the same test case.

Also clarify (or, if you prefer, reinterpret) the calling conventions
for _SPI_begin_call and _SPI_end_call.  The memory context cleanup
in the latter means that these have always had the flavor of a matched
resource-management pair, but they weren't documented that way before.

Per report from Ben Chobot.

Back-patch to 9.4 where logical decoding came in.  In principle,
the SPI changes should go all the way back, since the problem dates
back to commit 7ec1c5a8.  But given the lack of field complaints
it seems few people are using internal subtransactions in this way.
So I don't feel a need to take any risks in 9.2/9.3.

Discussion: https://postgr.es/m/73FBA179-C68C-4540-9473-71E865408B15@silentmedia.com

contrib/test_decoding/expected/decoding_into_rel.out

@@ -59,6 +59,31 @@ SELECT * FROM changeresult;
 
 DROP TABLE changeresult;
 DROP TABLE somechange;
+-- check calling logical decoding from pl/pgsql
+CREATE FUNCTION slot_changes_wrapper(slot_name name) RETURNS SETOF TEXT AS $$
+BEGIN
+  RETURN QUERY
+    SELECT data FROM pg_logical_slot_peek_changes(slot_name, NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
+END$$ LANGUAGE plpgsql;
+SELECT * FROM slot_changes_wrapper('regression_slot');
+                                                                                          slot_changes_wrapper                                                                                          
+--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
+ BEGIN
+ table public.changeresult: INSERT: data[text]:'BEGIN'
+ table public.changeresult: INSERT: data[text]:'table public.changeresult: INSERT: data[text]:''BEGIN'''
+ table public.changeresult: INSERT: data[text]:'table public.changeresult: INSERT: data[text]:''table public.somechange: INSERT: id[integer]:1'''
+ table public.changeresult: INSERT: data[text]:'table public.changeresult: INSERT: data[text]:''COMMIT'''
+ table public.changeresult: INSERT: data[text]:'COMMIT'
+ table public.changeresult: INSERT: data[text]:'BEGIN'
+ table public.changeresult: INSERT: data[text]:'table public.changeresult: INSERT: data[text]:''BEGIN'''
+ table public.changeresult: INSERT: data[text]:'table public.changeresult: INSERT: data[text]:''table public.changeresult: INSERT: data[text]:''''BEGIN'''''''
+ table public.changeresult: INSERT: data[text]:'table public.changeresult: INSERT: data[text]:''table public.changeresult: INSERT: data[text]:''''table public.somechange: INSERT: id[integer]:1'''''''
+ table public.changeresult: INSERT: data[text]:'table public.changeresult: INSERT: data[text]:''table public.changeresult: INSERT: data[text]:''''COMMIT'''''''
+ table public.changeresult: INSERT: data[text]:'table public.changeresult: INSERT: data[text]:''COMMIT'''
+ table public.changeresult: INSERT: data[text]:'COMMIT'
+ COMMIT
+(14 rows)
+
 SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
                                                                                                   data                                                                                                  
 --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

contrib/test_decoding/sql/decoding_into_rel.sql

@@ -27,5 +27,16 @@ INSERT INTO changeresult
 SELECT * FROM changeresult;
 DROP TABLE changeresult;
 DROP TABLE somechange;
+
+-- check calling logical decoding from pl/pgsql
+CREATE FUNCTION slot_changes_wrapper(slot_name name) RETURNS SETOF TEXT AS $$
+BEGIN
+  RETURN QUERY
+    SELECT data FROM pg_logical_slot_peek_changes(slot_name, NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
+END$$ LANGUAGE plpgsql;
+
+SELECT * FROM slot_changes_wrapper('regression_slot');
+
 SELECT data FROM pg_logical_slot_get_changes('regression_slot', NULL, NULL, 'include-xids', '0', 'skip-empty-xacts', '1');
+
 SELECT 'stop' FROM pg_drop_replication_slot('regression_slot');

src/backend/executor/spi.c

@@ -71,8 +71,8 @@ static void _SPI_cursor_operation(Portal portal,
 static SPIPlanPtr _SPI_make_plan_non_temp(SPIPlanPtr plan);
 static SPIPlanPtr _SPI_save_plan(SPIPlanPtr plan);
 
