Re: mergetool: what to do about deleting precious files?

From: Junio C Hamano <gitster@pobox.com>
Date: May 30, 2017, 00:52 UTC
Message-ID: <xmqq1sr7f9nb.fsf@gitster.mtv.corp.google.com>
In-Reply-To: <DE497A69E68043DABF9C3A7D9198B277@PhilipOakley>

"Philip Oakley" <philipoakley@iee.org> writes:

Show 8 quoted lines

> If I now understand correctly, the merge process flow is:
>
> * canonicalise content (eol, smudge-clean, $id, renormalise, etc)
> * diff the content (internal, or GIT_EXTERNAL_DIFF)
> * apply the diff
> * if conflicts, only then use merge-driver/tool
>
> Would that be a correct interpretation?

Not quite.  There are a lot more going on before any of those steps:

 * Find the common ancestor commit (which could be many).

 * Walk the three trees (the common ancestor's, ours and theirs) in
   parallel, noticing what happened to each path.  Depending on what
   happened to the path in each branch, the merge may or may not
   "conflict" (e.g. when both sides added exactly the same contents
   to the same path, they are not counted as conflicting.  when we
   removed while they modified, they show as conflicting).

 * For paths that are conflicting, feed the canonicalized content of
   the versions from common, ours and theirs to the file-level merge
   driver.  The builtin file-level merge driver takes two xdiff (one
   between ancestor and ours, the other between ancestore and
   theirs) and reconciles them to produce the result.  But that is
   irrelevant in the context of "custom merge driver"; the builtin
   one is skipped altogether and the custom contents merge driver
   the user specified via the attributes is used instead.

Notice that the second step above has no customization knobs. Any path the second step deems not to conflict is "merged cleanly" without even triggering the "oops, ours and theirs did conflicting changes, to the content; let's see how the final content should look like" (aka the third step). This is *not* because "Git knows the best"; it is merely that nobody felt the need for a mechanism to allow customizing the second step.

And that is why I said you need a new customization mechanism if you want to affect the outcome of the scenario that started this thread.