git-read-tree(1) Manual Page

NAME

git - read-tree - Reads tree information into the directory cache

SYNOPSIS

git-read-tree (<tree-ish> | [-m [-u]] <tree-ish1> [<tree-ish2> [<tree-ish3>]])

DESCRIPTION

Reads the tree information given by <tree-ish> into the directory cache, but does not actually update any of the files it "caches". (see: git-checkout-cache)

Optionally, it can merge a tree into the cache, perform a fast-forward (i.e. 2-way) merge, or a 3-way merge, with the -m flag. When used with -m, the -u flag causes it to also update the files in the work tree with the result of the merge.

Trivial merges are done by "git-read-tree" itself. Only conflicting paths will be in unmerged state when "git-read-tree" returns.

OPTIONS

-m
Perform a merge, not just a read.
-u
After a successful merge, update the files in the work tree with the result of the merge.
<tree-ish#>
The id of the tree object(s) to be read/merged.

Merging

If -m is specified, "git-read-tree" can performs 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 cache has an entry for a given pathname; and the contents of the path matches with the tree being read, the stat info from the cache is used. (In other words, the cache'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-cache -f -u -a", the "git-checkout-cache" 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:

  I (index)           H        M        Result
 -------------------------------------------------------
0 nothing             nothing  nothing  (does not happen)
1 nothing             nothing  exists   use M
2 nothing             exists   nothing  remove path from cache
3 nothing             exists   exists   use 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 cache
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 cache entry stays as in the original index file. If the entry were 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 what "local changes" you made are carried forward by running "git-diff-cache —cached $M". Note that this does not necessarily match "git-diff-cache —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-cache —cached $H" would have told you about the change before this merge, but it would not show in "git-diff-cache —cached $M" output after two-tree merge.

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".

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":

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:

You would normally use "git-merge-cache" with supplied "git-merge-one-file-script" to do this last step. The script does not touch the files in the work tree, and the entire merge happens in the index file. In other words, there is no need to worry about what is in the working directory, since it is never shown and never used.

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. To illustrate, suppose you start from what has been commited last to your repository:

$ JC=`cat .git/HEAD`
$ git-checkout-cache -f -u -a $JC

You do random edits, without running git-update-cache. And then you notice that the tip of your "upstream" tree has advanced since you pulled from him:

$ git-fetch-script rsync://.... linus
$ LT=`cat .git/MERGE_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 cache 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-cache git-merge-one-file-script -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.

See Also

git-write-tree(1); git-ls-files(1)

Author

Written by Linus Torvalds <torvalds@osdl.org>

Documentation

Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.

GIT

Part of the git(7) suite