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Documentation/git-merge.txt: Partial rewrite of How Merge Works

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The git-merge documentation's "HOW MERGE WORKS" section is confusingly
composed and actually omits the most interesting part, the merging of
the arguments into HEAD itself, surprisingly not actually mentioning
the fast-forward merge anywhere.

This patch replaces the "[NOTE]" screenful of highly technical details
by a single sentence summing up the interesting information, and instead
explains how are the arguments compared with HEAD and the three possible
inclusion states that are named "Already up-to-date", "Fast-forward"
and "True merge". It also makes it clear that the rest of the section
talks only about the true merge situation, and slightly expands the
talk on solving conflicts.

Junio initiated the removal of the Note screenful altogether and
offered many stylistical fixes.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
This commit is contained in:
Petr Baudis 2008-07-19 20:17:22 +02:00 committed by Junio C Hamano
parent 8575ea559e
commit c0be8aa06b
1 changed files with 24 additions and 42 deletions
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66
Documentation/git-merge.txt
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@ -57,50 +57,31 @@ HOW MERGE WORKS
A merge is always between the current `HEAD` and one or more
commits (usually, branch head or tag), and the index file must
exactly match the
tree of `HEAD` commit (i.e. the contents of the last commit) when
it happens. In other words, `git diff --cached HEAD` must
report no changes.
match the tree of `HEAD` commit (i.e. the contents of the last commit)
when it starts out. In other words, `git diff --cached HEAD` must
report no changes. (One exception is when the changed index
entries are already in the same state that would result from
the merge anyway.)
[NOTE]
This is a bit of a lie. In certain special cases, your index is
allowed to be different from the tree of the `HEAD` commit. The most
notable case is when your `HEAD` commit is already ahead of what
is being merged, in which case your index can have arbitrary
differences from your `HEAD` commit. Also, your index entries
may have differences from your `HEAD` commit that match
the result of a trivial merge (e.g. you received the same patch
from an external source to produce the same result as what you are
merging). For example, if a path did not exist in the common
ancestor and your head commit but exists in the tree you are
merging into your repository, and if you already happen to have
that path exactly in your index, the merge does not have to
fail.
Three kinds of merge can happen:
Otherwise, merge will refuse to do any harm to your repository
(that is, it may fetch the objects from remote, and it may even
update the local branch used to keep track of the remote branch
with `git pull remote rbranch:lbranch`, but your working tree,
`.git/HEAD` pointer and index file are left intact). In addition,
merge always sets `.git/ORIG_HEAD` to the original state of HEAD so
a problematic merge can be removed by using `git reset ORIG_HEAD`.
* The merged commit is already contained in `HEAD`. This is the
simplest case, called "Already up-to-date."
You may have local modifications in the working tree files. In
other words, 'git-diff' is allowed to report changes.
However, the merge uses your working tree as the working area,
and in order to prevent the merge operation from losing such
changes, it makes sure that they do not interfere with the
merge. Those complex tables in read-tree documentation define
what it means for a path to "interfere with the merge". And if
your local modifications interfere with the merge, again, it
stops before touching anything.
* `HEAD` is already contained in the merged commit. This is the
most common case especially when involved through 'git pull':
you are tracking an upstream repository, committed no local
changes and now you want to update to a newer upstream revision.
Your `HEAD` (and the index) is updated to at point the merged
commit, without creating an extra merge commit. This is
called "Fast-forward".
So in the above two "failed merge" case, you do not have to
worry about loss of data --- you simply were not ready to do
a merge, so no merge happened at all. You may want to finish
whatever you were in the middle of doing, and retry the same
pull after you are done and ready.
* Both the merged commit and `HEAD` are independent and must be
tied together by a merge commit that has them both as its parents.
The rest of this section describes this "True merge" case.
The chosen merge strategy merges the two commits into a single
new source tree.
When things cleanly merge, these things happen:
1. The results are updated both in the index file and in your
@ -142,12 +123,13 @@ After seeing a conflict, you can do two things:
* Decide not to merge. The only clean-up you need are to reset
the index file to the `HEAD` commit to reverse 2. and to clean
up working tree changes made by 2. and 3.; 'git-reset' can
up working tree changes made by 2. and 3.; 'git-reset --hard' can
be used for this.
* Resolve the conflicts. `git diff` would report only the
conflicting paths because of the above 2. and 3. Edit the
working tree files into a desirable shape, 'git-add' or 'git-rm'
conflicting paths because of the above 2. and 3.
Edit the working tree files into a desirable shape
('git mergetool' can ease this task), 'git-add' or 'git-rm'
them, to make the index file contain what the merge result
should be, and run 'git-commit' to commit the result.