If -m is specified, git read-tree can perform 3 kinds of merge, a
single tree merge if only 1 tree is given, a fast-forward merge
with 2 trees, or a 3-way merge if 3 or more trees are provided.
Single Tree Merge
If only 1 tree is specified, git read-tree operates as if the
user did not specify -m, except that if the original index has an
entry for a given pathname, and the contents of the path match
with the tree being read, the stat info from the index is used.
(In other words, the index's stat()s take precedence over the
merged tree's).
That means that if you do a git read-tree -m <newtree> followed
by a git checkout-index -f -u -a, the git checkout-index only
checks out the stuff that really changed.
This is used to avoid unnecessary false hits when git diff-files
is run after git read-tree.
Two Tree Merge
Typically, this is invoked as git read-tree -m $H $M, where $H is
the head commit of the current repository, and $M is the head of
a foreign tree, which is simply ahead of $H (i.e. we are in a
fast-forward situation).
When two trees are specified, the user is telling git read-tree
the following:
1. The current index and work tree is derived from $H, but the
user may have local changes in them since $H.
2. The user wants to fast-forward to $M.
In this case, the git read-tree -m $H $M command makes sure that
no local change is lost as the result of this "merge". Here are
the "carry forward" rules, where "I" denotes the index, "clean"
means that index and work tree coincide, and "exists"/"nothing"
refer to the presence of a path in the specified commit:
I H M Result
-------------------------------------------------------
0 nothing nothing nothing (does not happen)
1 nothing nothing exists use M
2 nothing exists nothing remove path from index
3 nothing exists exists, use M if "initial checkout",
H == M keep index otherwise
exists, fail
H != M
clean I==H I==M
------------------
4 yes N/A N/A nothing nothing keep index
5 no N/A N/A nothing nothing keep index
6 yes N/A yes nothing exists keep index
7 no N/A yes nothing exists keep index
8 yes N/A no nothing exists fail
9 no N/A no nothing exists fail
10 yes yes N/A exists nothing remove path from index
11 no yes N/A exists nothing fail
12 yes no N/A exists nothing fail
13 no no N/A exists nothing fail
clean (H==M)
------
14 yes exists exists keep index
15 no exists exists keep index
clean I==H I==M (H!=M)
------------------
16 yes no no exists exists fail
17 no no no exists exists fail
18 yes no yes exists exists keep index
19 no no yes exists exists keep index
20 yes yes no exists exists use M
21 no yes no exists exists fail
In all "keep index" cases, the index entry stays as in the
original index file. If the entry is not up to date, git
read-tree keeps the copy in the work tree intact when operating
under the -u flag.
When this form of git read-tree returns successfully, you can see
which of the "local changes" that you made were carried forward
by running git diff-index --cached $M. Note that this does not
necessarily match what git diff-index --cached $H would have
produced before such a two tree merge. This is because of cases
18 and 19 --- if you already had the changes in $M (e.g. maybe
you picked it up via e-mail in a patch form), git diff-index
--cached $H would have told you about the change before this
merge, but it would not show in git diff-index --cached $M output
after the two-tree merge.
Case 3 is slightly tricky and needs explanation. The result from
this rule logically should be to remove the path if the user
staged the removal of the path and then switching to a new
branch. That however will prevent the initial checkout from
happening, so the rule is modified to use M (new tree) only when
the content of the index is empty. Otherwise the removal of the
path is kept as long as $H and $M are the same.
3-Way Merge
Each "index" entry has two bits worth of "stage" state. stage 0
is the normal one, and is the only one you'd see in any kind of
normal use.
However, when you do git read-tree with three trees, the "stage"
starts out at 1.
This means that you can do
$ git read-tree -m <tree1> <tree2> <tree3>
and you will end up with an index with all of the <tree1> entries
in "stage1", all of the <tree2> entries in "stage2" and all of
the <tree3> entries in "stage3". When performing a merge of
another branch into the current branch, we use the common
ancestor tree as <tree1>, the current branch head as <tree2>, and
the other branch head as <tree3>.
