git-commit: add a prepare-commit-msg hook

The prepare-commit-msg hook is run whenever a "fresh" commit message
is prepared, just before it is shown in the editor (if it is).
Its purpose is to modify the commit message in-place.

It takes one to three parameters.  The first is the name of the file that
the commit log message.  The second is the source of the commit message,
and can be: "message" (if a -m or -F option was given); "template" (if a
-t option was given or the configuration option commit.template is set);
"merge" (if the commit is a merge or a .git/MERGE_MSG file exists);
"squash" (if a .git/SQUASH_MSG file exists); or "commit", followed by
a commit SHA1 as the third parameter (if a -c, -C or --amend option
was given).

If its exit status is non-zero, git-commit will abort.  The hook is
not suppressed by the --no-verify option, so it should not be used
as a replacement for the pre-commit hook.

The sample prepare-commit-msg comments out the `Conflicts:` part of
a merge's commit message; other examples are commented out, including
adding a Signed-off-by line at the bottom of the commit messsage,
that the user can then edit or discard altogether.

Signed-off-by: Paolo Bonzini <bonzini@gnu.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

6 files changed, 242 insertions, 2 deletions

diff --git a/Documentation/git-commit.txt b/Documentation/git-commit.txt
index c3725b2ed9..b4ae61ff46 100644
--- a/Documentation/git-commit.txt
+++ b/Documentation/git-commit.txt
@@ -280,8 +280,8 @@ order).
 
 HOOKS
 -----
-This command can run `commit-msg`, `pre-commit`, and
-`post-commit` hooks.  See link:hooks.html[hooks] for more
+This command can run `commit-msg`, `prepare-commit-msg`, `pre-commit`,
+and `post-commit` hooks.  See link:hooks.html[hooks] for more
 information.
 
 
diff --git a/Documentation/hooks.txt b/Documentation/hooks.txt
index e8d80cfcdb..76b8d77460 100644
--- a/Documentation/hooks.txt
+++ b/Documentation/hooks.txt
@@ -65,6 +65,31 @@ All the `git-commit` hooks are invoked with the environment
 variable `GIT_EDITOR=:` if the command will not bring up an editor
 to modify the commit message.
 
+prepare-commit-msg
+------------------
+
+This hook is invoked by `git-commit` right after preparing the
+default log message, and before the editor is started.
+
+It takes one to three parameters.  The first is the name of the file
+that the commit log message.  The second is the source of the commit
+message, and can be: `message` (if a `\-m` or `\-F` option was
+given); `template` (if a `\-t` option was given or the
+configuration option `commit.template` is set); `merge` (if the
+commit is a merge or a `.git/MERGE_MSG` file exists); `squash`
+(if a `.git/SQUASH_MSG` file exists); or `commit`, followed by
+a commit SHA1 (if a `\-c`, `\-C` or `\--amend` option was given).
+
+If the exit status is non-zero, `git-commit` will abort.
+
+The purpose of the hook is to edit the message file in place, and
+it is not suppressed by the `\--no-verify` option.  A non-zero exit
+means a failure of the hook and aborts the commit.  It should not
+be used as replacement for pre-commit hook.
+
+The sample `prepare-commit-msg` hook that comes with git comments
+out the `Conflicts:` part of a merge's commit message.
+
 commit-msg
 ----------
 
diff --git a/builtin-commit.c b/builtin-commit.c
index e8cb320590..03caa30f0e 100644
--- a/builtin-commit.c
+++ b/builtin-commit.c
@@ -394,6 +394,8 @@ static int prepare_to_commit(const char *index_file, const char *prefix)
 	struct strbuf sb;
 	char *buffer;
 	FILE *fp;
+	const char *hook_arg1 = NULL;
+	const char *hook_arg2 = NULL;
 
