Re: [PATCH v6 13/13] read-cache: make sure file handles are not inherited by child processes

From: Junio C Hamano <gitster@pobox.com>
Date: Aug 29, 2016, 19:45 UTC
Message-ID: <xmqq37lnbbpk.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <4D9E5AED-7003-4707-8791-1C25432DB558@gmail.com>

Lars Schneider <larsxschneider@gmail.com> writes:

> I see. Thanks for the explanation.

I expect the updated log message to explain the issue correctly then.

Show 10 quoted lines

>> And even on POSIX systems, if you are doing a long-running helper
>> any open file descriptor in the parent process when the long-running
>> helper is spawned will become leaked fd.  CLOEXEC is a possible
>> solution (but not necessarily the only or the best one) to the fd
>> leak in this case.
>> 
>> How much does the code that spawns these long-running helpers know
>> about the file descriptors that happen to be open?
>
> Nothing really.

OK.

Show 8 quoted lines

>> The parent is
>> very likely to have pack windows open into .pack files and they need
>> to be closed on the child side after fork(2) starts the child
>> process but before execve(2) runs the helper, if we want to avoid
>> file descriptor leaks.
>
> I think I understand what you are saying. However, during my tests
> .pack file fd's were never a problem.

I do not expect during the lifetime of your long-running helper anybody would try to unlink an existing packfile, so it is unlikely that "cannot unlink an open file on Windows" issue to come up. And the cross-platform problem I pointed out is a fd leak; leaks would not show up until you run out of the resource, just like you wouldn't notice small memory leak here and there UNLESS you actively look for them. I would be surprised if your "tests" found anything.

> How would I find the open .pack file fd's?  Should I go through
> /proc/PID/fd? Why is this no problem for other longer running
> commands such as the git-credential-cache--daemon or git-daemon?

Nobody said this is "no problem for others". While discussing the patch that started this thread, we noticed that any open file descriptor the main process has when the long-running clean/smudge helper is spawned _is_ a problem. Other helpers may share the same problem, and if they do, that is all the more reason that fixing the file descriptor leak is a good thing.

The primary thing I was wondering was if we should open the window into packfiles with CLOEXEC, just like we recently started opening the tempfiles and lockfiles with the flag. The reason why I asked if the site that spawns (i.e. run_command API) has an easy access to the list of file descriptors that we opened ONLY for ourselves is because that would make it possible to consider an alternative approach close them before execve(2) in run_commands API. That would save us from having to sprinkle existing calls to open(2) with CLOEXEC. But if that is not the case, opening them with CLOEXEC is a much better solution than going outside the program to "find" them in non-portable ways.

Thanks.