[PATCH v2 1/4] git-merge: Honor pre-merge hook

[Josh Steadmon <steadmon@google.com>]

From: Michael J Gruber <git@grubix.eu>

git-merge does not honor the pre-commit hook when doing automatic merge
commits, and for compatibility reasons this is going to stay.

Introduce a pre-merge hook which is called for an automatic merge commit
just like pre-commit is called for a non-automatic merge commit (or any
other commit).

[js: addressed review comments:
     * clarified that hook should write messages to stderr
     * only discard the index if the hook is actually present
]

Signed-off-by: Michael J Gruber <git@grubix.eu>
Signed-off-by: Josh Steadmon <steadmon@google.com>
---
 Documentation/githooks.txt        |  7 +++++++
 builtin/merge.c                   | 12 ++++++++++++
 templates/hooks--pre-merge.sample | 13 +++++++++++++
 3 files changed, 32 insertions(+)
 create mode 100755 templates/hooks--pre-merge.sample

diff --git a/Documentation/githooks.txt b/Documentation/githooks.txt
index 786e778ab8..dcc6606d44 100644
--- a/Documentation/githooks.txt
+++ b/Documentation/githooks.txt
@@ -103,6 +103,13 @@ The default 'pre-commit' hook, when enabled--and with the
 `hooks.allownonascii` config option unset or set to false--prevents
 the use of non-ASCII filenames.
 
+pre-merge
+~~~~~~~~~
+
+This hook is invoked by 'git merge' when doing an automatic merge
+commit; it is equivalent to 'pre-commit' for a non-automatic commit
+for a merge.
+
 prepare-commit-msg
 ~~~~~~~~~~~~~~~~~~
 
diff --git a/builtin/merge.c b/builtin/merge.c
index 6e99aead46..5cd7752191 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -816,6 +816,18 @@ static void write_merge_heads(struct commit_list *);
 static void prepare_to_commit(struct commit_list *remoteheads)
 {
 	struct strbuf msg = STRBUF_INIT;
+	const char *index_file = get_index_file();
+
+	if (run_commit_hook(0 < option_edit, index_file, "pre-merge", NULL))
+		abort_commit(remoteheads, NULL);
+	/*
+	 * Re-read the index as pre-merge hook could have updated it,
+	 * and write it out as a tree.  We must do this before we invoke
+	 * the editor and after we invoke run_status above.
+	 */
+	if (find_hook("pre-merge"))
+		discard_cache();
+	read_cache_from(index_file);
 	strbuf_addbuf(&msg, &merge_msg);
 	if (squash)
 		BUG("the control must not reach here under --squash");
diff --git a/templates/hooks--pre-merge.sample b/templates/hooks--pre-merge.sample
new file mode 100755
index 0000000000..f459b3db44
--- /dev/null
+++ b/templates/hooks--pre-merge.sample
@@ -0,0 +1,13 @@
+#!/bin/sh
+#
+# An example hook script to verify what is about to be committed.
+# Called by "git merge" with no arguments.  The hook should
+# exit with non-zero status after issuing an appropriate message to
+# stderr if it wants to stop the merge commit.
+#
+# To enable this hook, rename this file to "pre-merge".
+
+. git-sh-setup
+test -x "$GIT_DIR/hooks/pre-commit" &&
+        exec "$GIT_DIR/hooks/pre-commit"
+:
-- 
2.22.0.657.g960e92d24f-goog