merge: use editor by default in interactive sessionsjc/pull-signed-tag

Traditionally, a cleanly resolved merge was committed by "git merge" using
the auto-generated merge commit log message with invoking the editor.

After 5 years of use in the field, it turns out that many people perform
too many unjustified backmerges of the upstream history into their topic
branches. These merges are not just useless, but they are more often than
not explained and making the end result unreadable when it gets time for
merging their history back to their upstream.

Earlier we added the "--edit" option to the command, so that people can
edit the log message to explain and justify their merge commits. Let's
take it one step further and spawn the editor by default when we are in an
interactive session (i.e. the standard input and the standard output are
pointing at the same tty device).

There may be existing scripts that leave the standard input and the
standard output of the "git merge" connected to whatever environment the
scripts were started, and such invocation might trigger the above
"interactive session" heuristics. Such scripts can export GIT_MERGE_LEGACY
environment variable set to "yes" to force the traditional behaviour.

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

2 files changed, 32 insertions, 5 deletions

diff --git a/builtin/merge.c b/builtin/merge.c
index 99f1429b35..6a80e1eb25 100644
--- a/builtin/merge.c
+++ b/builtin/merge.c
@@ -46,7 +46,7 @@ static const char * const builtin_merge_usage[] = {
 
 static int show_diffstat = 1, shortlog_len, squash;
 static int option_commit = 1, allow_fast_forward = 1;
-static int fast_forward_only, option_edit;
+static int fast_forward_only, option_edit = -1;
 static int allow_trivial = 1, have_message;
 static struct strbuf merge_msg;
 static struct commit_list *remoteheads;
@@ -189,7 +189,7 @@ static struct option builtin_merge_options[] = {
 		"create a single commit instead of doing a merge"),
 	OPT_BOOLEAN(0, "commit", &option_commit,
 		"perform a commit if the merge succeeds (default)"),
-	OPT_BOOLEAN('e', "edit", &option_edit,
+	OPT_BOOL('e', "edit", &option_edit,
 		"edit message before committing"),
 	OPT_BOOLEAN(0, "ff", &allow_fast_forward,
 		"allow fast-forward (default)"),
@@ -877,12 +877,12 @@ static void prepare_to_commit(void)
 	write_merge_msg(&msg);
 	run_hook(get_index_file(), "prepare-commit-msg",
 		 git_path("MERGE_MSG"), "merge", NULL, NULL);
-	if (option_edit) {
+	if (0 < option_edit) {
 		if (launch_editor(git_path("MERGE_MSG"), NULL, NULL))
 			abort_commit(NULL);
 	}
 	read_merge_msg(&msg);
-	stripspace(&msg, option_edit);
+	stripspace(&msg, 0 < option_edit);
 	if (!msg.len)
 		abort_commit(_("Empty commit message."));
 	strbuf_release(&merge_msg);
@@ -1076,6 +1076,29 @@ static void write_merge_state(void)
 	close(fd);
 }
 
+static int default_edit_option(void)
+{
+	static const char name[] = "GIT_MERGE_LEGACY";
+	const char *e = getenv(name);
+	struct stat st_stdin, st_stdout;
+
+	if (e) {
+		int v = git_config_maybe_bool(name, e);
+		if (v < 0)
+			die("Bad value '%s' in environment '%s'", e, name);
+		return !v;
+	}
+
+	/* Use editor if stdin and stdout are the same and is a tty */
+	return (!fstat(0, &st_stdin) &&
+		!fstat(1, &st_stdout) &&
+		isatty(0) &&
+		st_stdin.st_dev == st_stdout.st_dev &&
+		st_stdin.st_ino == st_stdout.st_ino &&
+		st_stdin.st_rdev == st_stdout.st_rdev);
+}
+
+
 int cmd_merge(int argc, const char **argv, const char *prefix)
 {
 	unsigned char result_tree[20];
@@ -1261,6 +1284,9 @@ int cmd_merge(int argc, const char **argv, const char *prefix)
 		}
 	}
 
+	if (option_edit < 0)
+		option_edit = default_edit_option();
+
 	if (!use_strategies) {
 		if (!remoteheads->next)
 			add_strategies(pull_twohead, DEFAULT_TWOHEAD);
diff --git a/t/test-lib.sh b/t/test-lib.sh
index bdd9513b84..439f19209f 100644
--- a/t/test-lib.sh
+++ b/t/test-lib.sh
@@ -63,7 +63,8 @@ GIT_AUTHOR_NAME='A U Thor'
 GIT_COMMITTER_EMAIL=committer@example.com
 GIT_COMMITTER_NAME='C O Mitter'
 GIT_MERGE_VERBOSITY=5
-export GIT_MERGE_VERBOSITY
+GIT_MERGE_LEGACY=yes
+export GIT_MERGE_VERBOSITY GIT_MERGE_LEGACY
 export GIT_AUTHOR_EMAIL GIT_AUTHOR_NAME
 export GIT_COMMITTER_EMAIL GIT_COMMITTER_NAME
 export EDITOR