SYNOPSIS
--------
-'git-read-tree' (<tree-ish> | -m <tree-ish1> [<tree-ish2> <tree-ish3>])"
+'git-read-tree' (<tree-ish> | [-m [-u]] <tree-ish1> [<tree-ish2> [<tree-ish3>]])
+
DESCRIPTION
-----------
-Reads the tree information given by <tree> into the directory cache,
+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 or perform a 3-way
-merge.
+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
+ 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" performs 2 kinds of merge, a single tree
-merge if only 1 tree is given or a 3-way merge if 3 trees are
+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 cache has an entry for a
-given pathname; and the contents of the path matches with the tree
+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)
+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 -a", the "git-checkout-cache" only checks out
+"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
"merged"), the original tree (stage 1, aka "orig"), and the two trees
you are trying to merge (stage 2 and 3 respectively).
-In fact, the way "git-read-tree" works, it's entirely agnostic about how
-you assign the stages, and you could really assign them any which way,
-and the above is just a suggested way to do it (except since
-"git-write-tree" refuses to write anything but stage0 entries, it makes
-sense to always consider stage 0 to be the "full merge" state).
-
-So what happens? Try it out. Select the original tree, and two trees
-to merge, and look how it works:
+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 the new git-read-tree.
+ 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 "script
+ 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. But since the index is always sorted, they're easy
- to find: they'll be clustered together.
+ 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
matching "stage1" entry if it exists too. .. all the normal
trivial rules ..
-Incidentally - it also means that you don't even have to have a
-separate subdirectory for this. All the information literally is in
-the index file, which is a temporary thing anyway. There is no need to
-worry about what is in the working directory, since it is never shown
-and never used.
+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
--------