merge-tree.c: allow specifying the merge-base when --stdin is passed

The previous commit added a `--merge-base` option in order to allow
using a specified merge-base for the merge.  Extend the input accepted
by `--stdin` to also allow a specified merge-base with each merge
requested.  For example:

    printf "<b3> -- <b1> <b2>" | git merge-tree --stdin

does a merge of b1 and b2, and uses b3 as the merge-base.

Signed-off-by: Kyle Zhao <kylezhao@tencent.com>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

Documentation/git-merge-tree.txt

@@ -66,7 +66,8 @@ OPTIONS
 
 --merge-base=<commit>::
 	Instead of finding the merge-bases for <branch1> and <branch2>,
-	specify a merge-base for the merge.
+	specify a merge-base for the merge. This option is incompatible
+	with `--stdin`.
 
 [[OUTPUT]]
 OUTPUT
@@ -220,6 +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
 -----------------
 

builtin/merge-tree.c

@@ -557,12 +557,29 @@ int cmd_merge_tree(int argc, const char **argv, const char *prefix)
 		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, merge_base, 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);

t/t4301-merge-tree-write-tree.sh

@@ -860,6 +860,13 @@ 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
@@ -887,4 +894,32 @@ test_expect_success 'specify merge-base as parent of branch2' '
 	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