git@vger.kernel.org list mirror (unofficial, one of many)
 help / color / mirror / code / Atom feed
From: Jeff Hostetler <git@jeffhostetler.com>
To: Jeff King <peff@peff.net>, Son Luong Ngoc <sluongng@gmail.com>
Cc: git@vger.kernel.org, Jeff.Hostetler@microsoft.com
Subject: Re: Git pull stuck when Trace2 target set to Unix Stream Socket
Date: Mon, 13 Apr 2020 14:42:43 -0400	[thread overview]
Message-ID: <dde14102-1eeb-71dc-5333-30b21a1ef17a@jeffhostetler.com> (raw)
In-Reply-To: <20200413171812.GA8183@coredump.intra.peff.net>



On 4/13/2020 1:18 PM, Jeff King wrote:
> On Mon, Apr 13, 2020 at 02:05:00PM +0200, Son Luong Ngoc wrote:
> 
>> I am trying to write a simple git trace2 event collector and I notice
>> that when git doing git pull with trace events being sent to a unix
>> stream socket, the entire operation halted.
>>
>> Reproduce as follow:
>> ```
>> cd git/git
>> git config trace2.eventTarget af_unix:stream:/tmp/git_trace.sock
>> git config trace2.eventBrief false
>> (rm /tmp/git_trace.sock | ) &&  nc -lkU /tmp/git_trace.sock
>>
>> # In a different terminal
>> git pull # Pull stuck and never complete
>> ```
> 
> I think the issue is the use of netcat as the server side.
> 
> Your git-pull involves multiple simultaneously-running Git processes.
> But "nc -k" will only accept() a new client once the old one has
> disconnected. So we'd deadlock any time we have this situation:
> 
>    - process A opens a stream to the socket, and keeps it open
> 
>    - process A spawns process B and waits for it to finish
> 
>    - process B tries to open a stream to the socket, which will block
>      waiting for netcat to accept()
> 
> Now A cannot make forward progress until B finishes, but B will not make
> forward progress until A closes the socket.
> 
> I was able to reproduce the issue locally, and process "A" was git-pull
> and process "B" was git-merge.

Thanks for the great explanation.  Yes, each Git command will open
its own connection to the socket, so you need your server to be
able to process multiple incoming requests, such as the usual listen()
loop.

There is a "trace2.destinationdebug" aka GIT_TRACE2_DST_DEBUG which
when set to a positive integer will print warning messages when
attempting to open the trace2 files or sockets.  This might help you
track down issues.  (These aren't on by default.)

You might find it easier to set the trace2 path to that of an existing
directory.  Then each Git command will create a file, so you don't have
to worry about interleaved output or having your server be alive at all
times.  You could just let the files accumulate in that directory and
have a cron job process and delete them periodically.

Jeff

      reply	other threads:[~2020-04-13 18:42 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-13 12:05 Son Luong Ngoc
2020-04-13 16:00 ` Taylor Blau
2020-04-13 17:19   ` Son Luong Ngoc
2020-04-13 17:23     ` Taylor Blau
2020-04-13 17:18 ` Jeff King
2020-04-13 18:42   ` Jeff Hostetler [this message]

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

  List information: http://vger.kernel.org/majordomo-info.html

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=dde14102-1eeb-71dc-5333-30b21a1ef17a@jeffhostetler.com \
    --to=git@jeffhostetler.com \
    --cc=Jeff.Hostetler@microsoft.com \
    --cc=git@vger.kernel.org \
    --cc=peff@peff.net \
    --cc=sluongng@gmail.com \
    --subject='Re: Git pull stuck when Trace2 target set to Unix Stream Socket' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Code repositories for project(s) associated with this inbox:

	https://80x24.org/mirrors/git.git

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).