git-read-tree - Online in the Cloud

PROGRAM:

NAME

git-read-tree - Reads tree information into the index

SYNOPSIS

git read-tree [[-m [--trivial] [--aggressive] | --reset | --prefix=<prefix>]
[-u [--exclude-per-directory=<gitignore>] | -i]]
[--index-output=<file>] [--no-sparse-checkout]
(--empty | <tree-ish1> [<tree-ish2> [<tree-ish3>]])

DESCRIPTION

Reads the tree information given by <tree-ish> into the index, but does not actually
update any of the files it "caches". (see: git-checkout-index(1))

Optionally, it can merge a tree into the index, 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. The command will refuse to run if your index file
has unmerged entries, indicating that you have not finished previous merge you
started.

--reset
Same as -m, except that unmerged entries are discarded instead of failing.

-u
After a successful merge, update the files in the work tree with the result of the
merge.

-i
Usually a merge requires the index file as well as the files in the working tree to be
up to date with the current head commit, in order not to lose local changes. This flag
disables the check with the working tree and is meant to be used when creating a merge
of trees that are not directly related to the current working tree status into a
temporary index file.

-n, --dry-run
Check if the command would error out, without updating the index or the files in the
working tree for real.

-v
Show the progress of checking files out.

--trivial
Restrict three-way merge by git read-tree to happen only if there is no file-level
merging required, instead of resolving merge for trivial cases and leaving conflicting
files unresolved in the index.

--aggressive
Usually a three-way merge by git read-tree resolves the merge for really trivial cases
and leaves other cases unresolved in the index, so that porcelains can implement
different merge policies. This flag makes the command resolve a few more cases
internally:

· when one side removes a path and the other side leaves the path unmodified. The
resolution is to remove that path.

· when both sides remove a path. The resolution is to remove that path.

· when both sides add a path identically. The resolution is to add that path.

--prefix=<prefix>/
Keep the current index contents, and read the contents of the named tree-ish under the
directory at <prefix>. The command will refuse to overwrite entries that already
existed in the original index file. Note that the <prefix>/ value must end with a
slash.

--exclude-per-directory=<gitignore>
When running the command with -u and -m options, the merge result may need to
overwrite paths that are not tracked in the current branch. The command usually
refuses to proceed with the merge to avoid losing such a path. However this safety
valve sometimes gets in the way. For example, it often happens that the other branch
added a file that used to be a generated file in your branch, and the safety valve
triggers when you try to switch to that branch after you ran make but before running
make clean to remove the generated file. This option tells the command to read
per-directory exclude file (usually .gitignore) and allows such an untracked but
explicitly ignored file to be overwritten.

--index-output=<file>
Instead of writing the results out to $GIT_INDEX_FILE, write the resulting index in
the named file. While the command is operating, the original index file is locked with
the same mechanism as usual. The file must allow to be rename(2)ed into from a
temporary file that is created next to the usual index file; typically this means it
needs to be on the same filesystem as the index file itself, and you need write
permission to the directories the index file and index output file are located in.

--no-sparse-checkout
Disable sparse checkout support even if core.sparseCheckout is true.

--empty
Instead of reading tree object(s) into the index, just empty it.

<tree-ish#>
The id of the tree object(s) to be read/merged.

MERGING

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.

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.

SPARSE CHECKOUT

"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:

/*
!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.

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