Subject: Re: About detached heads

From: Adam Piatyszek <[email-masked]>
Date: Fri, 14 Mar 2008 13:28:57 +0100

* Jakub Narebski [14 III 2008 11:52]:
 > Lets check what git does in each of scenarios. Let's assume that
 > current branch is named 'master'.
 >
 > At beginning we have:
 >
 >    1---2---3---4---5    <--- master <--- HEAD
 >
 > HEAD contents is "ref: refs/heads/master"
 >
 > 1. Now, "git checkout 3...", which is equivalent to "git checkout 3",
 > detaches HEAD because commit '3' is not a head (is not a branch), so
 > we have:
 >
 >    1---2---3---4---5    <--- master
 >            ^
 >             \
 >              \-------------- HEAD
 >
 > HEAD contents is "<sha1 of 3>"
 >
 >
 > 2. If we did "git reset --hard 3" we would rewind the history,
 > resulting in the following situation:
 >
 >    1---2---3           <--- master <--- HEAD
 >             \
 >              \-4---5   <... master@{1}, ORIG_HEAD, HEAD@{1}
 >
 > and now commits 4 and 5 are referenced only by reflogs, and by the
 > (temporary) "last position of HEAD" reference named ORIG_HEAD.
 >
 >
 > 3. Now, if you have published 1..5 history you would not want
 > (usually) to rewind published branch. If you do the following:
 >
 >   $ git revert --no-commit 5
 >   $ git revert 4
 >
 > you would get the following:
 >
 >    1---2---3---4---5---(5^-1 4^-1 => 3)  <--- master <--- HEAD
 >
 > git-revert applies reversal of changes in given commit, in the
 > "patch -R" ("patch --reverse") sense. Using '--no-commit' option
 > allows to squash reverting two commits into one commit. The ordering
 > of reverting ensures that there are no merge conflicts.
 >
 >
 > 4. Or you can just put the _contents_ of revision 3 into your working
 > tree, either using plumbing command git-read-tree, or by checking out
 > or resetting to top tree: "git checkout 3^{tree}", or
 > "git checkout 3 -- .", or equivalent git-reset invocation.
 >
 > This way you would get exactly
 >
 >    1---2---3---4---5---3   <--- master <--- HEAD
 >
 > but the relation of 5---3 parentage is unclear: you would have to
 > explain it in the commit mesage.

I suggest one should add the above nice explanation to FAQ or some wiki 
material.

BR,
/Adam

-- 
.:.  Adam Piatyszek (ediap)  .:.....................................:.
.:.  [email-masked]  .:................................:.