Re: [PATCH 4/5] lock_file: make function-local locks non-static

From: Martin Ågren <martin.agren@gmail.com>
Date: May 6, 2018, 19:32 UTC
Message-ID: <CAN0heSpA5H7Gwwx0TEY9=iFJrgKb0SPXqKOwHK=4NxPYoGjZ7A@mail.gmail.com>
In-Reply-To: <CACsJy8Btuc2J4aCTymkvLYyMV5zJrdMUdtV5NDnPqXOjsTVw4w@mail.gmail.com>

On 6 May 2018 at 19:42, Duy Nguyen <pclouds@gmail.com> wrote:

> On Sun, May 6, 2018 at 7:26 PM, Duy Nguyen <pclouds@gmail.com> wrote:
>> On Sun, May 6, 2018 at 4:10 PM, Martin Ågren <martin.agren@gmail.com> wrote:
>>> These `struct lock_file`s are local to their respective functions and we
>>> can drop their staticness.

Show 18 quoted lines

>>> -       static struct lock_file lock;
>>> +       struct lock_file lock = LOCK_INIT;
>>
>> Is it really safe to do this? I vaguely remember something about
>> (global) linked list and signal handling which could trigger any time
>> and probably at atexit() time too (i.e. die()). You don't want to
>> depend on stack-based variables in that case.
>
> So I dug in a bit more about this. The original implementation does
> not allow stack-based lock files at all in 415e96c8b7 ([PATCH]
> Implement git-checkout-cache -u to update stat information in the
> cache. - 2005-05-15). The situation has changed since 422a21c6a0
> (tempfile: remove deactivated list entries - 2017-09-05). At the end
> of that second commit, Jeff mentioned "We can clean them up
> individually" which I guess is what these patches do. Though I do not
> know if we need to make sure to call "release" function or something/
> Either way you need more explanation and assurance than just "we can
> drop their staticness" in the commit mesage.

Thank you Duy for your comments. How about I write the commit message like so:

  After 076aa2cbd (tempfile: auto-allocate tempfiles on heap, 2017-09-05),
  we can have lockfiles on the stack. These `struct lock_file`s are local
  to their respective functions and we can drop their staticness.

  Each of these users either commits or rolls back the lock in every
  codepath, with these possible exceptions:

    * We bail using a call to `die()` or `exit()`. The lock will be
      cleaned up automatically.

    * We return early from a function `cmd_foo()` in builtin/, i.e., we
      are just about to exit. The lock will be cleaned up automatically.

  If I have missed some codepath where we do not exit, yet leave a locked
  lock around, that was so also before this patch. If we would later
  re-enter the same function, then before this patch, we would be retaking
  a lock for the very same `struct lock_file`, which feels awkward, but to
  the best of my reading has well-defined behavior. Whereas after this
  patch, we would attempt to take the lock with a completely fresh `struct
  lock_file`. In both cases, the result would simply be that the lock can
  not be taken, which is a situation we already handle.