fix(cancel): stop a cancel from stranding the job that replaced it

app/lib/services/worker_manager.dart

@@ -29,6 +29,28 @@ class WorkerManager {
   /// indistinguishable from the hang reported in #50.
   bool _completionEmitted = false;
 
+  /// Which job the manager is currently running.
+  ///
+  /// Incremented by every [startJob]. Anything asynchronous that outlives a job
+  /// — the exit handler, the tail of [cancel] — captures this and does nothing
+  /// if it no longer matches, because by then it would be acting on someone
+  /// else's job.
+  ///
+  /// This matters because cancelling now waits for the worker to genuinely exit,
+  /// which can take seconds. Cancellation emits a completion, the queue advances
+  /// and starts the next job on that event, and the tail of the *cancelling*
+  /// call then ran `_cleanup()` — nulling `_process` and cancelling the new job's
+  /// stdout subscription. The job ran to completion with nobody listening, so the
+  /// progress dialog span forever with no updates.
+  int _generation = 0;
+
+  /// Whether the job identified by [_generation] is being cancelled.
+  ///
+  /// The exit handler is registered on `exitCode` before [cancel] awaits it, so
+  /// it reports first. Without this it describes a deliberate cancellation as
+  /// "Worker exited with code 143".
+  bool _cancelRequested = false;
+
   /// Stream of progress updates from the worker.
   final _progressController = StreamController<ProgressInfo>.broadcast();
   Stream<ProgressInfo> get progressStream => _progressController.stream;
@@ -53,6 +75,8 @@ class WorkerManager {
     }
 
     _completionEmitted = false;
+    _cancelRequested = false;
+    final generation = ++_generation;
 
     final toolLocator = ToolLocator.instance;
     final workerPath = toolLocator.workerPath;
@@ -89,19 +113,26 @@ class WorkerManager {
 
       // Wait for process to exit
       _process!.exitCode.then((exitCode) {
-        // Clean up config file
+        // Clean up config file — safe regardless of whose job this is.
         configFile.delete().catchError((_) => configFile);
 
+        // A newer job has since started; reporting its completion or tearing
+        // down its plumbing here would strand it silently.
+        if (generation != _generation) return;
+
         // The worker sends a `complete` message on both success and failure, so
         // _pendingCompletion is normally set. If it isn't, the exit still has to
         // be reported — see [_completionEmitted].
         _emitCompletion(_pendingCompletion ??
             CompletionResult(
               success: false,
-              errorMessage: _lastErrorMessage ??
-                  (exitCode == 0
-                      ? 'Worker exited without reporting a result'
-                      : 'Worker exited with code $exitCode'),
+              cancelled: _cancelRequested,
+              errorMessage: _cancelRequested
+                  ? 'Job cancelled by user'
+                  : _lastErrorMessage ??
+                      (exitCode == 0
+                          ? 'Worker exited without reporting a result'
+                          : 'Worker exited with code $exitCode'),
             ));
 
         _cleanup();
@@ -188,6 +219,10 @@ class WorkerManager {
     // running" check was never actually false.
     final process = _process;
     if (process == null) return;
+    // Which job this call is cancelling. Everything after the await below is
+    // conditional on it still being the current one.
+    final generation = _generation;
+    _cancelRequested = true;
 
     if (Platform.isWindows) {
       // No SIGTERM on Windows, and Process.kill maps to TerminateProcess, which
@@ -223,6 +258,13 @@ class WorkerManager {
       }
     }
 
+    // Waiting above can take seconds, and cancelling emits a completion that
+    // the queue acts on by starting the next job. If that has happened, this
+    // call must not touch anything: `_cleanup()` would null `_process` and
+    // cancel the new job's stdout subscription, leaving it running with nobody
+    // listening and the progress dialog spinning forever.
+    if (generation != _generation) return;
+
     _cleanup();
 
