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> -- Thanks. -- Max
