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Merge branch 'maint' of git:// into maint

* 'maint' of git://
user-manual: recovering from corruption
user-manual: clarify language about "modifying" old commits
user-manual: failed push to public repository
user-manual: define "branch" and "working tree" at start

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 The initial clone may be time-consuming for a large project, but you
 will only need to clone once.
-The clone command creates a new directory named after the project
-("git" or "linux-2.6" in the examples above).  After you cd into this
+The clone command creates a new directory named after the project ("git"
+or "linux-2.6" in the examples above).  After you cd into this
 directory, you will see that it contains a copy of the project files,
-together with a special top-level directory named ".git", which
-contains all the information about the history of the project.
+called the <<def_working_tree,working tree>>, together with a special
+top-level directory named ".git", which contains all the information
+about the history of the project.
 How to check out a different version of a project
 interrelated snapshots of the project's contents.  In git each such
 version is called a <<def_commit,commit>>.
-A single git repository may contain multiple branches.  It keeps track
-of them by keeping a list of <<def_head,heads>> which reference the
+Those snapshots aren't necessarily all arranged in a single line from
+oldest to newest; instead, work may simultaneously proceed along
+parallel lines of development, called <def_branch,branches>>, which may
+merge and diverge.
+A single git repository can track development on multiple branches.  It
+does this by keeping a list of <<def_head,heads>> which reference the
 latest commit on each branch; the gitlink:git-branch[1] command shows
 you the list of branch heads:
 conflicts manually, just as in the case of <<resolving-a-merge,
 resolving a merge>>.
-Fixing a mistake by editing history
+Fixing a mistake by rewriting history
 If the problematic commit is the most recent commit, and you have not
 been merged into another branch; use gitlink:git-revert[1] instead in
 that case.
-It is also possible to edit commits further back in the history, but
+It is also possible to replace commits further back in the history, but
 this is an advanced topic to be left for
 <<cleaning-up-history,another chapter>>.
 git-gc when run without any options), it is not safe to prune while
 other git operations are in progress in the same repository.
+If gitlink:git-fsck[1] complains about sha1 mismatches or missing
+objects, you may have a much more serious problem; your best option is
+probably restoring from backups.  See
+<<recovering-from-repository-corruption>> for a detailed discussion.
 Recovering lost changes
 $ git push ssh:// master
-As with git-fetch, git-push will complain if this does not result in
-a <<fast-forwards,fast forward>>.  Normally this is a sign of
-something wrong.  However, if you are sure you know what you're
-doing, you may force git-push to perform the update anyway by
-proceeding the branch name by a plus sign:
-$ git push ssh:// +master
+As with git-fetch, git-push will complain if this does not result in a
+<<fast-forwards,fast forward>>; see the following section for details on
+handling this case.
 Note that the target of a "push" is normally a
 <<def_bare_repository,bare>> repository.  You can also push to a
 and remote.<name>.push options in gitlink:git-config[1] for
+What to do when a push fails
+If a push would not result in a <<fast-forwards,fast forward>> of the
+remote branch, then it will fail with an error like:
+error: remote 'refs/heads/master' is not an ancestor of
+ local  'refs/heads/master'.
+ Maybe you are not up-to-date and need to pull first?
+error: failed to push to 'ssh://'
+This can happen, for example, if you:
+	- use `git reset --hard` to remove already-published commits, or
+	- use `git commit --amend` to replace already-published commits
+	  (as in <<fixing-a-mistake-by-rewriting-history>>), or
+	- use `git rebase` to rebase any already-published commits (as
+	  in <<using-git-rebase>>).
+You may force git-push to perform the update anyway by preceding the
+branch name with a plus sign:
+$ git push ssh:// +master
+Normally whenever a branch head in a public repository is modified, it
+is modified to point to a descendent of the commit that it pointed to
+before.  By forcing a push in this situation, you break that convention.
+(See <<problems-with-rewriting-history>>.)
+Nevertheless, this is a common practice for people that need a simple
+way to publish a work-in-progress patch series, and it is an acceptable
+compromise as long as you warn other developers that this is how you
+intend to manage the branch.
+It's also possible for a push to fail in this way when other people have
+the right to push to the same repository.  In that case, the correct
+solution is to retry the push after first updating your work by either a
+pull or a fetch followed by a rebase; see the
+<<setting-up-a-shared-repository,next section>> and
+link:cvs-migration.html[git for CVS users] for more.
