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directory-rename-detection.txt: technical docs on abilities and limit…
…ations Signed-off-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>
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Directory rename detection | ||
========================== | ||
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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. | ||
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Scope of abilities | ||
------------------ | ||
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It is perhaps easiest to start with an example: | ||
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* 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'. | ||
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More interesting possibilities exist, though, such as: | ||
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* 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. | ||
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* 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. | ||
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* both 'x' and 'y' being merged into a single directory 'z', with a | ||
directory rename being detected for both x->z and y->z. | ||
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* 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'. | ||
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* 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). | ||
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* combinations of the above; see t/t6043-merge-rename-directories.sh for | ||
various interesting cases. | ||
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Limitations -- applicability of directory renames | ||
------------------------------------------------- | ||
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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: | ||
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1) If a given directory still exists on both sides of a merge, we do | ||
not consider it to have been renamed. | ||
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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. | ||
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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). | ||
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Limitations -- detailed rules and testcases | ||
------------------------------------------- | ||
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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: | ||
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a) If renames split a directory into two or more others, the directory | ||
with the most renames, "wins". | ||
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b) Avoid directory-rename-detection for a path, if that path is the | ||
source of a rename on either side of a merge. | ||
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c) Only apply implicit directory renames to directories if the other side | ||
of history is the one doing the renaming. | ||
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Limitations -- support in different commands | ||
-------------------------------------------- | ||
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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: | ||
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* diff | ||
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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. |