On Thu, Aug 09, 2018 at 01:49:52PM -0700, Junio C Hamano wrote:
> Jeff King <peff@xxxxxxxx> writes:
>
> > 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.
>
> Correct. But I am more worried about the "mixed/overwriting"
> breakage, if there is one; it means we may need to be prepared for
> systems that lack O_APPEND that works correctly. I initially just
> assumed that it was what Dscho was seeing, but after re-reading his
> message, I am not sure anymore.
>
> I think the "do not trace the other side" approach you suggest for
> these tests that only care about one side is more appropriate
> solution for this particular case. We then do not have to worry
> about overwriting or output from both sides mixed randomly.
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@xxxxxx>
Signed-off-by: Jeff King <peff@xxxxxxxx>
---
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
--
2.18.0.1058.g0926f0b71f
