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Diffstat (limited to 'Documentation/git-merge-base.txt')
| -rw-r--r-- | Documentation/git-merge-base.txt | 144 |
1 files changed, 104 insertions, 40 deletions
diff --git a/Documentation/git-merge-base.txt b/Documentation/git-merge-base.txt index b968b64c38..2d944e0851 100644 --- a/Documentation/git-merge-base.txt +++ b/Documentation/git-merge-base.txt @@ -80,9 +80,11 @@ which is reachable from both 'A' and 'B' through the parent relationship. For example, with this topology: - o---o---o---B - / - ---o---1---o---o---o---A +.... + o---o---o---B + / +---o---1---o---o---o---A +.... the merge base between 'A' and 'B' is '1'. @@ -90,21 +92,25 @@ Given three commits 'A', 'B' and 'C', `git merge-base A B C` will compute the merge base between 'A' and a hypothetical commit 'M', which is a merge between 'B' and 'C'. For example, with this topology: - o---o---o---o---C - / - / o---o---o---B - / / - ---2---1---o---o---o---A +.... + o---o---o---o---C + / + / o---o---o---B + / / +---2---1---o---o---o---A +.... the result of `git merge-base A B C` is '1'. This is because the equivalent topology with a merge commit 'M' between 'B' and 'C' is: - o---o---o---o---o - / \ - / o---o---o---o---M - / / - ---2---1---o---o---o---A +.... + o---o---o---o---o + / \ + / o---o---o---o---M + / / +---2---1---o---o---o---A +.... and the result of `git merge-base A M` is '1'. Commit '2' is also a common ancestor between 'A' and 'M', but '1' is a better common ancestor, @@ -116,11 +122,13 @@ the best common ancestor of all commits. When the history involves criss-cross merges, there can be more than one 'best' common ancestor for two commits. For example, with this topology: - ---1---o---A - \ / - X - / \ - ---2---o---o---B +.... +---1---o---A + \ / + X + / \ +---2---o---o---B +.... both '1' and '2' are merge-bases of A and B. Neither one is better than the other (both are 'best' merge bases). When the `--all` option is not given, @@ -131,46 +139,102 @@ and B is (or at least used to be) to compute the merge base between A and B, and check if it is the same as A, in which case, A is an ancestor of B. You will see this idiom used often in older scripts. - A=$(git rev-parse --verify A) - if test "$A" = "$(git merge-base A B)" - then - ... A is an ancestor of B ... - fi +.... +A=$(git rev-parse --verify A) +if test "$A" = "$(git merge-base A B)" +then + ... A is an ancestor of B ... +fi +.... In modern git, you can say this in a more direct way: - if git merge-base --is-ancestor A B - then - ... A is an ancestor of B ... - fi +.... +if git merge-base --is-ancestor A B +then + ... A is an ancestor of B ... +fi +.... instead. Discussion on fork-point mode ----------------------------- -After working on the `topic` branch created with `git checkout -b +After working on the `topic` branch created with `git switch -c topic origin/master`, the history of remote-tracking branch `origin/master` may have been rewound and rebuilt, leading to a history of this shape: - o---B1 - / - ---o---o---B2--o---o---o---B (origin/master) - \ - B3 - \ - Derived (topic) - -where `origin/master` used to point at commits B3, B2, B1 and now it +.... + o---B2 + / +---o---o---B1--o---o---o---B (origin/master) + \ + B0 + \ + D0---D1---D (topic) +.... + +where `origin/master` used to point at commits B0, B1, B2 and now it points at B, and your `topic` branch was started on top of it back -when `origin/master` was at B3. This mode uses the reflog of -`origin/master` to find B3 as the fork point, so that the `topic` -can be rebased on top of the updated `origin/master` by: +when `origin/master` was at B0, and you built three commits, D0, D1, +and D, on top of it. Imagine that you now want to rebase the work +you did on the topic on top of the updated origin/master. + +In such a case, `git merge-base origin/master topic` would return the +parent of B0 in the above picture, but B0^..D is *not* the range of +commits you would want to replay on top of B (it includes B0, which +is not what you wrote; it is a commit the other side discarded when +it moved its tip from B0 to B1). + +`git merge-base --fork-point origin/master topic` is designed to +help in such a case. It takes not only B but also B0, B1, and B2 +(i.e. old tips of the remote-tracking branches your repository's +reflog knows about) into account to see on which commit your topic +branch was built and finds B0, allowing you to replay only the +commits on your topic, excluding the commits the other side later +discarded. + +Hence $ fork_point=$(git merge-base --fork-point origin/master topic) + +will find B0, and + $ git rebase --onto origin/master $fork_point topic +will replay D0, D1 and D on top of B to create a new history of this +shape: + +.... + o---B2 + / +---o---o---B1--o---o---o---B (origin/master) + \ \ + B0 D0'--D1'--D' (topic - updated) + \ + D0---D1---D (topic - old) +.... + +A caveat is that older reflog entries in your repository may be +expired by `git gc`. If B0 no longer appears in the reflog of the +remote-tracking branch `origin/master`, the `--fork-point` mode +obviously cannot find it and fails, avoiding to give a random and +useless result (such as the parent of B0, like the same command +without the `--fork-point` option gives). + +Also, the remote-tracking branch you use the `--fork-point` mode +with must be the one your topic forked from its tip. If you forked +from an older commit than the tip, this mode would not find the fork +point (imagine in the above sample history B0 did not exist, +origin/master started at B1, moved to B2 and then B, and you forked +your topic at origin/master^ when origin/master was B1; the shape of +the history would be the same as above, without B0, and the parent +of B1 is what `git merge-base origin/master topic` correctly finds, +but the `--fork-point` mode will not, because it is not one of the +commits that used to be at the tip of origin/master). + See also -------- |