82 lines
3.2 KiB
Markdown
82 lines
3.2 KiB
Markdown
Worked on automatic merge conflict resolution today. I had expected to be
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able to use git's merge driver interface for this, but that interface is
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not sufficient. There are two problems with it:
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1. The merge program is run when git is in the middle of an operation
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that locks the index. So it cannot delete or stage files. I need to
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do both as part of my conflict resolution strategy.
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2. The merge program is not run at all when the merge conflict is caused
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by one side deleting a file, and the other side modifying it. This is
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an important case to handle.
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So, instead, git-annex will use a regular `git merge`, and if it fails, it
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will fix up the conflicts.
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That presented its own difficulty, of finding which files in the tree
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conflict. `git ls-files --unmerged` is the way to do that, but its output
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is a quite raw form:
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120000 3594e94c04db171e2767224db355f514b13715c5 1 foo
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120000 35ec3b9d7586b46c0fd3450ba21e30ef666cfcd6 3 foo
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100644 1eabec834c255a127e2e835dadc2d7733742ed9a 2 bar
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100644 36902d4d842a114e8b8912c02d239b2d7059c02b 3 bar
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I had to stare at the rather impenetrable documentation for hours and
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write a lot of parsing and processing code to get from that to these mostly
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self explanatory data types:
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data Conflicting v = Conflicting
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{ valUs :: Maybe v
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, valThem :: Maybe v
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} deriving (Show)
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data Unmerged = Unmerged
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{ unmergedFile :: FilePath
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, unmergedBlobType :: Conflicting BlobType
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, unmergedSha :: Conflicting Sha
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} deriving (Show)
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Not the first time I've whined here about time spent parsing unix command
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output, is it? :)
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From there, it was relatively easy to write the actual conflict cleanup
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code, and make `git annex sync` use it. Here's how it looks:
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$ ls -1
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foo.png
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bar.png
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$ git annex sync
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commit
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# On branch master
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nothing to commit (working directory clean)
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ok
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merge synced/master
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CONFLICT (modify/delete): bar.png deleted in refs/heads/synced/master and modified in HEAD. Version HEAD of bar.png left in tree.
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Automatic merge failed; fix conflicts and then commit the result.
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bar.png: needs merge
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(Recording state in git...)
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[master 0354a67] git-annex automatic merge conflict fix
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ok
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$ ls -1
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foo.png
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bar.variant-a1fe.png
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bar.variant-93a1.png
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There are very few options for ways for the conflict resolution code to
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name conflicting variants of files. The conflict resolver can only use data
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present in git to generate the names, because the same conflict needs to
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be resolved the same everywhere.
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So I had to choose between using the full key name in the filenames produced
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when resolving a merge, and using a shorter checksum of the key, that would be
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more user-friendly, but could theoretically collide with another key.
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I chose the checksum, and weakened it horribly by only using 32 bits of it!
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Surprisingly, I think this is a safe choice. The worst that can
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happens if such a collision happens is another conflict, and the conflict
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resolution code will work on conflicts produced by the conflict resolution
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code! In such a case, it does fall back to putting the whole key in
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the filename:
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"bar.variant-SHA256-s2550--2c09deac21fa93607be0844fefa870b2878a304a7714684c4cc8f800fda5e16b.png"
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Still need to hook this code into `git annex assistant`.
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