Don't crash on reference to an un-available system column.

Adopt a more consistent policy about what slot-type-specific
getsysattr functions should do when system attributes are not
available.  To wit, they should all throw the same user-oriented
error, rather than variously crashing or emitting developer-oriented
messages.

This closes a identifiable problem in commits a71cfc56b and
3fb93103a (in v13 and v12), so back-patch into those branches,
along with a test case to try to ensure we don't break it again.
It is not known that any of the former crash cases are reachable
in HEAD, but this seems like a good safety improvement in any case.

Discussion: https://postgr.es/m/141051591267657@mail.yandex.ru

1 files changed, 35 insertions, 5 deletions

diff --git a/src/backend/executor/execTuples.c b/src/backend/executor/execTuples.c
index 73c35df9c96..f4478028431 100644
--- a/src/backend/executor/execTuples.c
+++ b/src/backend/executor/execTuples.c
@@ -122,9 +122,8 @@ tts_virtual_clear(TupleTableSlot *slot)
 }
 
 /*
- * Attribute values are readily available in tts_values and tts_isnull array
- * in a VirtualTupleTableSlot. So there should be no need to call either of the
- * following two functions.
+ * VirtualTupleTableSlots always have fully populated tts_values and
+ * tts_isnull arrays.  So this function should never be called.
  */
 static void
 tts_virtual_getsomeattrs(TupleTableSlot *slot, int natts)
@@ -132,10 +131,19 @@ tts_virtual_getsomeattrs(TupleTableSlot *slot, int natts)
 	elog(ERROR, "getsomeattrs is not required to be called on a virtual tuple table slot");
 }
 
+/*
+ * VirtualTupleTableSlots never provide system attributes (except those
+ * handled generically, such as tableoid).  We generally shouldn't get
+ * here, but provide a user-friendly message if we do.
+ */
 static Datum
 tts_virtual_getsysattr(TupleTableSlot *slot, int attnum, bool *isnull)
 {
-	elog(ERROR, "virtual tuple table slot does not have system attributes");
+	Assert(!TTS_EMPTY(slot));
+
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("cannot retrieve a system column in this context")));
 
 	return 0;					/* silence compiler warnings */
 }
@@ -335,6 +343,15 @@ tts_heap_getsysattr(TupleTableSlot *slot, int attnum, bool *isnull)
 
 	Assert(!TTS_EMPTY(slot));
 
+	/*
+	 * In some code paths it's possible to get here with a non-materialized
+	 * slot, in which case we can't retrieve system columns.
+	 */
+	if (!hslot->tuple)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot retrieve a system column in this context")));
+
 	return heap_getsysattr(hslot->tuple, attnum,
 						   slot->tts_tupleDescriptor, isnull);
 }
@@ -497,7 +514,11 @@ tts_minimal_getsomeattrs(TupleTableSlot *slot, int natts)
 static Datum
 tts_minimal_getsysattr(TupleTableSlot *slot, int attnum, bool *isnull)
 {
-	elog(ERROR, "minimal tuple table slot does not have system attributes");
+	Assert(!TTS_EMPTY(slot));
+
+	ereport(ERROR,
+			(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+			 errmsg("cannot retrieve a system column in this context")));
 
 	return 0;					/* silence compiler warnings */
 }
@@ -681,6 +702,15 @@ tts_buffer_heap_getsysattr(TupleTableSlot *slot, int attnum, bool *isnull)
 
 	Assert(!TTS_EMPTY(slot));
 
+	/*
+	 * In some code paths it's possible to get here with a non-materialized
+	 * slot, in which case we can't retrieve system columns.
+	 */
+	if (!bslot->base.tuple)
+		ereport(ERROR,
+				(errcode(ERRCODE_FEATURE_NOT_SUPPORTED),
+				 errmsg("cannot retrieve a system column in this context")));
+
 	return heap_getsysattr(bslot->base.tuple, attnum,
 						   slot->tts_tupleDescriptor, isnull);
 }