fix(download): suffix only colliding quality variants

go_backend/extension_fallback_output_test.go

@@ -0,0 +1,35 @@
+package gobackend
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestShouldReuseExistingOutputProtectsCompletedFile(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "Artist - Track.flac")
+	if err := os.WriteFile(path, []byte("existing audio"), 0o644); err != nil {
+		t.Fatalf("write existing output: %v", err)
+	}
+
+	if !shouldReuseExistingOutput(DownloadRequest{}, path) {
+		t.Fatal("expected completed output to be reused when variants are disabled")
+	}
+	if shouldReuseExistingOutput(
+		DownloadRequest{AllowQualityVariant: true},
+		path,
+	) {
+		t.Fatal("quality-variant staging output must remain independently writable")
+	}
+}
+
+func TestShouldReuseExistingOutputIgnoresEmptyStagingFile(t *testing.T) {
+	path := filepath.Join(t.TempDir(), "native_saf_work.flac")
+	if err := os.WriteFile(path, nil, 0o644); err != nil {
+		t.Fatalf("write staging output: %v", err)
+	}
+
+	if shouldReuseExistingOutput(DownloadRequest{}, path) {
+		t.Fatal("empty staging output must not be treated as a completed download")
+	}
+}