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

[Michael Haggerty <mhagger@alum.mit.edu>]

On 04/02/2014 06:53 PM, Junio C Hamano wrote:
> Jeff King <peff@peff.net> writes:
> 
>> 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).

It's good that you and Peff asked questions about this sort of thing.

We reuse lock_file structures *all over the place*; for example, just
search for "static struct lock_file".  It has to be safe...

...and yet it isn't.  Look in the definition of lock_file() (before my
changes):

static int lock_file(struct lock_file *lk, const char *path, int flags)
{
	...
	strcpy(lk->filename, path);
	if (!(flags & LOCK_NODEREF))
		resolve_symlink(lk->filename, max_path_len);
	strcat(lk->filename, ".lock");

Remember that a reused lock_file structure is already in lock_file_list,
and there is already a signal handler registered that will call
remove_lock_file(), which looks like:

static void remove_lock_file(void)
{
	pid_t me = getpid();

	while (lock_file_list) {
		if (lock_file_list->owner == me &&
		    lock_file_list->filename[0]) {
			if (lock_file_list->fd >= 0)
				close(lock_file_list->fd);
			unlink_or_warn(lock_file_list->filename);
		}
		lock_file_list = lock_file_list->next;
	}
}

So, if the process gets a signal during the call to resolve_symlink(),
the atexit() cleanup routine will delete the valuable file (the one
being locked)!

It definitely looks like this area needs more work.

Michael

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