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diff --git a/Documentation/git-read-tree.txt b/Documentation/git-read-tree.txt new file mode 100644 index 0000000000..e1be6cc0bc --- /dev/null +++ b/Documentation/git-read-tree.txt @@ -0,0 +1,268 @@ +git-read-tree(1) +================ +v0.1, May 2005 + +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 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 +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": + + - stage 2 and 3 are the same; take one or the other (it makes no + difference - the same work has been done on stage 2 and 3) + + - stage 1 and stage 2 are the same and stage 3 is different; take + stage 3 (some work has been done on stage 3) + + - stage 1 and stage 3 are the same and stage 2 is different take + stage 2 (some work has been done on stage 2) + +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 (ie "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-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 +-------- +link:git-write-tree.html[git-write-tree]; link:git-ls-files.html[git-ls-files] + + +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 link:git.html[git] suite + |