git-read-tree(1) ================ v0.1, May 2005 NAME ---- git-read-tree - Reads tree information into the directory cache SYNOPSIS -------- 'git-read-tree' ( | -m [ ])" DESCRIPTION ----------- Reads the tree information given by 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. 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 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 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 " followed by a "git-checkout-cache -f -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 and you will end up with an index with all of the entries in "stage1", all of the entries in "stage2" and all of the 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). 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: - if a file exists in identical format in all three trees, it will automatically collapse to "merged" state by the new 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 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. - 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 .. 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. See Also -------- link:git-write-tree.html[git-write-tree]; link:git-ls-files.html[git-ls-files] Author ------ Written by Linus Torvalds Documentation -------------- Documentation by David Greaves, Junio C Hamano and the git-list . GIT --- Part of the link:git.html[git] suite