Re: [PATCH 0/4] t5552: fix flakiness by introducing proper locking for GIT_TRACE

> Here it is as a patch on top of jt/fetch-negotiator-skipping, which lets
> us pursue any fix for interleaved trace output on Windows without the
> pressure of an impending flaky test.
>
> My gut says that looking into making O_APPEND work there is going to be
> the nicest solution, but my gut is not very well versed in Windows
> subtleties. ;)

> -- >8 --
> Subject: [PATCH] t5552: suppress upload-pack trace output
>
> The t5552 test script uses GIT_TRACE_PACKET to monitor what
> git-fetch sends and receives. However, because we're
> accessing a local repository, the child upload-pack also
> sends trace output to the same file.
>
> On Linux, this works out OK. We open the trace file with
> O_APPEND, so all writes are atomically positioned at the end
> of the file. No data can be overwritten or omitted. And
> since we prepare our small writes in a strbuf and write them
> with a single write(), we should see each line as an atomic
> unit. The order of lines between the two processes is
> undefined, but the test script greps only for "fetch>" or
> "fetch<" lines. So under Linux, the test results are
> deterministic.
>
> The test fails intermittently on Windows, however,
> reportedly even overwriting bits of the output file (i.e.,
> O_APPEND does not seem to give us an atomic position+write).
>
> Since the test only cares about the trace output from fetch,
> we can just disable the output from upload-pack. That
> doesn't solve the greater question of O_APPEND/trace issues
> under Windows, but it easily fixes the flakiness from this
> test.
>
> Reported-by: Johannes Schindelin <johannes.schindelin@gmx.de>
> Signed-off-by: Jeff King <peff@peff.net>
> ---
> I'm assuming that this really isn't triggerable on Linux. I tried and
> couldn't manage to get it to fail, and the reasoning above explains why.
> But I wasn't 100% clear that Dscho hadn't seen it fail on non-Windows.
>
>  t/t5552-skipping-fetch-negotiator.sh | 23 ++++++++++++++++++-----
>  1 file changed, 18 insertions(+), 5 deletions(-)
>
> diff --git a/t/t5552-skipping-fetch-negotiator.sh b/t/t5552-skipping-fetch-negotiator.sh
> index 0a8e0e42ed..0ad50dd839 100755
> --- a/t/t5552-skipping-fetch-negotiator.sh
> +++ b/t/t5552-skipping-fetch-negotiator.sh
> @@ -28,6 +28,19 @@ have_not_sent () {
>  	done
>  }
>  
> +# trace_fetch <client_dir> <server_dir> [args]
> +#
> +# Trace the packet output of fetch, but make sure we disable the variable
> +# in the child upload-pack, so we don't combine the results in the same file.
> +trace_fetch () {
> +	client=$1; shift
> +	server=$1; shift
> +	GIT_TRACE_PACKET="$(pwd)/trace" \
> +	git -C "$client" fetch \
> +	  --upload-pack 'unset GIT_TRACE_PACKET; git-upload-pack' \
> +	  "$server" "$@"
> +}
> +
>  test_expect_success 'commits with no parents are sent regardless of skip distance' '
>  	git init server &&
>  	test_commit -C server to_fetch &&
> @@ -42,7 +55,7 @@ test_expect_success 'commits with no parents are sent regardless of skip distanc
>  	# "c1" has no parent, it is still sent as "have" even though it would
>  	# normally be skipped.
>  	test_config -C client fetch.negotiationalgorithm skipping &&
> -	GIT_TRACE_PACKET="$(pwd)/trace" git -C client fetch "$(pwd)/server" &&
> +	trace_fetch client "$(pwd)/server" &&
>  	have_sent c7 c5 c2 c1 &&
>  	have_not_sent c6 c4 c3
>  '
> @@ -65,7 +78,7 @@ test_expect_success 'when two skips collide, favor the larger one' '
>  	# the next "have" sent will be "c1" (from "c6" skip 4) and not "c4"
>  	# (from "c5side" skip 1).
>  	test_config -C client fetch.negotiationalgorithm skipping &&
> -	GIT_TRACE_PACKET="$(pwd)/trace" git -C client fetch "$(pwd)/server" &&
> +	trace_fetch client "$(pwd)/server" &&
>  	have_sent c5side c11 c9 c6 c1 &&
>  	have_not_sent c10 c8 c7 c5 c4 c3 c2
>  '
> @@ -91,7 +104,7 @@ test_expect_success 'use ref advertisement to filter out commits' '
>  	# not need to send any ancestors of "c3", but we still need to send "c3"
>  	# itself.
>  	test_config -C client fetch.negotiationalgorithm skipping &&
> -	GIT_TRACE_PACKET="$(pwd)/trace" git -C client fetch origin to_fetch &&
> +	trace_fetch client origin to_fetch &&
>  	have_sent c5 c4^ c2side &&
>  	have_not_sent c4 c4^^ c4^^^
>  '
> @@ -121,7 +134,7 @@ test_expect_success 'handle clock skew' '
>  	# and sent, because (due to clock skew) its only parent has already been
>  	# popped off the priority queue.
>  	test_config -C client fetch.negotiationalgorithm skipping &&
> -	GIT_TRACE_PACKET="$(pwd)/trace" git -C client fetch "$(pwd)/server" &&
> +	trace_fetch client "$(pwd)/server" &&
>  	have_sent c2 c1 old4 old2 old1 &&
>  	have_not_sent old3
>  '
> @@ -153,7 +166,7 @@ test_expect_success 'do not send "have" with ancestors of commits that server AC
>  	test_commit -C server commit-on-b1 &&
>  
>  	test_config -C client fetch.negotiationalgorithm skipping &&
> -	GIT_TRACE_PACKET="$(pwd)/trace" git -C client fetch "$(pwd)/server" to_fetch &&
> +	trace_fetch client "$(pwd)/server" to_fetch &&
>  	grep "  fetch" trace &&
>  
>  	# fetch-pack sends 2 requests each containing 16 "have" lines before