 Setting up a shared repository
 $ git rebase --abort
-Modifying a single commit
+Rewriting a single commit
-We saw in <<fixing-a-mistake-by-editing-history>> that you can replace the
+We saw in <<fixing-a-mistake-by-rewriting-history>> that you can replace the
 most recent commit using
 which will replace the old commit by a new commit incorporating your
 changes, giving you a chance to edit the old commit message first.
-You can also use a combination of this and gitlink:git-rebase[1] to edit
-commits further back in your history.  First, tag the problematic commit with
+You can also use a combination of this and gitlink:git-rebase[1] to
+replace a commit further back in your history and recreate the
+intervening changes on top of it.  First, tag the problematic commit
 $ git tag bad mywork~5
 contrast, running "git prune" while somebody is actively changing the
 repository is a *BAD* idea).
+Recovering from repository corruption
+By design, git treats data trusted to it with caution.  However, even in
+the absence of bugs in git itself, it is still possible that hardware or
+operating system errors could corrupt data.
+The first defense against such problems is backups.  You can back up a
+git directory using clone, or just using cp, tar, or any other backup
+As a last resort, you can search for the corrupted objects and attempt
+to replace them by hand.  Back up your repository before attempting this
+in case you corrupt things even more in the process.
+We'll assume that the problem is a single missing or corrupted blob,
+which is sometimes a solveable problem.  (Recovering missing trees and
+especially commits is *much* harder).
+Before starting, verify that there is corruption, and figure out where
+it is with gitlink:git-fsck[1]; this may be time-consuming.
+Assume the output looks like this:
+$ git-fsck --full
+broken link from    tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
+              to    blob 4b9458b3786228369c63936db65827de3cc06200
+missing blob 4b9458b3786228369c63936db65827de3cc06200
+(Typically there will be some "dangling object" messages too, but they
+aren't interesting.)
+Now you know that blob 4b9458b3 is missing, and that the tree 2d9263c6
+points to it.  If you could find just one copy of that missing blob
+object, possibly in some other repository, you could move it into
+.git/objects/4b/9458b3... and be done.  Suppose you can't.  You can
+still examine the tree that pointed to it with gitlink:git-ls-tree[1],
+which might output something like:
+$ git ls-tree 2d9263c6d23595e7cb2a21e5ebbb53655278dff8
+100644 blob 8d14531846b95bfa3564b58ccfb7913a034323b8	.gitignore
+100644 blob ebf9bf84da0aab5ed944264a5db2a65fe3a3e883	.mailmap
+100644 blob ca442d313d86dc67e0a2e5d584b465bd382cbf5c	COPYING
+100644 blob 4b9458b3786228369c63936db65827de3cc06200	myfile
+So now you know that the missing blob was the data for a file named
+"myfile".  And chances are you can also identify the directory--let's
+say it's in "somedirectory".  If you're lucky the missing copy might be
+the same as the copy you have checked out in your working tree at
+"somedirectory/myfile"; you can test whether that's right with
+$ git hash-object -w somedirectory/myfile
+which will create and store a blob object with the contents of
+somedirectory/myfile, and output the sha1 of that object.  if you're
+extremely lucky it might be 4b9458b3786228369c63936db65827de3cc06200, in
+which case you've guessed right, and the corruption is fixed!
+Otherwise, you need more information.  How do you tell which version of
+the file has been lost?
+The easiest way to do this is with:
+$ git log --raw --all --full-history -- somedirectory/myfile
+Because you're asking for raw output, you'll now get something like
+commit abc
+:100644 100644 4b9458b... newsha... M somedirectory/myfile
+commit xyz
+:100644 100644 oldsha... 4b9458b... M somedirectory/myfile
+This tells you that the immediately preceding version of the file was
+"newsha", and that the immediately following version was "oldsha".
+You also know the commit messages that went with the change from oldsha
+to 4b9458b and with the change from 4b9458b to newsha.
+If you've been committing small enough changes, you may now have a good
+shot at reconstructing the contents of the in-between state 4b9458b.
+If you can do that, you can now recreate the missing object with
+$ git hash-object -w <recreated-file>
+and your repository is good again!
+(Btw, you could have ignored the fsck, and started with doing a 
+$ git log --raw --all
+and just looked for the sha of the missing object (4b9458b..) in that 
+whole thing. It's up to you - git does *have* a lot of information, it is 
+just missing one particular blob version.
 The index
 Alternates, clone -reference, etc.
-git unpack-objects -r for recovery
+More on recovery from repository corruption.  See:
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