fix(download): skip AC-4 repair for non-MP4 decrypt output

Decrypt can emit a raw FLAC stream; the AC-4 repair step fed it to the
MP4 box parser, which failed with a garbage atom type whose control
bytes then broke the unescaped Kotlin error JSON (FormatException in
Dart). Guard the repair call by extension, sniff the box type in Go as
a second layer, and build platform error JSON with JSONObject.quote.

android/app/src/main/kotlin/com/zarz/spotiflac/MainActivity.kt

@@ -2677,7 +2677,7 @@ class MainActivity: FlutterFragmentActivity() {
                                     }
                                 } catch (e: Exception) {
                                     android.util.Log.e("SpotiFLAC", "readFileMetadata failed: ${e.message}", e)
-                                    """{"error":"${e.message?.replace("\"", "'")}"}"""
+                                    """{"error":${org.json.JSONObject.quote(e.message ?: "unknown")}}"""
                                 }
                             }
                             result.success(response)
@@ -2718,7 +2718,7 @@ class MainActivity: FlutterFragmentActivity() {
                                     }
                                 } catch (e: Exception) {
                                     android.util.Log.e("SpotiFLAC", "editFileMetadata failed: ${e.message}", e)
-                                    """{"error":"${e.message?.replace("\"", "'")}"}"""
+                                    """{"error":${org.json.JSONObject.quote(e.message ?: "unknown")}}"""
                                 }
                             }
                             result.success(response)
@@ -2731,7 +2731,7 @@ class MainActivity: FlutterFragmentActivity() {
                                     Gobackend.writeM4AFreeformTags(filePath, metadataJson)
                                 } catch (e: Exception) {
                                     android.util.Log.e("SpotiFLAC", "writeM4AFreeformTags failed: ${e.message}", e)
-                                    """{"error":"${e.message?.replace("\"", "'")}"}"""
+                                    """{"error":${org.json.JSONObject.quote(e.message ?: "unknown")}}"""
                                 }
                             }
                             result.success(response)
@@ -2744,7 +2744,7 @@ class MainActivity: FlutterFragmentActivity() {
                                     Gobackend.ensureAC4Config(filePath, sourcePath)
                                 } catch (e: Exception) {
                                     android.util.Log.e("SpotiFLAC", "ensureAC4Config failed: ${e.message}", e)
-                                    """{"error":"${e.message?.replace("\"", "'")}"}"""
+                                    """{"error":${org.json.JSONObject.quote(e.message ?: "unknown")}}"""
                                 }
                             }
                             result.success(response)
@@ -2758,7 +2758,7 @@ class MainActivity: FlutterFragmentActivity() {
                                     Gobackend.writeAC4Metadata(filePath, metadataJson, coverPath)
                                 } catch (e: Exception) {
                                     android.util.Log.e("SpotiFLAC", "writeAC4Metadata failed: ${e.message}", e)
-                                    """{"error":"${e.message?.replace("\"", "'")}"}"""
+                                    """{"error":${org.json.JSONObject.quote(e.message ?: "unknown")}}"""
                                 }
                             }
                             result.success(response)
@@ -2966,7 +2966,7 @@ class MainActivity: FlutterFragmentActivity() {
                                         Gobackend.reEnrichFile(requestJson)
                                     }
                                 } catch (e: Exception) {
-                                    """{"error":"${e.message?.replace("\"", "'")}"}"""
+                                    """{"error":${org.json.JSONObject.quote(e.message ?: "unknown")}}"""
                                 }
                             }
                             result.success(response)
@@ -3685,7 +3685,7 @@ class MainActivity: FlutterFragmentActivity() {
                                         Gobackend.readAudioMetadataJSON(filePath)
                                     }
                                 } catch (e: Exception) {
-                                    """{"error":"${e.message?.replace("\"", "'")}"}"""
+                                    """{"error":${org.json.JSONObject.quote(e.message ?: "unknown")}}"""
                                 }
                             }
                             result.success(response)
@@ -3759,7 +3759,7 @@ class MainActivity: FlutterFragmentActivity() {
                                         Gobackend.parseCueSheet(cuePath, audioDir)
                                     }
                                 } catch (e: Exception) {
-                                    """{"error":"${e.message?.replace("\"", "'")}"}"""
+                                    """{"error":${org.json.JSONObject.quote(e.message ?: "unknown")}}"""
                                 }
                             }
                             result.success(response)

go_backend/ac4_config.go

@@ -3,6 +3,7 @@ package gobackend
 import (
 	"encoding/binary"
 	"fmt"
+	"io"
 	"os"
 )
 
@@ -295,6 +296,23 @@ func EnsureAC4ConfigBox(decryptedPath, sourcePath string) error {
 		f.Close()
 		return err
 	}
+	// A non-MP4 decrypt output (e.g. a raw FLAC stream) is not an AC-4 file;
+	// bail out before the box parser reports it as a corrupt MP4. A real
+	// ISO-BMFF box type is four printable ASCII bytes.
+	var head [8]byte
+	if _, err := f.ReadAt(head[:], 0); err != nil {
+		f.Close()
+		if err == io.EOF || err == io.ErrUnexpectedEOF {
+			return nil
+		}
+		return err
+	}
+	for _, c := range head[4:8] {
+		if c < 0x20 || c > 0x7e {
+			f.Close()
+			return nil
+		}
+	}
 	moovBuf, moovOffset, moovFound, err := loadTopLevelMP4Box(f, info.Size(), "moov")
 	if err != nil || !moovFound {
 		f.Close()

lib/providers/download_queue_provider_finalization.dart

@@ -11,6 +11,13 @@ class _DecryptOutcome {
   const _DecryptOutcome(this.path, {this.newFileName, this.failStage});
 }
 
+/// AC-4 repair only applies to MP4 containers; decrypt can also emit raw
+/// FLAC, which the native MP4 box parser would reject as corrupt.
+bool _isMp4Container(String path) {
+  final lower = path.toLowerCase();
+  return lower.endsWith('.m4a') || lower.endsWith('.mp4');
+}
+
 extension _DownloadQueueFinalization on DownloadQueueNotifier {
   /// Builds the [DownloadHistoryItem] shared by the native-worker and inline
   /// completion paths. Fields whose source/derivation legitimately differs
@@ -297,7 +304,7 @@ extension _DownloadQueueFinalization on DownloadQueueNotifier {
             return null;
           }
           addCleanup(decryptedTempPath);
-          if (repairAc4) {
+          if (repairAc4 && _isMp4Container(decryptedTempPath)) {
             try {
               await PlatformBridge.ensureAC4Config(decryptedTempPath, tempPath);
             } catch (e) {
@@ -345,10 +352,12 @@ extension _DownloadQueueFinalization on DownloadQueueNotifier {
           failStage: DownloadQueueNotifier._decryptStageDecrypt,
         );
       }
-      try {
-        await PlatformBridge.ensureAC4Config(decryptedPath, filePath);
-      } catch (e) {
-        _log.w('AC-4 container repair skipped: $e');
+      if (_isMp4Container(decryptedPath)) {
+        try {
+          await PlatformBridge.ensureAC4Config(decryptedPath, filePath);
+        } catch (e) {
+          _log.w('AC-4 container repair skipped: $e');
+        }
       }
       try {
         await deleteFile(filePath);