Merge branch 'jc/merge-base-fork-point-doc'

Clarify and enhance documentation for "merge-base --fork-point", as
it was clear what it computed but not why/what for.

* jc/merge-base-fork-point-doc:
  merge-base --fork-point doc: clarify the example and failure modes
This commit is contained in:
Junio C Hamano 2017-11-27 11:06:32 +09:00
commit 022dd4a0d3

View file

@ -154,23 +154,71 @@ 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---B2
/
---o---o---B2--o---o---o---B (origin/master)
---o---o---B1--o---o---o---B (origin/master)
\
B3
B0
\
Derived (topic)
D0---D1---D (topic)
where `origin/master` used to point at commits B3, B2, B1 and now it
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
--------