perf(analysis): serialize and cancel owned FFmpeg sessions

lib/services/audio_analysis_jobs.dart

@@ -0,0 +1,64 @@
+import 'dart:async';
+
+class AudioAnalysisCancelled implements Exception {
+  const AudioAnalysisCancelled();
+}
+
+/// Serializes native analysis sessions and cancels only the owned session.
+/// A replacement waits for native completion before creating its output files.
+class AudioAnalysisJobs<T> {
+  final Future<({int id, Future<T> completed})> Function(List<String>) start;
+  final Future<void> Function(int) cancel;
+  Future<void> _tail = Future<void>.value();
+  int _generation = 0;
+  int? _activeId;
+  bool _disposed = false;
+
+  AudioAnalysisJobs({required this.start, required this.cancel});
+
+  void invalidate() {
+    _generation++;
+    final id = _activeId;
+    if (id != null) unawaited(_cancel(id));
+  }
+
+  void dispose() {
+    _disposed = true;
+    invalidate();
+  }
+
+  Future<void> _cancel(int id) async {
+    try {
+      await cancel(id);
+    } catch (_) {
+      // Await the native completion even if the cancellation request fails.
+    }
+  }
+
+  Future<T> run(List<String> arguments) {
+    final generation = _generation;
+    void checkCurrent() {
+      if (_disposed || generation != _generation) {
+        throw const AudioAnalysisCancelled();
+      }
+    }
+
+    final result = _tail.then((_) async {
+      checkCurrent();
+      final session = await start(arguments);
+      _activeId = session.id;
+      try {
+        if (_disposed || generation != _generation) {
+          await _cancel(session.id);
+        }
+        final value = await session.completed;
+        checkCurrent();
+        return value;
+      } finally {
+        _activeId = null;
+      }
+    });
+    _tail = result.then<void>((_) {}, onError: (Object _, StackTrace _) {});
+    return result;
+  }
+}