3 files changed, 131 insertions, 12 deletions
diff --git a/Documentation/git-merge-tree.txt b/Documentation/git-merge-tree.txt
index 04bcc416e6..88ee942101 100644
--- a/Documentation/git-merge-tree.txt
+++ b/Documentation/git-merge-tree.txt
@@ -64,6 +64,11 @@ OPTIONS
 	share no common history.  This flag can be given to override that
 	check and make the merge proceed anyway.
 
+--merge-base=<commit>::
+	Instead of finding the merge-bases for <branch1> and <branch2>,
+	specify a merge-base for the merge, and specifying multiple bases is
+	currently not supported. This option is incompatible with `--stdin`.
+
 [[OUTPUT]]
 OUTPUT
 ------
@@ -216,6 +221,17 @@ with linkgit:git-merge[1]:
   * any messages that would have been printed to stdout (the
     <<IM,Informational messages>>)
 
+INPUT FORMAT
+------------
+'git merge-tree --stdin' input format is fully text based. Each line
+has this format:
+
+	[<base-commit> -- ]<branch1> <branch2>
+
+If one line is separated by `--`, the string before the separator is
+used for specifying a merge-base for the merge and the string after
+the separator describes the branches to be merged.
+
 MISTAKES TO AVOID
 -----------------
 
diff --git a/builtin/merge-tree.c b/builtin/merge-tree.c
index e3767087bb..c8265e0aec 100644
--- a/builtin/merge-tree.c
+++ b/builtin/merge-tree.c
@@ -3,6 +3,7 @@
 #include "tree-walk.h"
 #include "xdiff-interface.h"
 #include "help.h"
+#include "commit.h"
 #include "commit-reach.h"
 #include "merge-ort.h"
 #include "object-store.h"
@@ -406,6 +407,7 @@ struct merge_tree_options {
 };
 
 static int real_merge(struct merge_tree_options *o,
+		      const char *merge_base,
 		      const char *branch1, const char *branch2,
 		      const char *prefix)
 {
@@ -432,16 +434,31 @@ static int real_merge(struct merge_tree_options *o,
 	opt.branch1 = branch1;
 	opt.branch2 = branch2;
 
-	/*
-	 * Get the merge bases, in reverse order; see comment above
-	 * merge_incore_recursive in merge-ort.h
-	 */
-	merge_bases = get_merge_bases(parent1, parent2);
-	if (!merge_bases && !o->allow_unrelated_histories)
-		die(_("refusing to merge unrelated histories"));
-	merge_bases = reverse_commit_list(merge_bases);
+	if (merge_base) {
+		struct commit *base_commit;
+		struct tree *base_tree, *parent1_tree, *parent2_tree;
+
+		base_commit = lookup_commit_reference_by_name(merge_base);
+		if (!base_commit)
+			die(_("could not lookup commit %s"), merge_base);
+
+		opt.ancestor = merge_base;
+		base_tree = get_commit_tree(base_commit);
+		parent1_tree = get_commit_tree(parent1);
+		parent2_tree = get_commit_tree(parent2);
+		merge_incore_nonrecursive(&opt, base_tree, parent1_tree, parent2_tree, &result);
+	} else {
+		/*
+		 * Get the merge bases, in reverse order; see comment above
+		 * merge_incore_recursive in merge-ort.h
+		 */
+		merge_bases = get_merge_bases(parent1, parent2);
+		if (!merge_bases && !o->allow_unrelated_histories)
+			die(_("refusing to merge unrelated histories"));
+		merge_bases = reverse_commit_list(merge_bases);
+		merge_incore_recursive(&opt, merge_bases, parent1, parent2, &result);
+	}
 
-	merge_incore_recursive(&opt, merge_bases, parent1, parent2, &result);
 	if (result.clean < 0)
 		die(_("failure to merge"));
 
@@ -487,6 +504,7 @@ int cmd_merge_tree(int argc, const char **argv, const char *prefix)
 	struct merge_tree_options o = { .show_messages = -1 };
 	int expected_remaining_argc;
 	int original_argc;
+	const char *merge_base = NULL;
 
 	const char * const merge_tree_usage[] = {
 		N_("git merge-tree [--write-tree] [<options>] <branch1> <branch2>"),
@@ -515,6 +533,10 @@ int cmd_merge_tree(int argc, const char **argv, const char *prefix)
 			   &o.use_stdin,
 			   N_("perform multiple merges, one per line of input"),
 			   PARSE_OPT_NONEG),
+		OPT_STRING(0, "merge-base",
+			   &merge_base,
+			   N_("commit"),
+			   N_("specify a merge-base for the merge")),
 		OPT_END()
 	};
 
