From: "Philip Oakley" <philipoakley@iee.org>
To: "Junio C Hamano" <gitster@pobox.com>
Cc: "Git List" <git@vger.kernel.org>
Subject: Re: mergetool: what to do about deleting precious files?
Date: Wed, 31 May 2017 00:04:46 +0100 [thread overview]
Message-ID: <E0D5AC60DB0F48DEBF865597910D9531@PhilipOakley> (raw)
In-Reply-To: xmqq1sr7f9nb.fsf@gitster.mtv.corp.google.com
From: "Junio C Hamano" <gitster@pobox.com>
Thanks for the replies. Let's see if I've got it...
> "Philip Oakley" <philipoakley@iee.org> writes:
>
>> 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).
IIUC Git selects one of them, rather than all if there are many (which then
may not be the optimum)
>
> * 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).
I'm assuming here that this is the sha-oid comparison, and then checking the
tree/blob names that match them. (the top tree not having a name). So here
"conflict free" is that the sha-oids match.
Also, I thnk this is saying that added or removed trees or blobs are in some
sense are 'conflict free' (though still subject to rename/move detection
etc). An added file/blob would be conflict free for merging into it's tree,
yes?
IIUC, the comparison is therefore using the in-repo sha-oids;
unless --renormalise was given which will do a smudge-clean washing cycle
and recomute fresh canonical sha-oids for the comparison (rather than doing
it later).
>
> * For paths that are conflicting, feed the canonicalized content of
> the versions from common, ours and theirs to the file-level merge
> driver.
So this is where any .gitattibutes settings come in, or is the merge driver
after the diff step? (which could also be a user diff?)
> 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.
Ok, I think I can see how I was confused between the "tree merge" (oid
conflict detection) and the more usual (to users) "file merge" (line by
line, etc.). I wasn't sure where to find that as someone relatively new to
Git.
Thanks for the explanations.
--
Philip
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next prev parent reply other threads:[~2017-05-30 23:05 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-05-27 10:03 mergetool: what to do about deleting precious files? Philip Oakley
2017-05-28 1:14 ` Junio C Hamano
2017-05-28 10:24 ` Philip Oakley
2017-05-28 13:06 ` Junio C Hamano
2017-05-29 12:57 ` Philip Oakley
2017-05-30 0:52 ` Junio C Hamano
2017-05-30 23:04 ` Philip Oakley [this message]
2017-05-31 0:02 ` Junio C Hamano
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