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Rename detection logic in diffcore-rename that checks for renames of individual files is aggregated and analyzed in merge-recursive for cases where combinations of renames indicate that a full directory has been renamed.
It is perhaps easiest to start with an example:
When all of x/a, x/b and x/c have moved to z/a, z/b and z/c, it is likely that x/d added in the meantime would also want to move to z/d by taking the hint that the entire directory x moved to z.
More interesting possibilities exist, though, such as:
one side of history renames x → z, and the other renames some file to x/e, causing the need for the merge to do a transitive rename.
one side of history renames x → z, but also renames all files within x. For example, x/a → z/alpha, x/b → z/bravo, etc.
both x and y being merged into a single directory z, with a directory rename being detected for both x→z and y→z.
not all files in a directory being renamed to the same location; i.e. perhaps most the files in x are now found under z, but a few are found under w.
a directory being renamed, which also contained a subdirectory that was renamed to some entirely different location. (And perhaps the inner directory itself contained inner directories that were renamed to yet other locations).
combinations of the above; see t/t6043-merge-rename-directories.sh for various interesting cases.
In order to prevent edge and corner cases resulting in either conflicts that cannot be represented in the index or which might be too complex for users to try to understand and resolve, a couple basic rules limit when directory rename detection applies:
1) If a given directory still exists on both sides of a merge, we do not consider it to have been renamed.
2) If a subset of to-be-renamed files have a file or directory in the way (or would be in the way of each other), "turn off" the directory rename for those specific sub-paths and report the conflict to the user.
3) If the other side of history did a directory rename to a path that your side of history renamed away, then ignore that particular rename from the other side of history for any implicit directory renames (but warn the user).
t/t6043-merge-rename-directories.sh contains extensive tests and commentary which generate and explore the rules listed above. It also lists a few additional rules:
a) If renames split a directory into two or more others, the directory with the most renames, "wins".
b) Avoid directory-rename-detection for a path, if that path is the source of a rename on either side of a merge.
c) Only apply implicit directory renames to directories if the other side of history is the one doing the renaming.
Directory rename detection is supported by merge and cherry-pick. Other git commands which users might be surprised to see limited or no directory rename detection support in:
Folks have requested in the past that `git diff` detect directory renames and somehow simplify its output. It is not clear whether this would be desirable or how the output should be simplified, so this was simply not implemented. Further, to implement this, directory rename detection logic would need to move from merge-recursive to diffcore-rename.
git-am tries to avoid a full three way merge, instead calling git-apply. That prevents us from detecting renames at all, which may defeat the directory rename detection. There is a fallback, though; if the initial git-apply fails and the user has specified the -3 option, git-am will fall back to a three way merge. However, git-am lacks the necessary information to do a "real" three way merge. Instead, it has to use build_fake_ancestor() to get a merge base that is missing files whose rename may have been important to detect for directory rename detection to function.
Since am-based rebases work by first generating a bunch of patches (which no longer record what the original commits were and thus don't have the necessary info from which we can find a real merge-base), and then calling git-am, this implies that am-based rebases will not always successfully detect directory renames either (see the 'am' section above). merged-based rebases (rebase -m) and cherry-pick-based rebases (rebase -i) are not affected by this shortcoming, and fully support directory rename detection.