Furthermore, git read-tree has special-case logic that says: if
you see a file that matches in all respects in the following
states, it "collapses" back to "stage0":
• stage 2 and 3 are the same; take one or the other (it makes
no difference - the same work has been done on our branch in
stage 2 and their branch in stage 3)
• stage 1 and stage 2 are the same and stage 3 is different;
take stage 3 (our branch in stage 2 did not do anything since
the ancestor in stage 1 while their branch in stage 3 worked
on it)
• stage 1 and stage 3 are the same and stage 2 is different
take stage 2 (we did something while they did nothing)
The git write-tree command refuses to write a nonsensical tree,
and it will complain about unmerged entries if it sees a single
entry that is not stage 0.
OK, this all sounds like a collection of totally nonsensical
rules, but it's actually exactly what you want in order to do a
fast merge. The different stages represent the "result tree"
(stage 0, aka "merged"), the original tree (stage 1, aka "orig"),
and the two trees you are trying to merge (stage 2 and 3
respectively).
The order of stages 1, 2 and 3 (hence the order of three
<tree-ish> command-line arguments) are significant when you start
a 3-way merge with an index file that is already populated. Here
is an outline of how the algorithm works:
• if a file exists in identical format in all three trees, it
will automatically collapse to "merged" state by git
read-tree.
• a file that has any difference what-so-ever in the three
trees will stay as separate entries in the index. It's up to
"porcelain policy" to determine how to remove the non-0
stages, and insert a merged version.
• the index file saves and restores with all this information,
so you can merge things incrementally, but as long as it has
entries in stages 1/2/3 (i.e., "unmerged entries") you can't
write the result. So now the merge algorithm ends up being
really simple:
• you walk the index in order, and ignore all entries of
stage 0, since they've already been done.
• if you find a "stage1", but no matching "stage2" or
"stage3", you know it's been removed from both trees (it
only existed in the original tree), and you remove that
entry.
• if you find a matching "stage2" and "stage3" tree, you
remove one of them, and turn the other into a "stage0"
entry. Remove any matching "stage1" entry if it exists
too. .. all the normal trivial rules ..
You would normally use git merge-index with supplied git
merge-one-file to do this last step. The script updates the files
in the working tree as it merges each path and at the end of a
successful merge.
When you start a 3-way merge with an index file that is already
populated, it is assumed that it represents the state of the
files in your work tree, and you can even have files with changes
unrecorded in the index file. It is further assumed that this
state is "derived" from the stage 2 tree. The 3-way merge refuses
to run if it finds an entry in the original index file that does
not match stage 2.
This is done to prevent you from losing your work-in-progress
changes, and mixing your random changes in an unrelated merge
commit. To illustrate, suppose you start from what has been
committed last to your repository:
$ JC=`git rev-parse --verify "HEAD^0"`
$ git checkout-index -f -u -a $JC
You do random edits, without running git update-index. And then
you notice that the tip of your "upstream" tree has advanced
since you pulled from him:
$ git fetch git://.... linus
$ LT=`git rev-parse FETCH_HEAD`
Your work tree is still based on your HEAD ($JC), but you have
some edits since. Three-way merge makes sure that you have not
added or modified index entries since $JC, and if you haven't,
then does the right thing. So with the following sequence:
$ git read-tree -m -u `git merge-base $JC $LT` $JC $LT
$ git merge-index git-merge-one-file -a
$ echo "Merge with Linus" | \
git commit-tree `git write-tree` -p $JC -p $LT
what you would commit is a pure merge between $JC and $LT without
your work-in-progress changes, and your work tree would be
updated to the result of the merge.
However, if you have local changes in the working tree that would
be overwritten by this merge, git read-tree will refuse to run to
prevent your changes from being lost.
In other words, there is no need to worry about what exists only
in the working tree. When you have local changes in a part of the
project that is not involved in the merge, your changes do not
interfere with the merge, and are kept intact. When they do
interfere, the merge does not even start (git read-tree complains
loudly and fails without modifying anything). In such a case, you
can simply continue doing what you were in the middle of doing,
and when your working tree is ready (i.e. you have finished your
work-in-progress), attempt the merge again.
