From: Jeff King <firstname.lastname@example.org> To: Son Luong Ngoc <email@example.com> Cc: firstname.lastname@example.org, Jeff.Hostetler@microsoft.com Subject: Re: Git pull stuck when Trace2 target set to Unix Stream Socket Date: Mon, 13 Apr 2020 13:18:12 -0400 [thread overview] Message-ID: <20200413171812.GA8183@coredump.intra.peff.net> (raw) 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: > 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
next prev parent reply other threads:[~2020-04-13 17:18 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 [this message] 2020-04-13 18:42 ` Jeff Hostetler
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=20200413171812.GA8183@coredump.intra.peff.net \ --email@example.com \ --cc=Jeff.Hostetler@microsoft.com \ --firstname.lastname@example.org \ --email@example.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).