rebase: fix GIT_REFLOG_ACTION regression

The scripted version of "rebase" honored the `GIT_REFLOG_ACTION`,
and some automation scripts expected the reflog entries to be
prefixed with "rebase -i", not "rebase", after running "rebase -i".
This regressed in the reimplementation in C.

Fix that, and add a regression test, both with `GIT_REFLOG_ACTION`
set and unset.

Note: the reflog message for "rebase finished" did *not* honor
GIT_REFLOG_ACTION, and as we are very late in the v2.20.0-rcN phase,
we leave that bug for later (as it seems that that bug has been with
us from the very beginning).

Reported by Ian Jackson.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -776,6 +776,23 @@ static void NORETURN error_on_missing_default_upstream(void)
 	exit(1);
 }
 
+static void set_reflog_action(struct rebase_options *options)
+{
+	const char *env;
+	struct strbuf buf = STRBUF_INIT;
+
+	if (!is_interactive(options))
+		return;
+
+	env = getenv(GIT_REFLOG_ACTION_ENVIRONMENT);
+	if (env && strcmp("rebase", env))
+		return; /* only override it if it is "rebase" */
+
+	strbuf_addf(&buf, "rebase -i (%s)", options->action);
+	setenv(GIT_REFLOG_ACTION_ENVIRONMENT, buf.buf, 1);
+	strbuf_release(&buf);
+}
+
 int cmd_rebase(int argc, const char **argv, const char *prefix)
 {
 	struct rebase_options options = {
@@ -978,6 +995,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 
 	if (action != NO_ACTION && !in_progress)
 		die(_("No rebase in progress?"));
+	setenv(GIT_REFLOG_ACTION_ENVIRONMENT, "rebase", 0);
 
 	if (action == ACTION_EDIT_TODO && !is_interactive(&options))
 		die(_("The --edit-todo action can only be used during "
@@ -990,6 +1008,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		int fd;
 
 		options.action = "continue";
+		set_reflog_action(&options);
 
 		/* Sanity check */
 		if (get_oid("HEAD", &head))
@@ -1018,6 +1037,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		struct string_list merge_rr = STRING_LIST_INIT_DUP;
 
 		options.action = "skip";
+		set_reflog_action(&options);
 
 		rerere_clear(&merge_rr);
 		string_list_clear(&merge_rr, 1);
@@ -1033,6 +1053,7 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	case ACTION_ABORT: {
 		struct string_list merge_rr = STRING_LIST_INIT_DUP;
 		options.action = "abort";
+		set_reflog_action(&options);
 
 		rerere_clear(&merge_rr);
 		string_list_clear(&merge_rr, 1);
@@ -1440,11 +1461,12 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 				}
 
 				strbuf_reset(&buf);
-				strbuf_addf(&buf, "rebase: checkout %s",
+				strbuf_addf(&buf, "%s: checkout %s",
+					    getenv(GIT_REFLOG_ACTION_ENVIRONMENT),
 					    options.switch_to);
 				if (reset_head(&oid, "checkout",
 					       options.head_name, 0,
-					       NULL, NULL) < 0) {
+					       NULL, buf.buf) < 0) {
 					ret = !!error(_("could not switch to "
 							"%s"),
 						      options.switch_to);
@@ -1508,7 +1530,8 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		printf(_("First, rewinding head to replay your work on top of "
 			 "it...\n"));
 
-	strbuf_addf(&msg, "rebase: checkout %s", options.onto_name);
+	strbuf_addf(&msg, "%s: checkout %s",
+		    getenv(GIT_REFLOG_ACTION_ENVIRONMENT), options.onto_name);
 	if (reset_head(&options.onto->object.oid, "checkout", NULL,
 		       RESET_HEAD_DETACH, NULL, msg.buf))
 		die(_("Could not detach HEAD"));

t/t3406-rebase-message.sh

@@ -91,4 +91,30 @@ test_expect_success 'error out early upon -C<n> or --whitespace=<bad>' '
 	test_i18ngrep "Invalid whitespace option" err
 '
 
+test_expect_success 'GIT_REFLOG_ACTION' '
+	git checkout start &&
+	test_commit reflog-onto &&
+	git checkout -b reflog-topic start &&
+	test_commit reflog-to-rebase &&
+
+	git rebase reflog-onto &&
+	git log -g --format=%gs -3 >actual &&
+	cat >expect <<-\EOF &&
+	rebase finished: returning to refs/heads/reflog-topic
+	rebase: reflog-to-rebase
+	rebase: checkout reflog-onto
+	EOF
+	test_cmp expect actual &&
+
+	git checkout -b reflog-prefix reflog-to-rebase &&
+	GIT_REFLOG_ACTION=change-the-reflog git rebase reflog-onto &&
+	git log -g --format=%gs -3 >actual &&
+	cat >expect <<-\EOF &&
+	rebase finished: returning to refs/heads/reflog-prefix
+	change-the-reflog: reflog-to-rebase
+	change-the-reflog: checkout reflog-onto
+	EOF
+	test_cmp expect actual
+'
+
 test_done