@@ -529,16 +551,35 @@ int cmd_merge_tree(int argc, const char **argv, const char *prefix)
 
 		if (o.mode == MODE_TRIVIAL)
 			die(_("--trivial-merge is incompatible with all other options"));
+		if (merge_base)
+			die(_("--merge-base is incompatible with --stdin"));
 		line_termination = '\0';
 		while (strbuf_getline_lf(&buf, stdin) != EOF) {
 			struct strbuf **split;
 			int result;
+			const char *input_merge_base = NULL;
 
 			split = strbuf_split(&buf, ' ');
-			if (!split[0] || !split[1] || split[2])
+			if (!split[0] || !split[1])
 				die(_("malformed input line: '%s'."), buf.buf);
 			strbuf_rtrim(split[0]);
-			result = real_merge(&o, split[0]->buf, split[1]->buf, prefix);
+			strbuf_rtrim(split[1]);
+
+			/* parse the merge-base */
+			if (!strcmp(split[1]->buf, "--")) {
+				input_merge_base = split[0]->buf;
+			}
+
+			if (input_merge_base && split[2] && split[3] && !split[4]) {
+				strbuf_rtrim(split[2]);
+				strbuf_rtrim(split[3]);
+				result = real_merge(&o, input_merge_base, split[2]->buf, split[3]->buf, prefix);
+			} else if (!input_merge_base && !split[2]) {
+				result = real_merge(&o, NULL, split[0]->buf, split[1]->buf, prefix);
+			} else {
+				die(_("malformed input line: '%s'."), buf.buf);
+			}
+
 			if (result < 0)
 				die(_("merging cannot continue; got unclean result of %d"), result);
 			strbuf_list_free(split);
@@ -581,7 +622,7 @@ int cmd_merge_tree(int argc, const char **argv, const char *prefix)
 
 	/* Do the relevant type of merge */
 	if (o.mode == MODE_REAL)
-		return real_merge(&o, argv[0], argv[1], prefix);
+		return real_merge(&o, merge_base, argv[0], argv[1], prefix);
 	else
 		return trivial_merge(argv[0], argv[1], argv[2]);
 }
diff --git a/t/t4301-merge-tree-write-tree.sh b/t/t4301-merge-tree-write-tree.sh
index cac85591b5..a8983a0edc 100755
--- a/t/t4301-merge-tree-write-tree.sh
+++ b/t/t4301-merge-tree-write-tree.sh
@@ -860,4 +860,66 @@ test_expect_success '--stdin with both a successful and a conflicted merge' '
 	test_cmp expect actual
 '
 
+
+test_expect_success '--merge-base is incompatible with --stdin' '
+	test_must_fail git merge-tree --merge-base=side1 --stdin 2>expect &&
+
+	grep "^fatal: --merge-base is incompatible with --stdin" expect
+'
+
+# specify merge-base as parent of branch2
+# git merge-tree --write-tree --merge-base=c2 c1 c3
+#   Commit c1: add file1
+#   Commit c2: add file2 after c1
+#   Commit c3: add file3 after c2
+#   Expected: add file3, and file2 does NOT appear
+
+test_expect_success 'specify merge-base as parent of branch2' '
+	# Setup
+	test_when_finished "rm -rf base-b2-p" &&
+	git init base-b2-p &&
+	test_commit -C base-b2-p c1 file1 &&
+	test_commit -C base-b2-p c2 file2 &&
+	test_commit -C base-b2-p c3 file3 &&
+
+	# Testing
+	TREE_OID=$(git -C base-b2-p merge-tree --write-tree --merge-base=c2 c1 c3) &&
+
+	q_to_tab <<-EOF >expect &&
+	100644 blob $(git -C base-b2-p rev-parse c1:file1)Qfile1
+	100644 blob $(git -C base-b2-p rev-parse c3:file3)Qfile3
+	EOF
+
+	git -C base-b2-p ls-tree $TREE_OID >actual &&
+	test_cmp expect actual
+'
+
+# Since the earlier tests have verified that individual merge-tree calls
+# are doing the right thing, this test case is only used to verify that
+# we can also trigger merges via --stdin, and that when we do we get
+# the same answer as running a bunch of separate merges.
+
+test_expect_success 'check the input format when --stdin is passed' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	test_commit -C repo c1 &&
+	test_commit -C repo c2 &&
+	test_commit -C repo c3 &&
+	printf "c1 c3\nc2 -- c1 c3\nc2 c3" | git -C repo merge-tree --stdin >actual &&
+
+	printf "1\0" >expect &&
+	git -C repo merge-tree --write-tree -z c1 c3 >>expect &&
+	printf "\0" >>expect &&
+
+	printf "1\0" >>expect &&
+	git -C repo merge-tree --write-tree -z --merge-base=c2 c1 c3 >>expect &&
+	printf "\0" >>expect &&
+
+	printf "1\0" >>expect &&
+	git -C repo merge-tree --write-tree -z c2 c3 >>expect &&
+	printf "\0" >>expect &&
+
+	test_cmp expect actual
+'
+
 test_done