merge: Parameterize merge functions + `git merge-file` on `marker_size` (with a stubbed lookup) #101

rwe · 2021-09-30T20:19:45Z

NOTE: For convenience, this branch is pre-resolved with https://github.com/rwe/git-revise/tree/rerere-skip-unsized-marker which is PR'd as #95 and would have a minor conflict once merged. The standalone change is available on https://github.com/rwe/git-revise/tree/conflict-marker.

This parameterizes the relevant pieces of merge.py on conflict_marker. The parameterization currently is dummied out to resolve to the default marker size, which is 7. However this refactoring makes that dependency explicit and therefore possible to extend to use git check-attr. See discussion here: #95 (comment), which describes a naïve and likely-not-performant example implementation of "real" resolution.

krobelus · 2021-09-30T21:10:06Z

gitrevise/merge.py

@@ -255,6 +267,9 @@ def merge_files(
            "--quiet",
            # Send results to stdout instead of overwriting "current file".
            "--stdout",
+            # Ensure markers are the expected length to ensure consistent rerere results.
+            "--marker-size",
+            str(marker_size),


nice, I thought there was no way to override attributes.
I'd probably put this in a separate commit, apart from the refactoring.

It's part of the "parameterize marker size" commit. I think that seems like the right place to me?

Note that this does not override attributes: they weren't and aren't used at all here. This is merging arbitrary temp files and git merge-file doesn't refer to the repo at all other than to check user preference for diff3 style, which isn't relevant here. (The rerere normalization strips those out, and otherwise we'd want to preserve their preference if the user wants to manually resolve).

The note about using git check-attr to determine conflict-marker-size is just so that the rerere conflict IDs here would be cross-compatible with the ones created and used by vanilla git rebase/git merge.

ah sorry I thought this commit already had a behavior change

krobelus · 2021-09-30T21:10:19Z

gitrevise/merge.py

@@ -284,7 +302,11 @@ def replay_recorded_resolution(
    ):
        return (b"", None, None)

-    (normalized_preimage, conflict_id) = normalize_conflicted_file(preimage)
+    (normalized_preimage, conflict_id) = normalize_conflicted_file(
+        marker_size=marker_size,


not sure if we are supposed to use kwarg-style calls for non-kwargs?

I'm not sure I follow what you mean by non-kwargs?

marker_size is a positional argument

Sorry, I'm still puzzling over what you mean here. Saying "it's a positional argument" on a line where it's passed by name doesn't make sense. normalize_conflicted_file does not have opaque *args: there are no positional-only parameters. There's nothing wrong with giving the names explicitly.

Oh sorry I mixed that up.
I was just wondering if there is a convention of when to pass the name along an argument.
I think I usually pass the name only for arguments that have default values (and kwargs).
However, we use the name when calling Repository.bool_config for example, this is consistent with your version.

I tend to prefer named arguments in general, unless the order is unambiguous (like something like call_with(fn, arg1, arg2)) or irrelevant (e.g. max(a, b)). They're particularly helpful during refactoring, since they're both resilient to re-ordering of parameters, their use makes the introduction of default parameters a little easier to check for, and having the names there can make a merge resolution much more obvious. Though, these are also ultimately justifications of habit :)

krobelus · 2021-09-30T21:10:45Z

gitrevise/merge.py

@@ -28,6 +28,12 @@
 DEFAULT_CONFLICT_MARKER_SIZE = 7


+def get_conflict_marker_size(__repo: Repository, __file: Path) -> int:
+    # TODO: Determine on a per-file basis by its `conflict-marker-size` attribute.
+    # See ll_merge_marker_size in git/ll-merge.c


Regarding your comment about performance concerns: here's a possible way to avoid them:

In merge_files(), run "git merge-file", without passing a marker size

Only if the above "git merge-file" failed, ask "git check-attr" for the conflict marker size, and re-run "git merge-file"

In the common case (no conflict) we're as fast as ever.

