On Thu, Aug 09, 2018 at 10:35:24AM -0700, Johannes Schindelin via GitGitGadget wrote:
> The culprit is that two processes try simultaneously to write to the same > file specified via GIT_TRACE_PACKET, and it is not well defined how that > should work, even on Linux. 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. > This patch series addresses that by locking the trace fd. I chose to use > flock() instead of fcntl() because the Win32 API LockFileEx() > [https://docs.microsoft.com/en-us/windows/desktop/api/fileapi/nf-fileapi-lockfileex] > > (which does exactly what I want in this context) has much more similar > semantics to the former than the latter. I must admit I'm not excited to see us adding extra locking and complexity when it is not necessarily needed. Is this a feature we actually care about delivering for normal users of GIT_TRACE, or do we just care about solving the test flakiness here? Because we should be able to do the latter with the patch below. I also wonder if we should be clearing GIT_TRACE as part of clearing repo-env. It would do what we want automatically in this case, though there are definitely cases where I prefer the current behavior (because I really do want to see the upload-pack side of it). Of course I could always use "--upload-pack 'GIT_TRACE_PACKET=... upload-pack'", so this is really just a default. 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
