Re: [PATCH 02/22] try_merge_strategy(): remove redundant lock_file allocation

From: Junio C Hamano <gitster@pobox.com>
Date: Apr 2, 2014, 16:53 UTC
Message-ID: <xmqqy4znwu4g.fsf@gitster.dls.corp.google.com>
In-Reply-To: <20140401195615.GB21715@sigill.intra.peff.net>

Jeff King <peff@peff.net> writes:

Show 16 quoted lines

> On Tue, Apr 01, 2014 at 05:58:10PM +0200, Michael Haggerty wrote:
>
>> By the time the "if" block is entered, the lock_file instance from the
>> main function block is no longer in use, so re-use that one instead of
>> allocating a second one.
>> 
>> Note that the "lock" variable in the "if" block used to shadow the
>> "lock" variable at function scope, so the only change needed is to
>> remove the inner definition.
>
> I wonder if this would also be simpler if "lock" were simply declared as
> a static variable, and we drop the allocation entirely. I suppose that
> does create more cognitive load, though, in that it is only correct if
> the function is not recursive. On the other hand, the current code makes
> a reader unfamiliar with "struct lock" wonder if there is a free(lock)
> missing.

Another thing that makes a reader wonder if this is a valid rewrite is if it is safe to reuse a lock_file structure, especially because the original gives a piece of memory _cleared_ with xcalloc(). The second invocation of hold_locked_index() is now done on a dirty piece of memory, and the reader needs to drill down the callchain to see if that is safe (and if not, hold_locked_index() and probably the underlying lock_file() needs to memset() it to NULs).