[PATCH] checkout: add 'pre-checkout' hook

[Sam Vilain <sam.vilain@catalyst.net.nz>]

Add a simple hook that will run before checkouts.

Signed-off-by: Sam Vilain <sam.vilain@catalyst.net.nz>
---
 Documentation/githooks.txt |   20 +++++++++++++++-----
 builtin-checkout.c         |   25 ++++++++++++++++++++++---
 2 files changed, 37 insertions(+), 8 deletions(-)

diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index 06e0f31..8dc3fbf 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -143,21 +143,31 @@ pre-rebase
 This hook is called by 'git-rebase' and can be used to prevent a branch
 from getting rebased.
 
+pre-checkout
+-----------
 
-post-checkout
-~~~~~~~~~~~~~
-
-This hook is invoked when a 'git-checkout' is run after having updated the
+This hook is invoked when a 'git-checkout' is run after before updating the
 worktree.  The hook is given three parameters: the ref of the previous HEAD,
 the ref of the new HEAD (which may or may not have changed), and a flag
 indicating whether the checkout was a branch checkout (changing branches,
 flag=1) or a file checkout (retrieving a file from the index, flag=0).
-This hook cannot affect the outcome of 'git-checkout'.
+This hook can prevent the checkout from proceeding by exiting with an
+error code.
 
 It is also run after 'git-clone', unless the --no-checkout (-n) option is
 used. The first parameter given to the hook is the null-ref, the second the
 ref of the new HEAD and the flag is always 1.
 
+This hook can be used to perform any clean-up deemed necessary before
+checking out the new branch/files.
+
+post-checkout
+-----------
+
+This hook is invoked when a 'git-checkout' is run after having updated the
+worktree.  It takes the same arguments as the 'pre-checkout' hook.
+This hook cannot affect the outcome of 'git-checkout'.
+
 This hook can be used to perform repository validity checks, auto-display
 differences from the previous HEAD if different, or set working dir metadata
 properties.
diff --git a/builtin-checkout.c b/builtin-checkout.c
index d050c37..b72a3cb 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -36,6 +36,17 @@ struct checkout_opts {
 	enum branch_track track;
 };
 
+static int pre_checkout_hook(struct commit *old, struct commit *new,
+			      int changed)
+{
+	return run_hook(NULL, "pre-checkout",
+			sha1_to_hex(old ? old->object.sha1 : null_sha1),
+			sha1_to_hex(new ? new->object.sha1 : null_sha1),
+			changed ? "1" : "0", NULL);
+	/* "new" can be NULL when checking out from the index before
+	   a commit exists. */
+}
+
 static int post_checkout_hook(struct commit *old, struct commit *new,
 			      int changed)
 {
@@ -256,6 +267,13 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec,
 	if (errs)
 		return 1;
 
+	/* Run the pre-checkout hook */
+	resolve_ref("HEAD", rev, 0, &flag);
+	head = lookup_commit_reference_gently(rev, 1);
+	errs = pre_checkout_hook(head, head, 0);
+	if (errs)
+		return 1;
+
 	/* Now we are committed to check them out */
 	memset(&state, 0, sizeof(state));
 	state.force = 1;
@@ -279,9 +297,6 @@ static int checkout_paths(struct tree *source_tree, const char **pathspec,
 	    commit_locked_index(lock_file))
 		die("unable to write new index file");
 
-	resolve_ref("HEAD", rev, 0, &flag);
-	head = lookup_commit_reference_gently(rev, 1);
-
 	errs |= post_checkout_hook(head, head, 0);
 	return errs;
 }
@@ -543,6 +558,10 @@ static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
 		parse_commit(new->commit);
 	}
 
+	ret = pre_checkout_hook(old.commit, new->commit, 1);
+	if (ret)
+		return ret;
+
 	ret = merge_working_tree(opts, &old, new);
 	if (ret)
 		return ret;
-- 
1.6.3.3