Re: t5570-git-daemon fails with SIGPIPE on OSX

>
> Travis CI changed its default OSX image to use XCode 9.4 on 2018-07-31
> [1].  Since then OSX build jobs fail rather frequently because of a
> SIGPIPE in the tests 'fetch notices corrupt pack' or 'fetch notices
> corrupt idx' in 't5570-git-daemon.sh' [2].  I think this is a symptom
> a real bug in Git affecting other platforms as well, but these tests
> are too lax to catch it.
>
> What it boils down to is this sequence:
>
>   - The test first prepares a repository containing a corrupt pack,
>     ready to be server via 'git daemon'.
>
>   - Then the test runs 'test_must_fail git fetch ....', which connects
>     to 'git daemon', which forks 'git upload-pack', which then
>     advertises refs (only HEAD) and capabilities.  So far so good.
>
>   - 'git fetch' eventually calls fetch-pack.c:find_common().  The
>     first half of this function assembles a request consisting of a
>     want and a flush pkt-line, and sends it via a send_request() call.
>
>     At this point the scheduling becomes important: let's suppose that
>     fetch is slow and upload-pack is fast.
>
>   - 'git upload-pack' receives the request, parses the want line,
>     notices the corrupt pack, responds with an 'ERR upload-pack: not
>     our ref' pkt-line, and die()s right away.
>
>   - 'git fetch' finally approaches the end of the function, where it
>     attempts to send a done pkt-line via another send_request() call
>     through the now closing TCP socket.
>
>   - What happens now seems to depend on the platform:
>
>     - On Linux, both on my machine and on Travis CI, it shows textbook
>       example behaviour: write() returns with error and sets errno to
>       ECONNRESET.  Since it happens in write_or_die(), 'git fetch'
>       die()s with 'fatal: write error: Connection reset by peer', and
>       doesn't show the error send by 'git upload-pack'; how could it,
>       it doesn't even get as far to receive upload-pack's ERR
>       pkt-line.
>
>       The test only checks that 'git fetch' fails, but it doesn't
>       check whether it failed with the right error message, so the
>       test still succeeds.  Had it checked the error message as well,
>       we most likely had noticed this issue already, it doesn't happen
>       all that rarely.
>
>     - On the new OSX images with XCode 9.4 on Travis CI the write()
>       triggers SIGPIPE right away, and 'test_must_fail' notices it and
>       fails the test.  I couldn't see any sign of an ECONNRESET or any
>       other error that we could act upon to avoid the SIGPIPE.
>
>     - On OSX with XCode 9.2 on Travis CI there is neither SIGPIPE, nor
>       ECONNRESET, but sending the request actually succeeds even
>       though there is no process on the other end of the socket
>       anymore.  'git fetch' then simply continues execution, reads and
>       parses the ERR pkt-line, and then dies()s with 'fatal: remote
>       error: upload-pack: not our ref'.  So, on the face of it, it
>       shows the desired behaviour, but I have no idea how that write()
>       could succeed instead of returning error.
>
> I don't know what happens on a real Mac as I don't have access to one;
> I figured out all the above by enabling packet tracing, adding a
> couple of well placed tracing printf() and sleep() calls, running a
> bunch of builds on Travis CI, and looking through their logs.  But
> without access to a debugger and netstat and what not I can't really
> go any further.  So I would now happily pass the baton to those who
> have a Mac and know a thing or two about its porting issues to first
> check whether OSX on a real Mac shows the same behaviour as it does in
> Travis CI's virtualized(?) environment.  And then they can pass the
> baton to those who know all the intricacies of the pack protocol and
> its implementation to decide what to do with this issue.
>
> For a mostly reliable reproduction recipe you might want to fetch this
> branch:
>
>   https://github.com/szeder/git t5570-git-daemon-sigpipe
>
> and then run 'make && cd t && ./t5570-git-daemon.sh -v -x'
>
>
> Have fun! ;)
>
>
> 1 - https://blog.travis-ci.com/2018-07-19-xcode9-4-default-announce
>
> 2 - On git.git's master:
>     https://travis-ci.org/git/git/jobs/411517552#L2717