[PATCH] rebase: use correct base for --keep-base when a branch is given

[Alex Henrie <alexhenrie24@gmail.com>]

--keep-base rebases onto the merge base of the given upstream and the
current HEAD regardless of whether a branch is given. This is contrary
to the documentation and to the option's intended purpose. Instead,
rebase onto the merge base of the given upstream and the given branch.

Signed-off-by: Alex Henrie <alexhenrie24@gmail.com>
---
 Documentation/git-rebase.txt     |  5 +--
 builtin/rebase.c                 | 55 ++++++++++++++++----------------
 t/t3416-rebase-onto-threedots.sh | 33 +++++++++++++++++++
 3 files changed, 64 insertions(+), 29 deletions(-)

diff --git a/Documentation/git-rebase.txt b/Documentation/git-rebase.txt
index 9da4647061..262fb01aec 100644
--- a/Documentation/git-rebase.txt
+++ b/Documentation/git-rebase.txt
@@ -215,9 +215,10 @@ leave out at most one of A and B, in which case it defaults to HEAD.
 
 --keep-base::
 	Set the starting point at which to create the new commits to the
-	merge base of <upstream> <branch>. Running
+	merge base of <upstream> and <branch>. Running
 	'git rebase --keep-base <upstream> <branch>' is equivalent to
-	running 'git rebase --onto <upstream>... <upstream>'.
+	running
+	'git rebase --onto <upstream>...<branch> <upstream> <branch>'.
 +
 This option is useful in the case where one is developing a feature on
 top of an upstream branch. While the feature is being worked on, the
diff --git a/builtin/rebase.c b/builtin/rebase.c
index 27fde7bf28..7f3bffc0a2 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -1583,33 +1583,6 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		options.upstream_arg = "--root";
 	}
 
-	/* Make sure the branch to rebase onto is valid. */
-	if (keep_base) {
-		strbuf_reset(&buf);
-		strbuf_addstr(&buf, options.upstream_name);
-		strbuf_addstr(&buf, "...");
-		options.onto_name = xstrdup(buf.buf);
-	} else if (!options.onto_name)
-		options.onto_name = options.upstream_name;
-	if (strstr(options.onto_name, "...")) {
-		if (get_oid_mb(options.onto_name, &merge_base) < 0) {
-			if (keep_base)
-				die(_("'%s': need exactly one merge base with branch"),
-				    options.upstream_name);
-			else
-				die(_("'%s': need exactly one merge base"),
-				    options.onto_name);
-		}
-		options.onto = lookup_commit_or_die(&merge_base,
-						    options.onto_name);
-	} else {
-		options.onto =
-			lookup_commit_reference_by_name(options.onto_name);
-		if (!options.onto)
-			die(_("Does not point to a valid commit '%s'"),
-				options.onto_name);
-	}
-
 	/*
 	 * If the branch to rebase is given, that is the branch we will rebase
 	 * branch_name -- branch/commit being rebased, or
@@ -1659,6 +1632,34 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 	} else
 		BUG("unexpected number of arguments left to parse");
 
+	/* Make sure the branch to rebase onto is valid. */
+	if (keep_base) {
+		strbuf_reset(&buf);
+		strbuf_addstr(&buf, options.upstream_name);
+		strbuf_addstr(&buf, "...");
+		strbuf_addstr(&buf, branch_name);
+		options.onto_name = xstrdup(buf.buf);
+	} else if (!options.onto_name)
+		options.onto_name = options.upstream_name;
+	if (strstr(options.onto_name, "...")) {
+		if (get_oid_mb(options.onto_name, &merge_base) < 0) {
+			if (keep_base)
+				die(_("'%s': need exactly one merge base with branch"),
+				    options.upstream_name);
+			else
+				die(_("'%s': need exactly one merge base"),
+				    options.onto_name);
+		}
+		options.onto = lookup_commit_or_die(&merge_base,
+						    options.onto_name);
+	} else {
+		options.onto =
+			lookup_commit_reference_by_name(options.onto_name);
+		if (!options.onto)
+			die(_("Does not point to a valid commit '%s'"),
+				options.onto_name);
+	}
+
 	if (options.fork_point > 0) {
 		struct commit *head =
 			lookup_commit_reference(the_repository,
diff --git a/t/t3416-rebase-onto-threedots.sh b/t/t3416-rebase-onto-threedots.sh
index 3716a42e81..d1db528e25 100755
--- a/t/t3416-rebase-onto-threedots.sh
+++ b/t/t3416-rebase-onto-threedots.sh
@@ -129,6 +129,22 @@ test_expect_success 'rebase --keep-base main from topic' '
 	test_cmp expect actual
 '
 
+test_expect_success 'rebase --keep-base main topic from main' '
+	git reset --hard &&
+	git checkout topic &&
+	git reset --hard G &&
+	git checkout main &&
+
+	git rebase --keep-base main topic &&
+	git rev-parse C >base.expect &&
+	git merge-base main HEAD >base.actual &&
+	test_cmp base.expect base.actual &&
+
+	git rev-parse HEAD~2 >actual &&
+	git rev-parse C^0 >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'rebase --keep-base main from side' '
 	git reset --hard &&
 	git checkout side &&
@@ -153,6 +169,23 @@ test_expect_success 'rebase -i --keep-base main from topic' '
 	test_cmp expect actual
 '
 
+test_expect_success 'rebase -i --keep-base main topic from main' '
+	git reset --hard &&
+	git checkout topic &&
+	git reset --hard G &&
+	git checkout main &&
+
+	set_fake_editor &&
+	EXPECT_COUNT=2 git rebase -i --keep-base main topic &&
+	git rev-parse C >base.expect &&
+	git merge-base main HEAD >base.actual &&
+	test_cmp base.expect base.actual &&
+
+	git rev-parse HEAD~2 >actual &&
+	git rev-parse C^0 >expect &&
+	test_cmp expect actual
+'
+
 test_expect_success 'rebase -i --keep-base main from side' '
 	git reset --hard &&
 	git checkout side &&
-- 
2.35.3