fix(download): suffix only colliding quality variants

go_backend/extension_fallback.go

@@ -26,6 +26,23 @@ func attemptExtensionDownload(
 	lastRetryAfterSeconds *int,
 ) (resp *DownloadResponse, cancelledOuter bool) {
 	outputPath := buildOutputPathForExtension(req, ext)
+	if shouldReuseExistingOutput(req, outputPath) {
+		result := DownloadResult{FilePath: outputPath}
+		enrichResultQualityFromFile(&result)
+		built := buildDownloadSuccessResponse(
+			req,
+			result,
+			providerLabel,
+			"File already exists",
+			outputPath,
+			true,
+		)
+		if req.ItemID != "" {
+			CompleteItemProgress(req.ItemID)
+		}
+		GoLog("[DownloadWithExtensionFallback] Keeping existing output instead of replacing it: %s\n", outputPath)
+		return &built, false
+	}
 	if req.ItemID != "" {
 		SetItemPreparingStage(req.ItemID, "resolving_stream")
 	}

go_backend/extension_fallback_output.go

@@ -83,6 +83,18 @@ func buildOutputPathForExtension(req DownloadRequest, ext *loadedExtension) stri
 	return filepath.Join(tempDir, buildDownloadFilename(req))
 }
 
+func shouldReuseExistingOutput(req DownloadRequest, outputPath string) bool {
+	if req.AllowQualityVariant || isFDOutput(req.OutputFD) {
+		return false
+	}
+	path := strings.TrimSpace(outputPath)
+	if path == "" || strings.HasPrefix(path, "content://") || strings.HasPrefix(path, "/proc/self/fd/") {
+		return false
+	}
+	info, err := os.Stat(path)
+	return err == nil && info.Mode().IsRegular() && info.Size() > 0
+}
+
 func canEmbedGenreLabel(filePath string) bool {
 	path := strings.TrimSpace(filePath)
 	if path == "" || strings.HasPrefix(path, "content://") || strings.HasPrefix(path, "/proc/self/fd/") {

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")
+	}
+}