Re: [PATCH] builtin-apply: keep information about files to be deleted

[Junio C Hamano <gitster@pobox.com>]

Michał Kiedrowicz <michal.kiedrowicz@gmail.com> writes:

> ... However, there are some cases when these two
> rules may cause problems:
>
> patch #1: rename A to B
> patch #2: rename C to A
> patch #3: modify A
>
> Should patch #3 modify B (which was A) or A (which was C)?
>
> patch #1: rename A to B
> patch #2: rename B to A
> patch #3: modify A
> patch #4: modify B
>
> Which files should be patched by #3 and #4?
>
> In my opinion both #3 and #4 should fail (or both should succeed) --
> with my patch only #3 will work and #4 will be rejected, because in #2
> B was marked as deleted.

Both of the examples above cannot be emitted as a single commit by
format-patch; the user is feeding a combined patch.  Perhaps renames
in each example sequence were came from one git commit but modifications
are from separate commit or handcrafted "follow-up" patch.

There are two stances we can take:

 (1) The user knows what he is doing.

     In the first example, if he wanted the change in #3 to end up in B,
     he would have arranged the patches in a different order, namely, 3 1
     2, but he didn't.  We should modify A (that came from C).

 (2) In situations like these when it is unusual and there is no clear and
     unambiguous answer, the rule has always been "fail and ask the user
     to clean up", because silently doing a wrong thing in an unusual
     situation that happens only once in a while is far worse than
     interrupting the user and forcing a manual intervention.

     In the first example, there is no clear answer.  Perhaps all three
     patches were independent patches (the first two obviously came from
     git because only we can do renames, but they may have been separate
     commits), and the user may have reordered them (or just picked a
     random order because he was linearizing a history with a merge).

The second one is even iffier.  If we _know_ that originally patch #1 and
#2 came from the same commit, then they represent swapping between A and
B, but if they came from different git commits, and if the user picked
patches in a random order, it may mean something completely different.

I am somewhat tempted to say that we should fail all of them, including
the original "single patch swapping files" brought up by Linus.

BUT

Can we make use simple rule to detect problematic cases?

 - An input to git-apply can contain more than one patch that affects a
   path; however

   - you cannot create a path that still exists, except for a path that
     _will_ be renamed away or removed (your patch fixes this by adding
     this "except for..." part to loosen the existing rule);

   - you cannot modify a path in a separate patch if it is involved in an
     either side of a rename (this will catch the ambiguity of patch #3 in
     your first example and #3 and #4 in your second example);

 - In addition:

   - the same path cannot be renamed from more than once (this will catch
     concatenation of two git generated patches);

With such a change, I think we can keep the safety of "when there are more
than one plausible outcomes, the tool shouldn't silently decide, nor make
progress that the user later needs to undo and redo", while allowing a
sane use of rename patches generated out of a git commit.