Re: Git pull stuck when Trace2 target set to Unix Stream Socket

From: Jeff King <peff@peff.net>
Date: Apr 13, 2020, 17:18 UTC
Message-ID: <20200413171812.GA8183@coredump.intra.peff.net>
In-Reply-To: <CAL3xRKceuniEOq+PqN15FC4-mxsWJd4+e2fu1Ku9q_+8JmR9pw@mail.gmail.com>

On Mon, Apr 13, 2020 at 02:05:00PM +0200, Son Luong Ngoc wrote:

Show 14 quoted lines

> 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.

Switching to using datagrams ("nc -u" and "af_unix:dgram") makes the problem go away, because the connections are able to interleave their packets.

> This does not happen when you set eventBrief to true
> ```
> git config trace2.eventBrief true
> ```

I don't know why this would matter (and it didn't seem to in my tests).

> Worth to note that if eventTarget is a file instead of a socket,
> everything works fine.

That makes sense, since their file writes can be interleaved.

-Peff