-static int	_SPI_begin_call(bool execmem);
-static int	_SPI_end_call(bool procmem);
+static int	_SPI_begin_call(bool use_exec);
+static int	_SPI_end_call(bool use_exec);
 static MemoryContext _SPI_execmem(void);
 static MemoryContext _SPI_procmem(void);
 static bool _SPI_checktuples(void);
@@ -118,6 +118,7 @@ SPI_connect(void)
 	_SPI_current->processed = 0;
 	_SPI_current->lastoid = InvalidOid;
 	_SPI_current->tuptable = NULL;
+	_SPI_current->execSubid = InvalidSubTransactionId;
 	slist_init(&_SPI_current->tuptables);
 	_SPI_current->procCxt = NULL;	/* in case we fail to create 'em */
 	_SPI_current->execCxt = NULL;
@@ -149,7 +150,7 @@ SPI_finish(void)
 {
 	int			res;
 
-	res = _SPI_begin_call(false);	/* live in procedure memory */
+	res = _SPI_begin_call(false);	/* just check we're connected */
 	if (res < 0)
 		return res;
 
@@ -268,8 +269,15 @@ AtEOSubXact_SPI(bool isCommit, SubTransactionId mySubid)
 	{
 		slist_mutable_iter siter;
 
-		/* free Executor memory the same as _SPI_end_call would do */
+		/*
+		 * Throw away executor state if current executor operation was started
+		 * within current subxact (essentially, force a _SPI_end_call(true)).
+		 */
+		if (_SPI_current->execSubid >= mySubid)
+		{
+			_SPI_current->execSubid = InvalidSubTransactionId;
 			MemoryContextResetAndDeleteChildren(_SPI_current->execCxt);
+		}
 
 		/* throw away any tuple tables created within current subxact */
 		slist_foreach_modify(siter, &_SPI_current->tuptables)
@@ -293,8 +301,6 @@ AtEOSubXact_SPI(bool isCommit, SubTransactionId mySubid)
 				MemoryContextDelete(tuptable->tuptabcxt);
 			}
 		}
-		/* in particular we should have gotten rid of any in-progress table */
-		Assert(_SPI_current->tuptable == NULL);
 	}
 }
 
@@ -2446,15 +2452,24 @@ _SPI_procmem(void)
 
 /*
  * _SPI_begin_call: begin a SPI operation within a connected procedure
+ *
+ * use_exec is true if we intend to make use of the procedure's execCxt
+ * during this SPI operation.  We'll switch into that context, and arrange
+ * for it to be cleaned up at _SPI_end_call or if an error occurs.
  */
 static int
-_SPI_begin_call(bool execmem)
+_SPI_begin_call(bool use_exec)
 {
 	if (_SPI_current == NULL)
 		return SPI_ERROR_UNCONNECTED;
 
-	if (execmem)				/* switch to the Executor memory context */
+	if (use_exec)
+	{
+		/* remember when the Executor operation started */
+		_SPI_current->execSubid = GetCurrentSubTransactionId();
+		/* switch to the Executor memory context */
 		_SPI_execmem();
+	}
 
 	return 0;
 }
@@ -2462,14 +2477,19 @@ _SPI_begin_call(bool execmem)
 /*
  * _SPI_end_call: end a SPI operation within a connected procedure
  *
+ * use_exec must be the same as in the previous _SPI_begin_call
+ *
  * Note: this currently has no failure return cases, so callers don't check
  */
 static int
-_SPI_end_call(bool procmem)
+_SPI_end_call(bool use_exec)
 {
-	if (procmem)				/* switch to the procedure memory context */
+	if (use_exec)
 	{
+		/* switch to the procedure memory context */
 		_SPI_procmem();
+		/* mark Executor context no longer in use */
+		_SPI_current->execSubid = InvalidSubTransactionId;
 		/* and free Executor memory */
 		MemoryContextResetAndDeleteChildren(_SPI_current->execCxt);
 	}

src/include/executor/spi_priv.h

@@ -26,6 +26,9 @@ typedef struct
 	Oid			lastoid;
 	SPITupleTable *tuptable;	/* tuptable currently being built */
 
+	/* subtransaction in which current Executor call was started */
+	SubTransactionId execSubid;
+
 	/* resources of this execution context */
 	slist_head	tuptables;		/* list of all live SPITupleTables */
 	MemoryContext procCxt;		/* procedure context */