I often use git-revise for rewrites that cause the same conflicts - that's
where rerere kicks in. However, there are typically very few of those
conflicts, so I don't think that matters. Also that's not the typical use case.

Doing an optimistic merge is a good idea to consider. I'd worry a little about being tempted (or tempting others) to look for or save cached resolutions for both marker sizes, which could lead to inconsistency. Probably avoidable, though! Or not a big deal.

I do wonder what the actual performance impact of batch-querying git check-attrs for each un-encountered entry per tree, e.g. using a persistent process like git cat-file is using. I suspect that the performance difference between that and re-merging is pretty small. (I was mainly leery of my hamfisted query-each-entry-individually 😄).

Probably avoidable, though!

Yeah, merge_files() would do two runs of "git merge-file" and if the first failed, callers would only ever see the results of the second.

Using a persistent process for git check-attrs sounds great, and there are probably other reasons to use that: we can detect attributes like file encoding and whether a file is binary.

krobelus · 2021-09-30T21:12:35Z

gitrevise/merge.py

@@ -28,6 +28,12 @@
 DEFAULT_CONFLICT_MARKER_SIZE = 7


+def get_conflict_marker_size(__repo: Repository, __file: Path) -> int:


this should maybe be a method of Repository (or at least the generic check-attr wrapper should be)

or at least the generic check-attr wrapper should be
Yep, I agree that a general check-attr wrapper thing would benefit from being lifted up, if implemented. It would make sense to allow that to be a persisted process and have those path → attributes cached.

As far as handling the "conflict-marker-size" attribute in particular, it might benefit from staying standalone. There's some extra validation vs other attributes (turning it into an int, and defaulting to 7 if non-positive), and it's very specific to this area of code dealing with merge-conflict text.

rwe · 2022-01-10T20:59:37Z

@mystor Just wanted to bump this. This branch has no conflicts with base and has been ready for a while. I've pushed up a no-op rebase in order to keep things fresh.

rwe mentioned this pull request Sep 30, 2021

rerere: Skip mismatched marker sizes like git-rerere does #95

Merged

krobelus reviewed Sep 30, 2021

View reviewed changes

rwe force-pushed the conflict-marker-size branch from 3292c35 to c393215 Compare January 10, 2022 03:59

rwe force-pushed the conflict-marker-size branch from c393215 to 068319d Compare January 10, 2022 21:31

rwe force-pushed the conflict-marker-size branch from 068319d to 3f2927c Compare February 6, 2022 20:08

rwe added 5 commits November 15, 2022 21:22

merge_blobs: Simplify post-merge marker detection

18cb980

merge/merge_files: Explicit option names to 'git merge-file'

657bcb4

merge/fmt: Consistent tmpdir argument ordering

2425715

merge: parameterize conflict-marker size

406d82d

merge: Dummy determination of conflict marker size

4359ed9

rwe force-pushed the conflict-marker-size branch from 3f2927c to 4359ed9 Compare November 16, 2022 21:06

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merge: Parameterize merge functions + `git merge-file` on `marker_size` (with a stubbed lookup) #101

merge: Parameterize merge functions + `git merge-file` on `marker_size` (with a stubbed lookup) #101

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rwe commented Jan 10, 2022

		@@ -28,6 +28,12 @@
		DEFAULT_CONFLICT_MARKER_SIZE = 7


		def get_conflict_marker_size(__repo: Repository, __file: Path) -> int:

merge: Parameterize merge functions + git merge-file on marker_size (with a stubbed lookup) #101

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merge: Parameterize merge functions + git merge-file on marker_size (with a stubbed lookup) #101

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rwe commented Jan 10, 2022

merge: Parameterize merge functions + `git merge-file` on `marker_size` (with a stubbed lookup) #101

merge: Parameterize merge functions + `git merge-file` on `marker_size` (with a stubbed lookup) #101

rwe commented Sep 30, 2021 •

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