receive-pack: support push-to-checkout hook

When receive.denyCurrentBranch is set to updateInstead, a push that
tries to update the branch that is currently checked out is accepted
only when the index and the working tree exactly matches the
currently checked out commit, in which case the index and the
working tree are updated to match the pushed commit.  Otherwise the
push is refused.

This hook can be used to customize this "push-to-deploy" logic.  The
hook receives the commit with which the tip of the current branch is
going to be updated, and can decide what kind of local changes are
acceptable and how to update the index and the working tree to match
the updated tip of the current branch.

For example, the hook can simply run `git read-tree -u -m HEAD "$1"`
in order to emulate 'git fetch' that is run in the reverse direction
with `git push`, as the two-tree form of `read-tree -u -m` is
essentially the same as `git checkout` that switches branches while
keeping the local changes in the working tree that do not interfere
with the difference between the branches.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 30 insertions, 0 deletions

diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index 9ef2469373..7ba0ac965d 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -341,6 +341,36 @@ Both standard output and standard error output are forwarded to
 'git send-pack' on the other end, so you can simply `echo` messages
 for the user.
 
+push-to-checkout
+~~~~~~~~~~~~~~~~
+
+This hook is invoked by 'git-receive-pack' on the remote repository,
+which happens when a 'git push' is done on a local repository, when
+the push tries to update the branch that is currently checked out
+and the `receive.denyCurrentBranch` configuration variable is set to
+`updateInstead`.  Such a push by default is refused if the working
+tree and the index of the remote repository has any difference from
+the currently checked out commit; when both the working tree and the
+index match the current commit, they are updated to match the newly
+pushed tip of the branch.  This hook is to be used to override the
+default behaviour.
+
+The hook receives the commit with which the tip of the current
+branch is going to be updated.  It can exit with a non-zero status
+to refuse the push (when it does so, it must not modify the index or
+the working tree).  Or it can make any necessary changes to the
+working tree and to the index to bring them to the desired state
+when the tip of the current branch is updated to the new commit, and
+exit with a zero status.
+
+For example, the hook can simply run `git read-tree -u -m HEAD "$1"`
+in order to emulate 'git fetch' that is run in the reverse direction
+with `git push`, as the two-tree form of `read-tree -u -m` is
+essentially the same as `git checkout` that switches branches while
+keeping the local changes in the working tree that do not interfere
+with the difference between the branches.
+
+
 pre-auto-gc
 ~~~~~~~~~~~