Re: [PATCH] trace2: refactor to avoid gcc warning under -O3

Ævar Arnfjörð Bjarmason  [off-list ref] writes:

Refactor tr2_dst_try_uds_connect() to avoid a gcc warning[1] that
appears under -O3 (but not -O2). This makes the build pass under
DEVELOPER=1 without needing a DEVOPTS=no-error.

This can be reproduced with GCC Debian 8.3.0-6, but not e.g. with
clang 7.0.1-8+deb10u2. We've had this warning since
ee4512ed481 (trace2: create new combined trace facility, 2019-02-22).

As noted in [2] this warning happens because the compiler doesn't
assume that errno must be non-zero after a failed syscall. Let's work
around it as suggested in that analysis. We now return -1 ourselves on
error, and save away the value of errno in a variable the caller
passes in.

1.

    trace2/tr2_dst.c: In function ‘tr2_dst_get_trace_fd.part.5’:
    trace2/tr2_dst.c:296:10: warning: ‘fd’ may be used uninitialized in this function [-Wmaybe-uninitialized]
      dst->fd = fd;
      ~~~~~~~~^~~~
    trace2/tr2_dst.c:229:6: note: ‘fd’ was declared here
      int fd;
          ^~
2. https://lore.kernel.org/git/20200404142131.GA679473@coredump.intra.peff.net/ (local)
---
 trace2/tr2_dst.c | 27 +++++++++++++++------------
 1 file changed, 15 insertions(+), 12 deletions(-)

What's the concensus if any on this topic?

In any case, this needs to be signed off before it gets carved into
our history.

Thanks.