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 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.
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.
"Sparse checkout" allows populating the working directory sparsely. It uses the skip-worktree bit (see git-update-index(1)) to tell Git whether a file in the working directory is worth looking at.
git read-tree and other merge-based commands (git merge, git checkout...) can help maintaining the skip-worktree bitmap and working directory update. $GIT_DIR/info/sparse-checkout is used to define the skip-worktree reference bitmap. When git read-tree needs to update the working directory, it resets the skip-worktree bit in the index based on this file, which uses the same syntax as .gitignore files. If an entry matches a pattern in this file, skip-worktree will not be set on that entry. Otherwise, skip-worktree will be set.
Then it compares the new skip-worktree value with the previous one. If skip-worktree turns from set to unset, it will add the corresponding file back. If it turns from unset to set, that file will be removed.
While $GIT_DIR/info/sparse-checkout is usually used to specify what files are in, you can also specify what files are not in, using negate patterns. For example, to remove the file unwanted:
Another tricky thing is fully repopulating the working directory when you no longer want sparse checkout. You cannot just disable "sparse checkout" because skip-worktree bits are still in the index and your working directory is still sparsely populated. You should re-populate the working directory with the $GIT_DIR/info/sparse-checkout file content as follows:
Then you can disable sparse checkout. Sparse checkout support in git read-tree and similar commands is disabled by default. You need to turn core.sparseCheckout on in order to have sparse checkout support.