122 lines
4.2 KiB
Plaintext
122 lines
4.2 KiB
Plaintext
Trivial merge rules
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===================
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This document describes the outcomes of the trivial merge logic in read-tree.
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One-way merge
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-------------
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This replaces the index with a different tree, keeping the stat info
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for entries that don't change, and allowing -u to make the minimum
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required changes to the working tree to have it match.
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Entries marked '+' have stat information. Spaces marked '*' don't
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affect the result.
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index tree result
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-----------------------
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* (empty) (empty)
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(empty) tree tree
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index+ tree tree
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index+ index index+
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Two-way merge
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-------------
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It is permitted for the index to lack an entry; this does not prevent
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any case from applying.
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If the index exists, it is an error for it not to match either the old
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or the result.
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If multiple cases apply, the one used is listed first.
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A result which changes the index is an error if the index is not empty
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and not up to date.
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Entries marked '+' have stat information. Spaces marked '*' don't
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affect the result.
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case index old new result
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-------------------------------------
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0/2 (empty) * (empty) (empty)
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1/3 (empty) * new new
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4/5 index+ (empty) (empty) index+
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6/7 index+ (empty) index index+
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10 index+ index (empty) (empty)
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14/15 index+ old old index+
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18/19 index+ old index index+
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20 index+ index new new
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Three-way merge
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---------------
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It is permitted for the index to lack an entry; this does not prevent
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any case from applying.
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If the index exists, it is an error for it not to match either the
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head or (if the merge is trivial) the result.
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If multiple cases apply, the one used is listed first.
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A result of "no merge" means that index is left in stage 0, ancest in
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stage 1, head in stage 2, and remote in stage 3 (if any of these are
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empty, no entry is left for that stage). Otherwise, the given entry is
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left in stage 0, and there are no other entries.
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A result of "no merge" is an error if the index is not empty and not
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up to date.
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*empty* means that the tree must not have a directory-file conflict
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with the entry.
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For multiple ancestors, a '+' means that this case applies even if
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only one ancestor or remote fits; a '^' means all of the ancestors
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must be the same.
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case ancest head remote result
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----------------------------------------
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1 (empty)+ (empty) (empty) (empty)
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2ALT (empty)+ *empty* remote remote
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2 (empty)^ (empty) remote no merge
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3ALT (empty)+ head *empty* head
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3 (empty)^ head (empty) no merge
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4 (empty)^ head remote no merge
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5ALT * head head head
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6 ancest+ (empty) (empty) no merge
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8 ancest^ (empty) ancest no merge
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7 ancest+ (empty) remote no merge
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10 ancest^ ancest (empty) no merge
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9 ancest+ head (empty) no merge
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16 anc1/anc2 anc1 anc2 no merge
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13 ancest+ head ancest head
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14 ancest+ ancest remote remote
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11 ancest+ head remote no merge
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Only #2ALT and #3ALT use *empty*, because these are the only cases
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where there can be conflicts that didn't exist before. Note that we
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allow directory-file conflicts between things in different stages
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after the trivial merge.
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A possible alternative for #6 is (empty), which would make it like
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#1. This is not used, due to the likelihood that it arises due to
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moving the file to multiple different locations or moving and deleting
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it in different branches.
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Case #1 is included for completeness, and also in case we decide to
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put on '+' markings; any path that is never mentioned at all isn't
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handled.
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Note that #16 is when both #13 and #14 apply; in this case, we refuse
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the trivial merge, because we can't tell from this data which is
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right. This is a case of a reverted patch (in some direction, maybe
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multiple times), and the right answer depends on looking at crossings
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of history or common ancestors of the ancestors.
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Note that, between #6, #7, #9, and #11, all cases not otherwise
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covered are handled in this table.
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For #8 and #10, there is alternative behavior, not currently
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implemented, where the result is (empty). As currently implemented,
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the automatic merge will generally give this effect.
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