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

[Jeff King <peff@peff.net>]

On Fri, Aug 10, 2018 at 11:34:59AM -0700, Junio C Hamano wrote:

> Johannes Sixt <j6t@kdbg.org> writes:
> 
> > As this buglet looks like a recurring theme, and a proper fix is
> > preferable over repeated work-arounds. To me it looks like we need
> > some sort of locking on Windows. Unless your friends at Microsoft have
> > an ace in their sleeves that lets us have atomic O_APPEND the POSIX
> > way...
> 
> Just to put the severity of the issue in context, we use O_APPEND in
> a few codepaths, and the trace thing for debugging is the only thing
> that could have multiple writers.  Other users of O_APPEND are:
> 
>  * refs/files-backend.c uses it so that a reflog entry can be
>    appended at the end, but because update to each ref is protected
>    from racing at a lot higher layer with a lock, no two people
>    would try to append to the same reflog file, so atomicity of
>    O_APPEND does not matter here.

Just an interesting side note, but I think one of the reasons we use
dot-locks and not flock() is that the former generally works on network
filesystems. I think O_APPEND also is not reliable for non-local files,
though, so short of actually dot-locking trace files (yuck) I don't
think there's a great solution there.

> It may make sense to allow GIT_TRACE to have a placeholder
> (e.g. "/tmp/traceout.$$") to help debuggers arrange to give
> different processes their own trace output file, which perhaps may
> be a simpler and less impactful to the performance solution than
> having to make locks at an application layer.

Heh, I actually wrote that patch several years ago, as part of an
abandoned effort to have config-triggered tracing (e.g., on the server
side where you're getting hundreds of requests and you want to enable
tracing on a repo for a slice of time).

One annoying thing for a case like the test in question is that you
don't actually know the pid of the process you're interested in. So we'd
generate two trace files, and then you'd have to figure out which one is
which. In my earlier work, I coupled it with some magic to allow
per-command config, like:

  [trace "fetch"]
  packet = /tmp/trace.%p

but you could also imagine a "%n" which uses the command name.

I don't remember why I abandoned it, but I think a lot had to do with
violating the "don't look at config" rules for our repo startup code. A
lot of that has been fixed since then, but I haven't really needed it.
If anybody is really interested, the patches are at:

  https://github.com/peff/git jk/trace-stdin

(my ultimate goal was to record stdin for pack-objects to replay slow
fetches, but I ended up hacking together a patch to pack-objects
instead).

-Peff