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

From: "Johannes Schindelin via GitGitGadget" <[email-masked]>
Date: Thu, 09 Aug 2018 10:35:24 -0700 (PDT)

I reported a couple of times that t5552 is not passing reliably. It has now
reached next, and will no doubt infect master soon.

Turns out that it is not a Windows-specific issue, even if it occurs a lot 
more often on Windows than elsewhere.

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.

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.

Of course, I have to admit that I am not super solid on flock() semantics,
and I also do not know which conditional blocks in config.mak.uname should
grow a HAVE_FLOCK = YesWeDo line, still. Reviewers knowledgeable in flock() 
semantics: I would be much indebted if you helped me there. Also: is it safe
to call flock() on file descriptors referring not to files, but, say, pipes
or an interactive terminal?

Johannes Schindelin (4):
  Introduce a function to lock/unlock file descriptors when appending
  mingw: implement lock_or_unlock_fd_for_appending()
  trace: lock the trace file to avoid racy trace_write() calls
  trace: verify that locking works

 Makefile               |   1 +
 compat/mingw.c         |  19 ++++++
 compat/mingw.h         |   3 +
 config.mak.uname       |   3 +
 git-compat-util.h      |   2 +
 t/helper/test-tool.c   |   1 +
 t/helper/test-tool.h   |   1 +
 t/helper/test-trace.c  | 130 +++++++++++++++++++++++++++++++++++++++++
 t/t0070-fundamental.sh |   6 ++
 trace.c                |  11 +++-
 wrapper.c              |  14 +++++
 11 files changed, 190 insertions(+), 1 deletion(-)
 create mode 100644 t/helper/test-trace.c

base-commit: 42cc7485a2ec49ecc440c921d2eb0cae4da80549
Published-As: https://github.com/gitgitgadget/git/releases/tags/pr-17%2Fdscho%2Ffetch-negotiator-skipping-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-17/dscho/fetch-negotiator-skipping-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/17
-- 
gitgitgadget