diff --git a/compat/mingw.h b/compat/mingw.h
index 5da45927a1..5148be2328 100644
--- a/compat/mingw.h
+++ b/compat/mingw.h
@@ -67,6 +67,10 @@ typedef int pid_t;
 #define F_SETFD 2
 #define FD_CLOEXEC 0x1
 
+#if !defined O_CLOEXEC && defined O_NOINHERIT
+#define O_CLOEXEC	O_NOINHERIT
+#endif
+
 #ifndef EAFNOSUPPORT
 #define EAFNOSUPPORT WSAEAFNOSUPPORT
 #endif
diff --git a/t/t6026-merge-attr.sh b/t/t6026-merge-attr.sh
index ef0cbceafe..9be8119008 100755
--- a/t/t6026-merge-attr.sh
+++ b/t/t6026-merge-attr.sh
@@ -181,4 +181,19 @@ test_expect_success 'up-to-date merge without common ancestor' '
 	)
 '
 
+test_expect_success 'custom merge does not lock index' '
+	git reset --hard anchor &&
+
+	write_script sleep-one-second.sh <<-\EOF &&
+		sleep 1 &
+	EOF
+
+	printf "* merge=ours\ntext merge=sleep-one-second\n" >.gitattributes &&
+
+	git config merge.ours.driver "true" &&
+	git config merge.sleep-one-second.driver "./sleep-one-second.sh" &&
+
+	git merge master
+'
+
 test_done
diff --git a/tempfile.c b/tempfile.c
index 0af7ebf016..db3981deab 100644
--- a/tempfile.c
+++ b/tempfile.c
@@ -120,7 +120,7 @@ int create_tempfile(struct tempfile *tempfile, const char *path)
 	prepare_tempfile_object(tempfile);
 
 	strbuf_add_absolute_path(&tempfile->filename, path);
-	tempfile->fd = open(tempfile->filename.buf, O_RDWR | O_CREAT | O_EXCL, 0666);
+	tempfile->fd = open(tempfile->filename.buf, O_RDWR | O_CREAT | O_EXCL | O_CLOEXEC, 0666);
 	if (tempfile->fd < 0) {
 		strbuf_reset(&tempfile->filename);
 		return -1;