Re: git-name-rev off-by-one bug

linux@horizon.com writes:

Thanks for the complaints. No sarcasm intended. Yours is exactly the kind of message we (people who've been around here for too long) need to hear.

Although the technical details are hidden in the documentation which needs reorganization to make them easier to find [*1*] as you point out, the guiding principle for merge is quite simple.

To the git barebone Porcelain layer (things that start with git-*, not with cg-*) [*2*], a merge is always between the current HEAD and one or more remote branch heads, and the index file must exactly match the tree of HEAD commit (i.e. the contents of the last commit) when it happens. In other words, "git-diff --cached HEAD" must report no changes [*3*]. So HEAD+1 must be HEAD in your above notation, or merge will refuse to do any harm to your repository (that is, it may fetch the objects from remote, and it may even update the local branch used to keep track of the remote branch with "git pull remote rbranch:lbranch", but your working tree, .git/HEAD pointer and index file are left intact).

You may have local modifications in the working tree files. In other words, "git-diff" is allowed to report changes (the difference between HEAD+2 and HEAD+1 in your notation). However, the merge uses your working tree as the working area, and in order to prevent the merge operation from losing such changes, it makes sure that they do not interfere with the merge. Those complex tables in read-tree documentation define what it means for a path to "interfere with the merge". And if your local modifications interfere with the merge, again, it stops before touching anything.

So in the above two "failed merge" case, you do not have to worry about lossage of data --- you simply were not ready to do a merge, so no merge happened at all. You may want to finish whatever you were in the middle of doing, and retry the same pull after you are done and ready.

When things cleanly merge, these things happen:

 (1) the results are updated both in the index file and in your
     working tree,
 (2) index file is written out as a tree,
 (3) the tree gets committed, and 
 (4) the HEAD pointer gets advanced.

Because of (2), we require that the original state of the index file to match exactly the current HEAD commit; otherwise we will write out your local changes already registered in your index file (the difference between HEAD+1 and HEAD in your notation) along with the merge result, which is not good. Because (1) involves only the paths different between your branch and the remote branch you are pulling from during the merge (which is typically a fraction of the whole tree), you can have local modifications in your working tree as long as they do not overlap with what the merge updates.

When there are conflicts, these things happen:

 (0) HEAD stays the same.

 (1) Cleanly merged paths are updated both in the index file and
     in your working tree.

 (2) For conflicting paths, the index file records the version
     from HEAD. The working tree files have the result of
     "merge" program; i.e. 3-way merge result with familiar
     conflict markers <<< === >>>.

 (3) No other changes are done.  In particular, the local
     modifications you had before you started merge will stay the
     same and the index entries for them stay as they were,
     i.e. matching HEAD.

After seeing a conflict, you can do two things:

 * Decide not to merge.  The only clean-up you need are to reset
   the index file to the HEAD commit to reverse (1) and to clean
   up working tree changes made by (1) and (2); "git-reset" can
   be used for this.

 * Resolve the conflicts.  "git-diff" would report only the
   conflicting paths because of the above (1) and (2).  Edit the
   working tree files into a desirable shape, git-update-index
   them, to make the index file contain what the merge result
   should be, and run "git-commit" to commit the result.

[Footnotes]

*1* It is a shame that the most comprehensive definition of 3-way read-tree semantics is in t/t1000-read-tree-m-3way.sh test script.

*2* Cogito (things that start with cg-*) seems to try to be cleverer. Pasky might want to brag about the rules in Cogito land.

*3* This is a bit of lie. In certain special cases, your index are allowed to be different from the tree of HEAD commit; basically your index entries are allowed to match the result of trivial merge already (e.g. you received the same patch from external source to produce the same result as what you are merging). For example, if a path did not exist in the common ancestor and your head commit but exists in the tree you are merging into your repository, and if you already happen to have that path exactly in your index, the merge does not have to fail. This is case #2 in the 3-way read-tree table in t/t1000.