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From:   Junio C Hamano <gitster@pobox.com>
To:     Jeff Hostetler <git@jeffhostetler.com>
Cc:     Chris Torek <chris.torek@gmail.com>, Jeff King <peff@peff.net>,
        Jeff Hostetler via GitGitGadget <gitgitgadget@gmail.com>,
        Git List <git@vger.kernel.org>,
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        Johannes Schindelin <Johannes.Schindelin@gmx.de>,
        Jeff Hostetler <jeffhost@microsoft.com>
Subject: Re: [PATCH v4 09/12] unix-socket: disallow chdir() when creating
 unix domain sockets
References: <pull.766.v3.git.1613174954.gitgitgadget@gmail.com>
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Date:   Fri, 05 Mar 2021 09:53:03 -0800
In-Reply-To: <449a73be-52e3-8363-b771-959f8114e3a0@jeffhostetler.com> (Jeff
        Hostetler's message of "Fri, 5 Mar 2021 12:33:11 -0500")
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Jeff Hostetler <git@jeffhostetler.com> writes:

> The original problem was that chdir() is not safe in a multi-threaded
> process because one thread calling chdir() will affect any concurrent
> file operations (open(), mkdir(), etc.) that use relative paths.
>
> I think Adding a fork() at this layer would just create new types of
> problems.  For example, if another thread was concurrently writing to
> a socket while we were setting up this new socket, we would suddenly
> have 1 thread in each process now writing to that socket and the
> receiver would get a mixture of output from both processes.  Right?

cf. https://pubs.opengroup.org/onlinepubs/9699919799/functions/fork.html

The fork() function shall create a new process. The new process
(child process) shall be an exact copy of the calling process
(parent process) except as detailed below:

...

 * A process shall be created with a single thread. If a
   multi-threaded process calls fork(), the new process shall
   contain a replica of the calling thread and its entire address
   space, possibly including the states of mutexes and other
   resources. Consequently, to avoid errors, the child process may
   only execute async-signal-safe operations until such time as one
   of the exec functions is called.

So, probably not.