lib/config, lib/ignore: Write Windows line endings (fixes #7115) (#8052)

lib/ignore/ignore.go

@@ -595,8 +595,9 @@ func WriteIgnores(filesystem fs.Filesystem, path string, content []string) error
 		return err
 	}
 
+	wr := osutil.LineEndingsWriter(fd)
 	for _, line := range content {
-		fmt.Fprintln(fd, line)
+		fmt.Fprintln(wr, line)
 	}
 
 	if err := fd.Close(); err != nil {

lib/ignore/ignore_test.go

@@ -1192,3 +1192,42 @@ func TestEmptyPatterns(t *testing.T) {
 		}
 	}
 }
+
+func TestWindowsLineEndings(t *testing.T) {
+	if runtime.GOOS != "windows" {
+		t.Skip("Windows specific")
+	}
+
+	lines := "foo\nbar\nbaz\n"
+
+	dir, err := os.MkdirTemp("", "syncthing-test")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll(dir)
+
+	ffs := fs.NewFilesystem(fs.FilesystemTypeBasic, dir)
+	m := New(ffs)
+	if err := m.Parse(strings.NewReader(lines), ".stignore"); err != nil {
+		t.Fatal(err)
+	}
+	if err := WriteIgnores(ffs, ".stignore", m.Lines()); err != nil {
+		t.Fatal(err)
+	}
+
+	fd, err := ffs.Open(".stignore")
+	if err != nil {
+		t.Fatal(err)
+	}
+	bs, err := io.ReadAll(fd)
+	fd.Close()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	unixLineEndings := bytes.Count(bs, []byte("\n"))
+	windowsLineEndings := bytes.Count(bs, []byte("\r\n"))
+	if unixLineEndings == 0 || windowsLineEndings != unixLineEndings {
+		t.Error("expected there to be a non-zero number of Windows line endings")
+	}
+}