 	if (!no_verify && run_hook(index_file, "pre-commit", NULL))
 		return 0;
@@ -401,32 +403,47 @@ static int prepare_to_commit(const char *index_file, const char *prefix)
 	strbuf_init(&sb, 0);
 	if (message.len) {
 		strbuf_addbuf(&sb, &message);
+		hook_arg1 = "message";
 	} else if (logfile && !strcmp(logfile, "-")) {
 		if (isatty(0))
 			fprintf(stderr, "(reading log message from standard input)\n");
 		if (strbuf_read(&sb, 0, 0) < 0)
 			die("could not read log from standard input");
+		hook_arg1 = "message";
 	} else if (logfile) {
 		if (strbuf_read_file(&sb, logfile, 0) < 0)
 			die("could not read log file '%s': %s",
 			    logfile, strerror(errno));
+		hook_arg1 = "message";
 	} else if (use_message) {
 		buffer = strstr(use_message_buffer, "\n\n");
 		if (!buffer || buffer[2] == '\0')
 			die("commit has empty message");
 		strbuf_add(&sb, buffer + 2, strlen(buffer + 2));
+		hook_arg1 = "commit";
+		hook_arg2 = use_message;
 	} else if (!stat(git_path("MERGE_MSG"), &statbuf)) {
 		if (strbuf_read_file(&sb, git_path("MERGE_MSG"), 0) < 0)
 			die("could not read MERGE_MSG: %s", strerror(errno));
+		hook_arg1 = "merge";
 	} else if (!stat(git_path("SQUASH_MSG"), &statbuf)) {
 		if (strbuf_read_file(&sb, git_path("SQUASH_MSG"), 0) < 0)
 			die("could not read SQUASH_MSG: %s", strerror(errno));
+		hook_arg1 = "squash";
 	} else if (template_file && !stat(template_file, &statbuf)) {
 		if (strbuf_read_file(&sb, template_file, 0) < 0)
 			die("could not read %s: %s",
 			    template_file, strerror(errno));
+		hook_arg1 = "template";
 	}
 
+	/*
+	 * This final case does not modify the template message,
+	 * it just sets the argument to the prepare-commit-msg hook.
+	 */
+	else if (in_merge)
+		hook_arg1 = "merge";
+
 	fp = fopen(git_path(commit_editmsg), "w");
 	if (fp == NULL)
 		die("could not open %s", git_path(commit_editmsg));
@@ -534,6 +551,10 @@ static int prepare_to_commit(const char *index_file, const char *prefix)
 		return 0;
 	}
 
+	if (run_hook(index_file, "prepare-commit-msg",
+		     git_path(commit_editmsg), hook_arg1, hook_arg2, NULL))
+		return 0;
+
 	if (use_editor) {
 		char index[PATH_MAX];
 		const char *env[2] = { index, NULL };
diff --git a/t/t7505-prepare-commit-msg-hook.sh b/t/t7505-prepare-commit-msg-hook.sh
new file mode 100755
index 0000000000..7ddec99a64
--- /dev/null
+++ b/t/t7505-prepare-commit-msg-hook.sh
@@ -0,0 +1,155 @@
+#!/bin/sh
+
+test_description='prepare-commit-msg hook'
+
+. ./test-lib.sh
+
+test_expect_success 'with no hook' '
+
+	echo "foo" > file &&
+	git add file &&
+	git commit -m "first"
+
+'
+
+# set up fake editor for interactive editing
+cat > fake-editor <<'EOF'
+#!/bin/sh
+exit 0
+EOF
+chmod +x fake-editor
+FAKE_EDITOR="$(pwd)/fake-editor"
+export FAKE_EDITOR
+
+# now install hook that always succeeds and adds a message
+HOOKDIR="$(git rev-parse --git-dir)/hooks"
+HOOK="$HOOKDIR/prepare-commit-msg"
+mkdir -p "$HOOKDIR"
+cat > "$HOOK" <<'EOF'
+#!/bin/sh
+if test "$2" = commit; then
+  source=$(git-rev-parse "$3")
+else
+  source=${2-default}
+fi
+if test "$GIT_EDITOR" = :; then
+  sed -e "1s/.*/$source (no editor)/" "$1" > msg.tmp
+else
+  sed -e "1s/.*/$source/" "$1" > msg.tmp
+fi
+mv msg.tmp "$1"
+exit 0
+EOF
+chmod +x "$HOOK"
+
+echo dummy template > "$(git rev-parse --git-dir)/template"
+
+test_expect_success 'with hook (-m)' '
+
+	echo "more" >> file &&
+	git add file &&
+	git commit -m "more" &&
+	test "`git log -1 --pretty=format:%s`" = "message (no editor)"
+
+'
+
+test_expect_success 'with hook (-m editor)' '
+
+	echo "more" >> file &&
+	git add file &&
+	GIT_EDITOR="$FAKE_EDITOR" git commit -e -m "more more" &&
+	test "`git log -1 --pretty=format:%s`" = message
+
+'
+
+test_expect_success 'with hook (-t)' '
+
+	echo "more" >> file &&
+	git add file &&
+	git commit -t "$(git rev-parse --git-dir)/template" &&
+	test "`git log -1 --pretty=format:%s`" = template
+
+'
+
+test_expect_success 'with hook (-F)' '
+
+	echo "more" >> file &&
+	git add file &&
+	(echo more | git commit -F -) &&
+	test "`git log -1 --pretty=format:%s`" = "message (no editor)"
+
+'
+
+test_expect_success 'with hook (-F editor)' '
+
+	echo "more" >> file &&
+	git add file &&
+	(echo more more | GIT_EDITOR="$FAKE_EDITOR" git commit -e -F -) &&
+	test "`git log -1 --pretty=format:%s`" = message
+
+'
+
+test_expect_success 'with hook (-C)' '
+
+	head=`git rev-parse HEAD` &&
+	echo "more" >> file &&
+	git add file &&
+	git commit -C $head &&
+	test "`git log -1 --pretty=format:%s`" = "$head (no editor)"
+
+'
+
+test_expect_success 'with hook (editor)' '
+
+	echo "more more" >> file &&
+	git add file &&
+	GIT_EDITOR="$FAKE_EDITOR" git commit &&
+	test "`git log -1 --pretty=format:%s`" = default
+
+'
+
+test_expect_success 'with hook (--amend)' '
+
+	head=`git rev-parse HEAD` &&
+	echo "more" >> file &&
+	git add file &&
+	GIT_EDITOR="$FAKE_EDITOR" git commit --amend &&
+	test "`git log -1 --pretty=format:%s`" = "$head"
+
+'
+
+test_expect_success 'with hook (-c)' '
+
+	head=`git rev-parse HEAD` &&
+	echo "more" >> file &&
+	git add file &&
+	GIT_EDITOR="$FAKE_EDITOR" git commit -c $head &&
+	test "`git log -1 --pretty=format:%s`" = "$head"
+
+'
+
+cat > "$HOOK" <<'EOF'
+#!/bin/sh
+exit 1
+EOF
+
+test_expect_success 'with failing hook' '
+
+	head=`git rev-parse HEAD` &&
+	echo "more" >> file &&
+	git add file &&
+	! GIT_EDITOR="$FAKE_EDITOR" git commit -c $head
+
+'
+
+test_expect_success 'with failing hook (--no-verify)' '
+
+	head=`git rev-parse HEAD` &&
+	echo "more" >> file &&
+	git add file &&
+	! GIT_EDITOR="$FAKE_EDITOR" git commit --no-verify -c $head
+
+'
+
+
+test_done
diff --git a/templates/hooks--commit-msg b/templates/hooks--commit-msg
index c5cdb9d7ee..4ef86eb244 100644
--- a/templates/hooks--commit-msg
+++ b/templates/hooks--commit-msg
@@ -9,6 +9,9 @@
 # To enable this hook, make this file executable.
 
