On 04/02/2014 06:53 PM, Junio C Hamano wrote: > Jeff King <p...@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 mhag...@alum.mit.edu http://softwareswirl.blogspot.com/ -- To unsubscribe from this list: send the line "unsubscribe git" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html