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diff --git a/Documentation/technical/trivial-merge.txt b/Documentation/technical/trivial-merge.txt new file mode 100644 index 0000000000..c79d4a7c47 --- /dev/null +++ b/Documentation/technical/trivial-merge.txt @@ -0,0 +1,121 @@ +Trivial merge rules +=================== + +This document describes the outcomes of the trivial merge logic in read-tree. + +One-way merge +------------- + +This replaces the index with a different tree, keeping the stat info +for entries that don't change, and allowing -u to make the minimum +required changes to the working tree to have it match. + +Entries marked '+' have stat information. Spaces marked '*' don't +affect the result. + + index tree result + ----------------------- + * (empty) (empty) + (empty) tree tree + index+ tree tree + index+ index index+ + +Two-way merge +------------- + +It is permitted for the index to lack an entry; this does not prevent +any case from applying. + +If the index exists, it is an error for it not to match either the old +or the result. + +If multiple cases apply, the one used is listed first. + +A result which changes the index is an error if the index is not empty +and not up-to-date. + +Entries marked '+' have stat information. Spaces marked '*' don't +affect the result. + + case index old new result + ------------------------------------- + 0/2 (empty) * (empty) (empty) + 1/3 (empty) * new new + 4/5 index+ (empty) (empty) index+ + 6/7 index+ (empty) index index+ + 10 index+ index (empty) (empty) + 14/15 index+ old old index+ + 18/19 index+ old index index+ + 20 index+ index new new + +Three-way merge +--------------- + +It is permitted for the index to lack an entry; this does not prevent +any case from applying. + +If the index exists, it is an error for it not to match either the +head or (if the merge is trivial) the result. + +If multiple cases apply, the one used is listed first. + +A result of "no merge" means that index is left in stage 0, ancest in +stage 1, head in stage 2, and remote in stage 3 (if any of these are +empty, no entry is left for that stage). Otherwise, the given entry is +left in stage 0, and there are no other entries. + +A result of "no merge" is an error if the index is not empty and not +up-to-date. + +*empty* means that the tree must not have a directory-file conflict + with the entry. + +For multiple ancestors, a '+' means that this case applies even if +only one ancestor or remote fits; a '^' means all of the ancestors +must be the same. + + case ancest head remote result + ---------------------------------------- + 1 (empty)+ (empty) (empty) (empty) + 2ALT (empty)+ *empty* remote remote + 2 (empty)^ (empty) remote no merge + 3ALT (empty)+ head *empty* head + 3 (empty)^ head (empty) no merge + 4 (empty)^ head remote no merge + 5ALT * head head head + 6 ancest+ (empty) (empty) no merge + 8 ancest^ (empty) ancest no merge + 7 ancest+ (empty) remote no merge + 10 ancest^ ancest (empty) no merge + 9 ancest+ head (empty) no merge + 16 anc1/anc2 anc1 anc2 no merge + 13 ancest+ head ancest head + 14 ancest+ ancest remote remote + 11 ancest+ head remote no merge + +Only #2ALT and #3ALT use *empty*, because these are the only cases +where there can be conflicts that didn't exist before. Note that we +allow directory-file conflicts between things in different stages +after the trivial merge. + +A possible alternative for #6 is (empty), which would make it like +#1. This is not used, due to the likelihood that it arises due to +moving the file to multiple different locations or moving and deleting +it in different branches. + +Case #1 is included for completeness, and also in case we decide to +put on '+' markings; any path that is never mentioned at all isn't +handled. + +Note that #16 is when both #13 and #14 apply; in this case, we refuse +the trivial merge, because we can't tell from this data which is +right. This is a case of a reverted patch (in some direction, maybe +multiple times), and the right answer depends on looking at crossings +of history or common ancestors of the ancestors. + +Note that, between #6, #7, #9, and #11, all cases not otherwise +covered are handled in this table. + +For #8 and #10, there is alternative behavior, not currently +implemented, where the result is (empty). As currently implemented, +the automatic merge will generally give this effect. |