From: "Ævar Arnfjörð Bjarmason" <avarab@gmail.com>
To: Mike Hommey <mh@glandium.org>
Cc: git@vger.kernel.org, Johannes Sixt <j6t@kdbg.org>
Subject: Re: Closing fds twice when using remote helpers
Date: Wed, 15 May 2019 13:43:10 +0200 [thread overview]
Message-ID: <87bm04vt81.fsf@evledraar.gmail.com> (raw)
In-Reply-To: <20190515105609.sucfjvuumeyyrmjb@glandium.org>
On Wed, May 15 2019, Mike Hommey wrote:
> Hi,
>
> I started getting a weird error message during some test case involving
> git-cinnabar, which is a remote-helper to access mercurial
> repositories.
>
> The error says:
> fatal: mmap failed: Bad file descriptor
>
> ... which was not making much sense. Some debugging later, and it turns
> out this is what happens:
>
> - start_command is called for fast-import
> - start_command is called again for git-remote-hg, passing the
> fast_import->out as cmd->in.
> - in start_command, we end up on the line of code that does
> close(cmd->in), so fast_import->out/cmd->in is now closed
> - much later, in disconnect_helper, we call close(data->helper->out),
> where data->helper is the cmd for fast-import, and that fd was already
> closed above.
> - Except, well, fds being what they are, we in fact just closed a fd
> from a packed_git->pack_fd. So, when use_pack is later called, and
> tries to mmap data from that pack, it fails because the file
> descriptor was closed.
>
> I'm not entirely sure how to address this... Any ideas?
>
> Relatedly, use_pack calls xmmap, which does its own error handling and
> die()s in case of error, but then goes on to do its own check with a
> different error message (which, in fact, could be more useful in other
> cases). It seems like it should call xmmap_gently instead.
The "obvious" hacky fix is to pass in some "I own it, don't close it"
new flag in the child_process struct.
In fact we used to have such a thing in the code, see e72ae28895
("start_command(), .in/.out/.err = -1: Callers must close the file
descriptor", 2008-02-16).
So we could bring it back, but I wonder if a better long-term solution
is to refactor the API to have explicit start_command() ->
free_command() steps, even if the free() is something that happens
implicitly unless some "gutsy" function is called.
next prev parent reply other threads:[~2019-05-15 11:43 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-05-15 10:56 Closing fds twice when using remote helpers Mike Hommey
2019-05-15 11:43 ` Ævar Arnfjörð Bjarmason [this message]
2019-05-15 17:59 ` Johannes Sixt
2019-05-15 22:08 ` Mike Hommey
2019-05-15 23:53 ` Jeff King
2019-05-16 0:48 ` Mike Hommey
2019-05-16 3:28 ` Jeff King
2019-05-16 8:35 ` Mike Hommey
2019-05-16 21:47 ` Jeff King
2019-05-16 22:02 ` Mike Hommey
2019-05-16 0:31 ` Mike Hommey
2019-05-16 0:37 ` [PATCH 1/2] dup() the input fd for fast-import used for " Mike Hommey
2019-05-16 0:37 ` [PATCH 2/2] Use xmmap_gently instead of xmmap in use_pack Mike Hommey
2019-05-16 3:34 ` Jeff King
2019-05-16 3:28 ` [PATCH 1/2] dup() the input fd for fast-import used for remote helpers Jeff King
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