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

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);
 }

src/test/regress/expected/update.out

@@ -836,6 +836,59 @@ DETAIL:  Failing row contains (a, 10).
 -- ok
 UPDATE list_default set a = 'x' WHERE a = 'd';
 DROP TABLE list_parted;
+-- Test retrieval of system columns with non-consistent partition row types.
+-- This is only partially supported, as seen in the results.
+create table utrtest (a int, b text) partition by list (a);
+create table utr1 (a int check (a in (1)), q text, b text);
+create table utr2 (a int check (a in (2)), b text);
+alter table utr1 drop column q;
+alter table utrtest attach partition utr1 for values in (1);
+alter table utrtest attach partition utr2 for values in (2);
+insert into utrtest values (1, 'foo')
+  returning *, tableoid::regclass, cmin;
+ a |  b  | tableoid | cmin 
+---+-----+----------+------
+ 1 | foo | utr1     |    0
+(1 row)
+
+insert into utrtest values (2, 'bar')
+  returning *, tableoid::regclass, cmin;  -- fails
+ERROR:  cannot retrieve a system column in this context
+insert into utrtest values (2, 'bar')
+  returning *, tableoid::regclass;
+ a |  b  | tableoid 
+---+-----+----------
+ 2 | bar | utr2
+(1 row)
+
+update utrtest set b = b || b from (values (1), (2)) s(x) where a = s.x
+  returning *, tableoid::regclass, cmin;
+ a |   b    | x | tableoid | cmin 
+---+--------+---+----------+------
+ 1 | foofoo | 1 | utr1     |    0
+ 2 | barbar | 2 | utr2     |    0
+(2 rows)
+
+update utrtest set a = 3 - a from (values (1), (2)) s(x) where a = s.x
+  returning *, tableoid::regclass, cmin;  -- fails
+ERROR:  cannot retrieve a system column in this context
+update utrtest set a = 3 - a from (values (1), (2)) s(x) where a = s.x
+  returning *, tableoid::regclass;
+ a |   b    | x | tableoid 
+---+--------+---+----------
+ 2 | foofoo | 1 | utr2
+ 1 | barbar | 2 | utr1
+(2 rows)
+
+delete from utrtest
+  returning *, tableoid::regclass, cmin;
+ a |   b    | tableoid | cmin 
+---+--------+----------+------
+ 1 | barbar | utr1     |    0
+ 2 | foofoo | utr2     |    0
+(2 rows)
+
+drop table utrtest;
 --------------
 -- Some more update-partition-key test scenarios below. This time use list
 -- partitions.

src/test/regress/sql/update.sql

@@ -529,6 +529,38 @@ UPDATE list_default set a = 'x' WHERE a = 'd';
 
 DROP TABLE list_parted;
 
+-- Test retrieval of system columns with non-consistent partition row types.
+-- This is only partially supported, as seen in the results.
+
+create table utrtest (a int, b text) partition by list (a);
+create table utr1 (a int check (a in (1)), q text, b text);
+create table utr2 (a int check (a in (2)), b text);
+alter table utr1 drop column q;
+alter table utrtest attach partition utr1 for values in (1);
+alter table utrtest attach partition utr2 for values in (2);
+
+insert into utrtest values (1, 'foo')
+  returning *, tableoid::regclass, cmin;
+insert into utrtest values (2, 'bar')
+  returning *, tableoid::regclass, cmin;  -- fails
+insert into utrtest values (2, 'bar')
+  returning *, tableoid::regclass;
+
+update utrtest set b = b || b from (values (1), (2)) s(x) where a = s.x
+  returning *, tableoid::regclass, cmin;
+
+update utrtest set a = 3 - a from (values (1), (2)) s(x) where a = s.x
+  returning *, tableoid::regclass, cmin;  -- fails
+
+update utrtest set a = 3 - a from (values (1), (2)) s(x) where a = s.x
+  returning *, tableoid::regclass;
+
+delete from utrtest
+  returning *, tableoid::regclass, cmin;
+
+drop table utrtest;
+
+
 --------------
 -- Some more update-partition-key test scenarios below. This time use list
 -- partitions.