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mergetools: add new conflict marker format with diffs in
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Authored by martinvonz on Dec 9 2020, 4:28 AM.

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Summary

I use 3-way conflict markers. Often when I resolve them, I manually
compare one the base with one side and apply the differences to the
other side. That can be hard when the conflict marker is large. This
patch introduces a new type of conflict marker, which I'm hoping will
make it easier to resolve conflicts.

The new format uses <<<<<<< and >>>>>>> to open and close the
markers, just like our existing 2-way and 3-way conflict
markers. Instead of having 2 or 3 snapshots (left+right or
left+base+right), it has a sequence of diffs. A diff looks like this:

------- base
+++++++ left
 a
-b
+c
 d

A diff that adds one side ("diff from nothing") has a ======= header
instead and does not have have + prefixed on its lines. A regular
3-way merge can be viewed as adding one side plus a diff between the
base and the other side. It thus has two ways of being represented,
depending on which side is being diffed:

<<<<<<<
======= left
contents
on
left
------- base
+++++++ right
 contents
 on
-left
+right
>>>>>>>

or

<<<<<<<
------- base
+++++++ left
 contents
 on
-right
+left
======= right
contents
on
right
>>>>>>>

I've made it so the new merge tool tries to pick a version that has
the most common lines (no difference in the example above).

I've called the new tool "mergediff" to stick to the convention of
starting with "merge" if the tool tries a regular 3-way merge.

The idea came from my pet VCS (placeholder name jj), which has
support for octopus merges and other ways of ending up with merges of
more than 3 versions. I wanted to be able to represent such conflicts
in the working copy and therefore thought of this format (although I
have not yet implemented it in my VCS). I then attended a meeting with
Larry McVoy, who said BitKeeper has an option (bk smerge -g) for
showing a similar format, which reminded me to actually attempt this
in Mercurial.

Diff Detail

Repository
rHG Mercurial
Lint
Automatic diff as part of commit; lint not applicable.
Unit
Automatic diff as part of commit; unit tests not applicable.

Event Timeline

martinvonz created this revision.Dec 9 2020, 4:28 AM
pulkit added a subscriber: pulkit.Dec 9 2020, 7:05 AM

Wow! +1 for the feature. I just skimmed through the code, running out of time and will review later if no one else beats me to it.

martinvonz added inline comments.Dec 9 2020, 11:59 AM
mercurial/simplemerge.py
565

I think the or b' ' bit was a leftover from an experiment. I'll see if I can remove that.

tests/test-conflict.t
316–318

I think ======= might actually be better for when the section is a full snapshot (not a diff). I'll change that.

martinvonz edited the summary of this revision. (Show Details)Dec 9 2020, 12:16 PM
martinvonz updated this revision to Diff 24143.
martinvonz edited the summary of this revision. (Show Details)Dec 9 2020, 1:28 PM
martinvonz updated this revision to Diff 24151.Dec 9 2020, 10:23 PM
pulkit accepted this revision.Dec 12 2020, 2:17 AM
This revision is now accepted and ready to land.Dec 12 2020, 2:17 AM