Re: pgsql: Fix blatantly broken record_image_cmp() logic for pass-by-value

Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Kevin Grittner <kgrittn(at)ymail(dot)com> writes:

>>  I don't get a warning on this with either of these compilers,
>>  either with or without asserts enabled:
>
> Perhaps you built with -O0?  At least in older versions of gcc, you need
> at least -O1 to get uninitialized-variable warnings.  (I've heard some
> claims that the latest versions of gcc don't require that anymore.)
> I don't recommend -O0 as your default optimization level, precisely
> because of this.

This is with default configure and compile options, which results
in this:

gcc -O2 -Wall -Wmissing-prototypes -Wpointer-arith -Wdeclaration-after-statement -Wendif-labels -Wmissing-format-attribute -Wformat-security -fno-strict-aliasing -fwrapv -fexcess-precision=standard -g -I../../../../src/include -D_GNU_SOURCE -I/usr/include/libxml2   -c -o rowtypes.o rowtypes.c -MMD -MP -MF .deps/rowtypes.Po

>                         if (GET_4_BYTES(values1[i1]) !=
>                             GET_4_BYTES(values2[i2]))
>                         {
>                             cmpresult = (GET_4_BYTES(values1[i1]) <
>                                          GET_4_BYTES(values2[i2])) ? -1 : 1;
>                         }
>
> if the two values are in fact equal then this falls through leaving
> cmpresult uninitialized, rather than setting it to zero as it should be;
> which is the case my patch was meant to correct.

I see it now.  It is pretty disturbing that the compilers in my
Linux distro don't recognize that as leaving the value
uninitialized.

> This perhaps escaped testing because we aren't using
> record_image_cmp in a way that exposes false nonequality results.

... or the usage pattern happens to leave zero on that location in
the stack when it has mattered.

> I'm not particularly excited about the possibility that attlen might
> not be either 1,2,4, or 8; I'm pretty sure there are lots of other
> places that would go belly-up with such a problem.  However, if that
> bothers you the fix is to replace the Assert with an elog(ERROR),
> not to remove the initialization.

Well, I was suggesting replacing the Assert with elog(ERROR); but I
was changing the variable declaration back to uninitialized in what
I was asking people to try so I could find out whether that was the
path causing the uninitialized variable report, to make sure I
understood the report.  Nobody who was seeing the warning had
passed along enough information to make that clear.

Now that I understand the problem, I'm fine with the state of
things as currently committed.

Thanks for following up.

-Kevin