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

> Are you sure that it's not well-defined? We open the path with O_APPEND,
> which means every write() will be atomically positioned at the end of
> file. So we would never lose or overwrite data.
>
> We do our own buffering in a strbuf, writing the result out in a single
> write() call (modulo the OS returning a short write, but that should not
> generally happen when writing short strings to a file). So we should get
> individual trace lines as atomic units.
>
> The order of lines from the two processes is undefined, of course.

> diff --git a/t/t5552-skipping-fetch-negotiator.sh b/t/t5552-skipping-fetch-negotiator.sh
> index 3b60bd44e0..5ad5bece55 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
>  '
> @@ -88,7 +101,7 @@ 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 "$(pwd)/server" &&
>  	have_sent c5side c11 c9 c6 c1 &&
>  	have_not_sent c10 c8 c7 c5 c4 c3 c2
>  '
> @@ -114,7 +127,7 @@ test_expect_success 'use ref advertisement to filter out commits' '
>  	# not need to send any ancestors of "c3", but we still need to send "c3"
>  	# itself.
>  	test_config -C client fetch.negotiationalgorithm skipping &&
> -	GIT_TRACE_PACKET="$(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^^^
>  '
> @@ -144,7 +157,7 @@ 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 "$(pwd)/server" &&
>  	have_sent c2 c1 old4 old2 old1 &&
>  	have_not_sent old3
>  '
> @@ -176,7 +189,7 @@ test_expect_success 'do not send "have" with ancestors of commits that server AC
>  	test_commit -C server commit-on-b1 &&
>  
>  	test_config -C client fetch.negotiationalgorithm skipping &&
> -	GIT_TRACE_PACKET="$(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