[v3,14/15] merge-tree: add a --allow-unrelated-histories flag

Commit Message

Elijah Newren Feb. 2, 2022, 7:34 a.m. UTC

From: Elijah Newren <[e-mail]>

Folks may want to merge histories that have no common ancestry; provide
a flag with the same name as used by `git merge` to allow this.

Signed-off-by: Elijah Newren <[e-mail]>
---
 Documentation/git-merge-tree.txt |  5 +++++
 builtin/merge-tree.c             |  7 ++++++-
 t/t4301-merge-tree-write-tree.sh | 24 +++++++++++++++++++++++-
 3 files changed, 34 insertions(+), 2 deletions(-)

Patch

diff --git a/Documentation/git-merge-tree.txt b/Documentation/git-merge-tree.txt
index 02f766716f9..e6a9ff2768b 100644
--- a/Documentation/git-merge-tree.txt
+++ b/Documentation/git-merge-tree.txt
@@ -55,6 +55,11 @@  OPTIONS
 	default is to include these messages if there are merge
 	conflicts, and to omit them otherwise.
 
+--allow-unrelated-histories::
+	merge-tree will by default error out if the two branches specified
+	share no common history.  This flag can be given to override that
+	check and make the merge proceed anyway.
+
 OUTPUT
 ------
 
diff --git a/builtin/merge-tree.c b/builtin/merge-tree.c
index 7e55f0fa301..58c0ddc5a32 100644
--- a/builtin/merge-tree.c
+++ b/builtin/merge-tree.c
@@ -393,6 +393,7 @@  static int trivial_merge(const char *base,
 
 struct merge_tree_options {
 	int mode;
+	int allow_unrelated_histories;
 	int show_messages;
 	int exclude_modes_oids_stages;
 };
@@ -430,7 +431,7 @@  static int real_merge(struct merge_tree_options *o,
 	 * merge_incore_recursive in merge-ort.h
 	 */
 	common = get_merge_bases(parent1, parent2);
-	if (!common)
+	if (!common && !o->allow_unrelated_histories)
 		die(_("refusing to merge unrelated histories"));
 	for (j = common; j; j = j->next)
 		commit_list_insert(j->item, &merge_bases);
@@ -496,6 +497,10 @@  int cmd_merge_tree(int argc, const char **argv, const char *prefix)
 			   &o.exclude_modes_oids_stages,
 			   N_("list conflicted files without modes/oids/stages"),
 			   PARSE_OPT_NONEG),
+		OPT_BOOL_F(0, "allow-unrelated-histories",
+			   &o.allow_unrelated_histories,
+			   N_("allow merging unrelated histories"),
+			   PARSE_OPT_NONEG),
 		OPT_END()
 	};
 
diff --git a/t/t4301-merge-tree-write-tree.sh b/t/t4301-merge-tree-write-tree.sh
index f89d87c26b7..4de089d976d 100755
--- a/t/t4301-merge-tree-write-tree.sh
+++ b/t/t4301-merge-tree-write-tree.sh
@@ -38,7 +38,13 @@  test_expect_success setup '
 	>whatever/empty &&
 	git add numbers greeting whatever/empty &&
 	test_tick &&
-	git commit -m other-modifications
+	git commit -m other-modifications &&
+
+	git switch --orphan unrelated &&
+	>something-else &&
+	git add something-else &&
+	test_tick &&
+	git commit -m first-commit
 '
 
 test_expect_success 'Content merge and a few conflicts' '
@@ -179,4 +185,20 @@  test_expect_success 'NUL terminated conflicted file "lines"' '
 	test_cmp expect actual
 '
 
+test_expect_success 'error out by default for unrelated histories' '
+	test_expect_code 128 git merge-tree --write-tree side1 unrelated 2>error &&
+
+	grep "refusing to merge unrelated histories" error
+'
+
+test_expect_success 'can override merge of unrelated histories' '
+	git merge-tree --write-tree --allow-unrelated-histories side1 unrelated >tree &&
+	TREE=$(cat tree) &&
+
+	git rev-parse side1:numbers side1:greeting side1:whatever unrelated:something-else >expect &&
+	git rev-parse $TREE:numbers $TREE:greeting $TREE:whatever $TREE:something-else >actual &&
+
+	test_cmp expect actual
+'
+
 test_done