From 7fa3338870d66dd3946c5c3a0bd09dadb798893d Mon Sep 17 00:00:00 2001
From: Elijah Newren <newren@gmail.com>
Date: Sat, 18 Jun 2022 00:20:52 +0000
Subject: merge-tree: provide a list of which files have conflicts

Callers of `git merge-tree --write-tree` will often want to know which
files had conflicts.  While they could potentially attempt to parse the
CONFLICT notices printed, those messages are not meant to be machine
readable.  Provide a simpler mechanism of just printing the files (in
the same format as `git ls-files` with quoting, but restricted to
unmerged files) in the output before the free-form messages.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

diff --git a/Documentation/git-merge-tree.txt b/Documentation/git-merge-tree.txt
index 25b462b..68a51c8 100644
--- a/Documentation/git-merge-tree.txt
+++ b/Documentation/git-merge-tree.txt
@@ -58,6 +58,7 @@ line:
 Whereas for a conflicted merge, the output is by default of the form:
 
 	<OID of toplevel tree>
+	<Conflicted file list>
 	<Informational messages>
 
 These are discussed individually below.
@@ -70,6 +71,14 @@ This is a tree object that represents what would be checked out in the
 working tree at the end of `git merge`.  If there were conflicts, then
 files within this tree may have embedded conflict markers.
 
+[[CFI]]
+Conflicted file list
+~~~~~~~~~~~~~~~~~~~~
+
+This is a sequence of lines containing a filename on each line, quoted
+as explained for the configuration variable `core.quotePath` (see
+linkgit:git-config[1]).
+
 [[IM]]
 Informational messages
 ~~~~~~~~~~~~~~~~~~~~~~
diff --git a/builtin/merge-tree.c b/builtin/merge-tree.c
index 831d9c7..13a9536 100644
--- a/builtin/merge-tree.c
+++ b/builtin/merge-tree.c
@@ -11,6 +11,9 @@
 #include "blob.h"
 #include "exec-cmd.h"
 #include "merge-blobs.h"
+#include "quote.h"
+
+static int line_termination = '\n';
 
 struct merge_list {
 	struct merge_list *next;
@@ -400,7 +403,8 @@ struct merge_tree_options {
 };
 
 static int real_merge(struct merge_tree_options *o,
-		      const char *branch1, const char *branch2)
+		      const char *branch1, const char *branch2,
+		      const char *prefix)
 {
 	struct commit *parent1, *parent2;
 	struct commit_list *merge_bases = NULL;
@@ -441,8 +445,24 @@ static int real_merge(struct merge_tree_options *o,
 		o->show_messages = !result.clean;
 
 	puts(oid_to_hex(&result.tree->object.oid));
+	if (!result.clean) {
+		struct string_list conflicted_files = STRING_LIST_INIT_NODUP;
+		const char *last = NULL;
+		int i;
+
+		merge_get_conflicted_files(&result, &conflicted_files);
+		for (i = 0; i < conflicted_files.nr; i++) {
+			const char *name = conflicted_files.items[i].string;
+			if (last && !strcmp(last, name))
+				continue;
+			write_name_quoted_relative(
+				name, prefix, stdout, line_termination);
+			last = name;
+		}
+		string_list_clear(&conflicted_files, 1);
+	}
 	if (o->show_messages) {
-		printf("\n");
+		putchar(line_termination);
 		merge_display_update_messages(&opt, &result);
 	}
 	merge_finalize(&opt, &result);
@@ -508,7 +528,7 @@ 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]);
+		return real_merge(&o, 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 719d81e..8e6dba4 100755
--- a/t/t4301-merge-tree-write-tree.sh
+++ b/t/t4301-merge-tree-write-tree.sh
@@ -117,6 +117,8 @@ test_expect_success 'test conflict notices and such' '
 	#   "whatever" has *both* a modify/delete and a file/directory conflict
 	cat <<-EOF >expect &&
 	HASH
+	greeting
+	whatever~side1
 
 	Auto-merging greeting
 	CONFLICT (content): Merge conflict in greeting
@@ -140,4 +142,13 @@ do
 	'
 done
 
+test_expect_success 'Just the conflicted files without the messages' '
+	test_expect_code 1 git merge-tree --write-tree --no-messages side1 side2 >out &&
+	anonymize_hash out >actual &&
+
+	test_write_lines HASH greeting whatever~side1 >expect &&
+
+	test_cmp expect actual
+'
+
 test_done
-- 
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