mirror of
https://github.com/git/git
synced 2024-08-26 19:19:18 +00:00
![Junio C Hamano](/assets/img/avatar_default.png)
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>
184 lines
5.6 KiB
Plaintext
184 lines
5.6 KiB
Plaintext
git-merge-base(1)
|
|
=================
|
|
|
|
NAME
|
|
----
|
|
git-merge-base - Find as good common ancestors as possible for a merge
|
|
|
|
|
|
SYNOPSIS
|
|
--------
|
|
[verse]
|
|
'git merge-base' [-a|--all] <commit> <commit>...
|
|
'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
|
|
-----------
|
|
|
|
'git merge-base' finds best common ancestor(s) between two commits to use
|
|
in a three-way merge. One common ancestor is 'better' than another common
|
|
ancestor if the latter is an ancestor of the former. A common ancestor
|
|
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 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.
|
|
|
|
More generally, among the two commits to compute the merge base from,
|
|
one is specified by the first commit argument on the command line;
|
|
the other commit is a (possibly hypothetical) commit that is a merge
|
|
across all the remaining commits on the command line.
|
|
|
|
As a consequence, the 'merge base' is not necessarily contained in each of the
|
|
commit arguments if more than two commits are specified. This is different
|
|
from linkgit:git-show-branch[1] when used with the `--merge-base` option.
|
|
|
|
--octopus::
|
|
Compute the best common ancestors of all supplied commits,
|
|
in preparation for an n-way merge. This mimics the behavior
|
|
of 'git show-branch --merge-base'.
|
|
|
|
--independent::
|
|
Instead of printing merge bases, print a minimal subset of
|
|
the supplied commits with the same ancestors. In other words,
|
|
among the commits given, list those which cannot be reached
|
|
from any other. This mimics the behavior of 'git show-branch
|
|
--independent'.
|
|
|
|
--is-ancestor::
|
|
Check if the first <commit> is an ancestor of the second <commit>,
|
|
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
|
|
-------
|
|
-a::
|
|
--all::
|
|
Output all merge bases for the commits, instead of just one.
|
|
|
|
DISCUSSION
|
|
----------
|
|
|
|
Given two commits 'A' and 'B', `git merge-base A B` will output a commit
|
|
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
|
|
|
|
the merge base between 'A' and 'B' is '1'.
|
|
|
|
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
|
|
|
|
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
|
|
|
|
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,
|
|
because '2' is an ancestor of '1'. Hence, '2' is not a merge base.
|
|
|
|
The result of `git merge-base --octopus A B C` is '2', because '2' is
|
|
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
|
|
|
|
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,
|
|
it is unspecified which best one is output.
|
|
|
|
A common idiom to check "fast-forward-ness" between two commits A
|
|
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
|
|
|
|
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
|
|
|
|
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
|
|
--------
|
|
linkgit:git-rev-list[1],
|
|
linkgit:git-show-branch[1],
|
|
linkgit:git-merge[1]
|
|
|
|
GIT
|
|
---
|
|
Part of the linkgit:git[1] suite
|