 # Uncomment the below to add a Signed-off-by line to the message.
+# Doing this in a hook is a bad idea in general, but the prepare-commit-msg
+# hook is more suited to it.
+#
 # SOB=$(git var GIT_AUTHOR_IDENT | sed -n 's/^\(.*>\).*$/Signed-off-by: \1/p')
 # grep -qs "^$SOB" "$1" || echo "$SOB" >> "$1"
 
diff --git a/templates/hooks--prepare-commit-msg b/templates/hooks--prepare-commit-msg
new file mode 100644
index 0000000000..ff0f42a1d9
--- /dev/null
+++ b/templates/hooks--prepare-commit-msg
@@ -0,0 +1,36 @@
+#!/bin/sh
+#
+# An example hook script to prepare the commit log message.
+# Called by git-commit with the name of the file that has the
+# commit message, followed by the description of the commit
+# message's source.  The hook's purpose is to edit the commit
+# message file.  If the hook fails with a non-zero status,
+# the commit is aborted.
+#
+# To enable this hook, make this file executable.
+
+# This hook includes three examples.  The first comments out the
+# "Conflicts:" part of a merge commit.
+#
+# The second includes the output of "git diff --name-status -r"
+# into the message, just before the "git status" output.  It is
+# commented because it doesn't cope with --amend or with squashed
+# commits.
+#
+# The third example adds a Signed-off-by line to the message, that can
+# still be edited.  This is rarely a good idea.
+
+case "$2 $3" in
+  merge)
+    sed -i '/^Conflicts:/,/#/!b;s/^/# &/;s/^# #/#/' "$1" ;;
+
+# ""|template)
+#   perl -i -pe '
+#      print "\n" . `git diff --cached --name-status -r`
+#	 if /^#/ && $first++ == 0' "$1" ;;
+
+  *) ;;
+esac
+
+# SOB=$(git var GIT_AUTHOR_IDENT | sed -n 's/^\(.*>\).*$/Signed-off-by: \1/p')
+# grep -qs "^$SOB" "$1" || echo "$SOB" >> "$1"