Merge branch 'ew/empty-merge-with-dirty-index'

"git merge -s recursive" did not correctly abort when the index is
dirty, if the merged tree happened to be the same as the current
HEAD, which has been fixed.

* ew/empty-merge-with-dirty-index:
  merge-recursive: avoid incorporating uncommitted changes in a merge
  move index_has_changes() from builtin/am.c to merge.c for reuse
  t6044: recursive can silently incorporate dirty changes in a merge

builtin/am.c

@@ -1146,43 +1146,6 @@ static void refresh_and_write_cache(void)
 		die(_("unable to write index file"));
 }
 
-/**
- * Returns 1 if the index differs from HEAD, 0 otherwise. When on an unborn
- * branch, returns 1 if there are entries in the index, 0 otherwise. If an
- * strbuf is provided, the space-separated list of files that differ will be
- * appended to it.
- */
-static int index_has_changes(struct strbuf *sb)
-{
-	struct object_id head;
-	int i;
-
-	if (!get_oid_tree("HEAD", &head)) {
-		struct diff_options opt;
-
-		diff_setup(&opt);
-		opt.flags.exit_with_status = 1;
-		if (!sb)
-			opt.flags.quick = 1;
-		do_diff_cache(&head, &opt);
-		diffcore_std(&opt);
-		for (i = 0; sb && i < diff_queued_diff.nr; i++) {
-			if (i)
-				strbuf_addch(sb, ' ');
-			strbuf_addstr(sb, diff_queued_diff.queue[i]->two->path);
-		}
-		diff_flush(&opt);
-		return opt.flags.has_changes != 0;
-	} else {
-		for (i = 0; sb && i < active_nr; i++) {
-			if (i)
-				strbuf_addch(sb, ' ');
-			strbuf_addstr(sb, active_cache[i]->name);
-		}
-		return !!active_nr;
-	}
-}
-
 /**
  * Dies with a user-friendly message on how to proceed after resolving the
  * problem. This message can be overridden with state->resolvemsg.

cache.h

@@ -644,6 +644,15 @@ extern int write_locked_index(struct index_state *, struct lock_file *lock, unsi
 extern int discard_index(struct index_state *);
 extern void move_index_extensions(struct index_state *dst, struct index_state *src);
 extern int unmerged_index(const struct index_state *);
+
+/**
+ * Returns 1 if the index differs from HEAD, 0 otherwise. When on an unborn
+ * branch, returns 1 if there are entries in the index, 0 otherwise. If an
+ * strbuf is provided, the space-separated list of files that differ will be
+ * appended to it.
+ */
+extern int index_has_changes(struct strbuf *sb);
+
 extern int verify_path(const char *path);
 extern int strcmp_offset(const char *s1, const char *s2, size_t *first_change);
 extern int index_dir_exists(struct index_state *istate, const char *name, int namelen);

merge-recursive.c

@@ -1952,6 +1952,13 @@ int merge_trees(struct merge_options *o,
 	}
 
 	if (oid_eq(&common->object.oid, &merge->object.oid)) {
+		struct strbuf sb = STRBUF_INIT;
+
+		if (index_has_changes(&sb)) {
+			err(o, _("Dirty index: cannot merge (dirty: %s)"),
+			    sb.buf);
+			return 0;
+		}
 		output(o, 0, _("Already up to date!"));
 		*result = head;
 		return 1;

merge.c

@@ -1,4 +1,6 @@
 #include "cache.h"
+#include "diff.h"
+#include "diffcore.h"
 #include "lockfile.h"
 #include "commit.h"
 #include "run-command.h"
@@ -15,6 +17,37 @@ static const char *merge_argument(struct commit *commit)
 		return EMPTY_TREE_SHA1_HEX;
 }
 
+int index_has_changes(struct strbuf *sb)
+{
+	struct object_id head;
+	int i;
+
+	if (!get_oid_tree("HEAD", &head)) {
+		struct diff_options opt;
+
+		diff_setup(&opt);
+		opt.flags.exit_with_status = 1;
+		if (!sb)
+			opt.flags.quick = 1;
+		do_diff_cache(&head, &opt);
+		diffcore_std(&opt);
+		for (i = 0; sb && i < diff_queued_diff.nr; i++) {
+			if (i)
+				strbuf_addch(sb, ' ');
+			strbuf_addstr(sb, diff_queued_diff.queue[i]->two->path);
+		}
+		diff_flush(&opt);
+		return opt.flags.has_changes != 0;
+	} else {
+		for (i = 0; sb && i < active_nr; i++) {
+			if (i)
+				strbuf_addch(sb, ' ');
+			strbuf_addstr(sb, active_cache[i]->name);
+		}
+		return !!active_nr;
+	}
+}
+
 int try_merge_command(const char *strategy, size_t xopts_nr,
 		      const char **xopts, struct commit_list *common,
 		      const char *head_arg, struct commit_list *remotes)

t/t6044-merge-unrelated-index-changes.sh

@@ -6,18 +6,21 @@ test_description="merges with unrelated index changes"
 
 # Testcase for some simple merges
 #   A
-#   o-----o B
+#   o-------o B
 #    \
-#     \---o C
+#     \-----o C
 #      \
-#       \-o D
+#       \---o D
 #        \
-#         o E
+#         \-o E
+#          \
+#           o F
 #   Commit A: some file a
 #   Commit B: adds file b, modifies end of a
 #   Commit C: adds file c
 #   Commit D: adds file d, modifies beginning of a
 #   Commit E: renames a->subdir/a, adds subdir/e
+#   Commit F: empty commit
 
 test_expect_success 'setup trivial merges' '
 	test_seq 1 10 >a &&
@@ -29,6 +32,7 @@ test_expect_success 'setup trivial merges' '
 	git branch C &&
 	git branch D &&
 	git branch E &&
+	git branch F &&
 
 	git checkout B &&
 	echo b >b &&
@@ -52,7 +56,10 @@ test_expect_success 'setup trivial merges' '
 	git mv a subdir/a &&
 	echo e >subdir/e &&
 	git add subdir &&
-	test_tick && git commit -m E
+	test_tick && git commit -m E &&
+
+	git checkout F &&
+	test_tick && git commit --allow-empty -m F
 '
 
 test_expect_success 'ff update' '
@@ -105,6 +112,15 @@ test_expect_success 'recursive' '
 	test_must_fail git merge -s recursive C^0
 '
 
+test_expect_success 'recursive, when merge branch matches merge base' '
+	git reset --hard &&
+	git checkout B^0 &&
+
+	touch random_file && git add random_file &&
+
+	test_must_fail git merge -s recursive F^0
+'
+
 test_expect_success 'octopus, unrelated file touched' '
 	git reset --hard &&
 	git checkout B^0 &&