Re: [PATCH 06/11] apply: move lockfile into `apply_state`

[Jeff King <peff@peff.net>]

On Sun, Oct 01, 2017 at 04:56:07PM +0200, Martin Ågren wrote:

> We have two users of `struct apply_state` and the related functionality
> in apply.c. Each user sets up its `apply_state` by handing over a
> pointer to its static `lock_file`. (Before 076aa2cbd (tempfile:
> auto-allocate tempfiles on heap, 2017-09-05), we could never free
> lockfiles, so making them static was a reasonable approach.)
> 
> Other than that, they never directly access their `lock_file`s, which
> are instead handled by the functionality in apply.c.
> 
> To make life easier for the caller and to make it less tempting for a
> future caller to mess with the lock, make apply.c fully responsible for
> setting up the `lock_file`. As mentioned above, it is now safe to free a
> `lock_file`, so we can make the `struct apply_state` contain an actual
> `struct lock_file` instead of a pointer to one.
> 
> The user in builtin/apply.c is rather simple. For builtin/am.c, we might
> worry that the lock state is actually meant to be inherited across
> calls. But the lock is only taken as `apply_all_patches()` executes, and
> code inspection shows that it will always be released.
> 
> Alternatively, we can observe that the lock itself is never queried
> directly. When we decide whether we should lock, we check a related
> variable `newfd`. That variable is not inherited, so from the point of
> view of apply.c, the state machine really is reset with each call to
> `init_apply_state()`. (It would be a bug if `newfd` and the lock status
> were not in sync. The duplication of information in `newfd` and the lock
> will be addressed in the next patch.)

Nicely explained. I didn't re-verify your inspection of
apply_all_patches(), but I think it would be fine regardless. We don't
init_apply_state() again, so both before and after your patch we are
always looking at the "same" lock struct (and so if somebody forgot to
unlock, in either case we'd hit the "this tempfile is already active"
assertion).

-Peff