Re: [PATCH] selftests/harness: Flush stdout before forking

[Shuah Khan]

On 9/16/20 10:53 PM, Max Filippov wrote:
> On Wed, Sep 16, 2020 at 9:16 PM Michael Ellerman <mpe@xxxxxxxxxxxxxx> wrote:
> > The test harness forks() a child to run each test. Both the parent and
> > the child print to stdout using libc functions. That can lead to
> > duplicated (or more) output if the libc buffers are not flushed before
> > forking.
> >
> > It's generally not seen when running programs directly, because stdout
> > will usually be line buffered when it's pointing to a terminal.
> >
> > This was noticed when running the seccomp_bpf test, eg:
> >
> >    $ ./seccomp_bpf | tee test.log
> >    $ grep -c "TAP version 13" test.log
> >    2
> >
> > But we only expect the TAP header to appear once.
> >
> > It can be exacerbated using stdbuf to increase the buffer size:
> >
> >    $ stdbuf -o 1MB ./seccomp_bpf > test.log
> >    $ grep -c "TAP version 13" test.log
> >    13
> >
> > The fix is simple, we just flush stdout & stderr before fork. Usually
> > stderr is unbuffered, but that can be changed, so flush it as well
> > just to be safe.
> >
> > Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
> > ---
> >   tools/testing/selftests/kselftest_harness.h | 5 +++++
> >   1 file changed, 5 insertions(+)
>
> Tested-by: Max Filippov <jcmvbkbc@xxxxxxxxx>

Thank you both. Applying to linux-kselftest fixes for 5.9-rc7

thanks,
-- Shuah