Re: BUG #16801: Invalid memory access on WITH RECURSIVE with nested WITHs

Hello Michael,
03.01.2021 08:15, Michael Paquier wrote:
> On Sat, Jan 02, 2021 at 03:00:00PM +0000, PG Bug reporting form wrote:
>> valgrind detects an invalid read:
>> ==00:00:00:04.145 217144== Invalid read of size 8
>> ==00:00:00:04.145 217144==    at 0x302CB7: makeDependencyGraphWalker
>> (parse_cte.c:549)
>> ==00:00:00:04.145 217144==    by 0x302EA1: makeDependencyGraph
>> (parse_cte.c:439)
>> ==00:00:00:04.145 217144==    by 0x304557: transformWithClause
>> (parse_cte.c:176)
>>
>> The first bad commit is 1cff1b95.
> The same kind of list manipulation is done in two places in
> parse_cte.c, and there are extra ones in split_pathtarget_walker().  I
> cannot reproduce that here, and I have just tried with different
> optimization levels on HEAD and REL_13_STABLE.  Are you using specific
> options for valgrind?
I've found out that the list implementation doesn't support the
following usage pattern:
List *testList = NIL;
ListCell *testCell;

testList = lcons(NIL, testList);
testCell = list_head(testList);
...
testList = lcons(NIL, testList);

elog(INFO, "lfirst(testCell): %p", lfirst(testCell)); // prints
0x7f7f7f7f7f7f7f7f when compiled with -DUSE_VALGRIND

(Such list manipulation is happening in that makeDependencyGraphWalker
call.)

Best regards,
Alexander