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authorJunio C Hamano <gitster@pobox.com>2013-10-23 16:47:32 -0700
committerJunio C Hamano <gitster@pobox.com>2013-10-29 13:06:08 -0700
commitd96855ff517560650c62eca51a8bb78263f6a3ef (patch)
tree6fb9e42186c21d95b9c86af87eb8f8ed7a80d028 /Documentation/git-merge-base.txt
parent16e57aec7f672fabe38fa0e4d39252eb9c7a1563 (diff)
downloadgit-d96855ff517560650c62eca51a8bb78263f6a3ef.tar.gz
merge-base: teach "--fork-point" modejc/merge-base-reflog
The "git pull --rebase" command computes the fork point of the branch being rebased using the reflog entries of the "base" branch (typically a remote-tracking branch) the branch's work was based on, in order to cope with the case in which the "base" branch has been rewound and rebuilt. For example, if the history looked like this: o---B1 / ---o---o---B2--o---o---o---Base \ B3 \ Derived where the current tip of the "base" branch is at Base, but earlier fetch observed that its tip used to be B3 and then B2 and then B1 before getting to the current commit, and the branch being rebased on top of the latest "base" is based on commit B3, it tries to find B3 by going through the output of "git rev-list --reflog base" (i.e. Base, B1, B2, B3) until it finds a commit that is an ancestor of the current tip "Derived". Internally, we have get_merge_bases_many() that can compute this with one-go. We would want a merge-base between Derived and a fictitious merge commit that would result by merging all the historical tips of "base". When such a commit exist, we should get a single result, which exactly match one of the reflog entries of "base". Teach "git merge-base" a new mode, "--fork-point", to compute exactly that. Helped-by: Martin von Zweigbergk <martinvonz@gmail.com> Helped-by: John Keeping <john@keeping.me.uk> Signed-off-by: Junio C Hamano <gitster@pobox.com>
Diffstat (limited to 'Documentation/git-merge-base.txt')
-rw-r--r--Documentation/git-merge-base.txt38
1 files changed, 36 insertions, 2 deletions
diff --git a/Documentation/git-merge-base.txt b/Documentation/git-merge-base.txt
index 87842e33f8..808426faac 100644
--- a/Documentation/git-merge-base.txt
+++ b/Documentation/git-merge-base.txt
@@ -13,6 +13,7 @@ SYNOPSIS
'git merge-base' [-a|--all] --octopus <commit>...
'git merge-base' --is-ancestor <commit> <commit>
'git merge-base' --independent <commit>...
+'git merge-base' --fork-point <ref> [<commit>]
DESCRIPTION
-----------
@@ -24,8 +25,8 @@ that does not have any better common ancestor is a 'best common
ancestor', i.e. a 'merge base'. Note that there can be more than one
merge base for a pair of commits.
-OPERATION MODE
---------------
+OPERATION MODES
+---------------
As the most common special case, specifying only two commits on the
command line means computing the merge base between the given two commits.
@@ -56,6 +57,14 @@ from linkgit:git-show-branch[1] when used with the `--merge-base` option.
and exit with status 0 if true, or with status 1 if not.
Errors are signaled by a non-zero status that is not 1.
+--fork-point::
+ Find the point at which a branch (or any history that leads
+ to <commit>) forked from another branch (or any reference)
+ <ref>. This does not just look for the common ancestor of
+ the two commits, but also takes into account the reflog of
+ <ref> to see if the history leading to <commit> forked from
+ an earlier incarnation of the branch <ref> (see discussion
+ on this mode below).
OPTIONS
-------
@@ -137,6 +146,31 @@ In modern git, you can say this in a more direct way:
instead.
+Discussion on fork-point mode
+-----------------------------
+
+After working on the `topic` branch created with `git checkout -b
+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
+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:
+
+ $ fork_point=$(git merge-base --fork-point origin/master topic)
+ $ git rebase --onto origin/master $fork_point topic
+
See also
--------