Re: BUG #7656: PL/Perl SPI_freetuptable() segfault

pgmail@joh.to writes:
> I have a reproducible segmentation fault in PL/Perl.  I have yet to narrow
> down the test case to something sensible, but I do have a backtrace:

> 219        while (context->firstchild != NULL)
> (gdb) bt
> #0  0x0000000104e90782 in MemoryContextDeleteChildren (context=0x1000002bd)
> at mcxt.c:219
> #1  0x0000000104e906a8 in MemoryContextDelete (context=0x1000002bd) at
> mcxt.c:174
> #2  0x0000000104bbefb5 in SPI_freetuptable (tuptable=0x7f9ae4289230) at
> spi.c:1003
> #3  0x000000011ec9928b in plperl_spi_execute_fetch_result
> (tuptable=0x7f9ae4289230, processed=1, status=-6) at plperl.c:2900
> #4  0x000000011ec98f27 in plperl_spi_exec (query=0x7f9ae4155f80
> "0x7f9ae3e3fe50", limit=-439796840) at plperl.c:2821
> #5  0x000000011ec9b5f7 in XS__spi_exec_query (my_perl=0x7f9ae40cce00,
> cv=0x7f9ae4148e90) at SPI.c:69

> While trying to narrow down the test case I noticed what the problem was: I
> was calling spi_execute_query() instead of spi_execute_prepared().

Hm.  It looks like SPI_execute failed as expected (note the status
passed to plperl_spi_execute_fetch_result is -6 which is
SPI_ERROR_ARGUMENT), but it did not reset SPI_tuptable, which led to
plperl_spi_execute_fetch_result trying to call SPI_freetuptable on what
was probably an already-deleted tuple table.

One theory we could adopt on this is that this is
plperl_spi_execute_fetch_result's fault and it shouldn't be trying to
free a tuple table unless status > 0.

Another theory we could adopt is that SPI functions that are capable of
setting SPI_tuptable ought to clear it at start, to ensure that they
return it as null on failure.

The latter seems like a "nicer" fix but I'm afraid it might have
unexpected side-effects.  It would certainly be a lot more invasive.

Thoughts?
        regards, tom lane