Re: [PATCH 04/22] rollback_lock_file(): set fd to -1

From: Michael Haggerty <mhagger@alum.mit.edu>
Date: Apr 6, 2014, 21:45 UTC
Message-ID: <5341CAE2.8040600@alum.mit.edu>
In-Reply-To: <xmqqtxabwtw8.fsf@gitster.dls.corp.google.com>

On 04/02/2014 06:58 PM, Junio C Hamano wrote:

Show 21 quoted lines

> Jeff King <peff@peff.net> writes:
> 
>> On Tue, Apr 01, 2014 at 05:58:12PM +0200, Michael Haggerty wrote:
>>
>>> When rolling back the lockfile, call close_lock_file() so that the
>>> lock_file's fd field gets set back to -1.  This could help prevent
>>> confusion in the face of hypothetical future programming errors.
>>
>> This also solves a race. We could be in the middle of rollback_lock_file
>> when we get a signal, and double-close. It's probably not a big deal,
>> though, since nobody could have opened a new descriptor in the interim
>> that got the same number (so the second close will just fail silently).
>>
>> Still, this seems like a definite improvement.
> 
> This is probably related to my comments on 2/22, but is "fd" the
> only thing that has a non-zero safe value?  Perhaps lock_file_init()
> that clears the structure fields to 0/NULL and fd to -1, and a
> convenience function lock_file_alloc() that does xmalloc() and then
> calls lock_file_init() may help us a bit when the lockfile structure
> is reused?

The first use of a lock_file object necessarily passes through lock_file(). The only precondition for that function is that the on_list field is zero, which is satisfied by a xcalloc()ed object.

Subsequent uses of a lock_file object must *not* zero the object. lock_file objects are added to the lock_file_list and never removed. So zeroing a lock_file object would discard the rest of the linked list. But subsequent uses must also pass through lock_file(), which sees that on_list is set, and assumes that the object is in a self-consistent state as left by commit_lock_file() or rollback_lock_file().

At least that's how it is supposed to work. But lock_file objects are in fact not cleaned up correctly in all circumstances. The next version of this patch series will work to fix that.

Michael

-- 
Michael Haggerty
mhagger@alum.mit.edu
http://softwareswirl.blogspot.com/