     _emitCompletion(CompletionResult(

app/test/worker_manager_generation_test.dart

@@ -0,0 +1,99 @@
+// Cancelling must not tear down the job that replaced it.
+//
+// `cancel()` waits for the worker to genuinely exit, which can take seconds.
+// Cancelling emits a completion, the queue acts on that by starting the next
+// job, and the tail of the *cancelling* call then ran `_cleanup()` — nulling
+// `_process` and cancelling the new job's stdout subscription. The new job ran
+// to completion with nobody listening, so the progress dialog span forever with
+// no updates. The same applies to the previous job's `exitCode` handler, which
+// fires on its own schedule.
+//
+// Both are now conditional on the generation counter still matching. These tests
+// pin that, because the symptom is a UI hang with no error anywhere: the worker
+// is healthy, the encode is progressing, and nothing is listening.
+
+import 'dart:io';
+
+import 'package:test/test.dart';
+
+void main() {
+  group('worker manager job scoping', () {
+    late String source;
+
+    setUpAll(() {
+      // Normalise line endings. git checks this file out CRLF on Windows, and
+      // every scan below is written against "\n" — so without this the whole
+      // suite fails there and nowhere else. Same trap as test_92 in
+      // filter_integration_test.rs; if you add an assertion here, do not embed
+      // a bare "\n" without remembering this line exists.
+      source = File('lib/services/worker_manager.dart')
+          .readAsStringSync()
+          .replaceAll('\r\n', '\n');
+    });
+
+    test('the exit handler does nothing once a newer job has started', () {
+      final start = source.indexOf('_process!.exitCode.then(');
+      expect(start, greaterThan(-1), reason: 'exit handler not found');
+      final body = source.substring(start, source.indexOf('} catch (e) {', start));
+
+      expect(body, contains('if (generation != _generation) return;'),
+          reason: 'the exit handler must bail out when superseded, or it emits '
+              "a stale completion and calls _cleanup() on someone else's job");
+
+      // The guard has to come before the teardown, not after it.
+      final guardAt = body.indexOf('if (generation != _generation) return;');
+      final cleanupAt = RegExp(r'^\s*_cleanup\(\);', multiLine: true)
+          .firstMatch(body)!
+          .start;
+      expect(guardAt, lessThan(cleanupAt),
+          reason: 'the guard must precede the _cleanup() call, or the damage is '
+              'done before it is checked');
+    });
+
+    test('cancel does not clean up a job it did not start', () {
+      final start = source.indexOf('Future<void> cancel() async {');
+      expect(start, greaterThan(-1));
+      final body = source.substring(start);
+      final end = body.indexOf('\n  }\n');
+      final cancelBody = body.substring(0, end);
+
+      expect(cancelBody, contains('final generation = _generation;'),
+          reason: 'cancel() must capture which job it is cancelling');
+      expect(cancelBody, contains('if (generation != _generation) return;'),
+          reason: 'cancel() waits for a real exit, so by the time it resumes the '
+              'queue may have started the next job — it must not clean that up');
+      // Match the call, not the word: cancel()'s own comments mention
+      // _cleanup(), and comparing against prose proved nothing.
+      final guardAt = cancelBody.indexOf('if (generation != _generation) return;');
+      final cleanupAt = RegExp(r'^\s*_cleanup\(\);', multiLine: true)
+          .firstMatch(cancelBody)!
+          .start;
+      expect(guardAt, lessThan(cleanupAt),
+          reason: 'the guard must precede the _cleanup() call');
+    });
+
+    test('a cancelled job is reported as cancelled, not as an exit code', () {
+      // The exit handler is registered on exitCode before cancel() awaits it, so
+      // it reports first. Without _cancelRequested it describes a deliberate
+      // cancellation as "Worker exited with code 143", and the UI cannot tell a
+      // cancellation from a crash.
+      final start = source.indexOf('_process!.exitCode.then(');
+      final body = source.substring(start, source.indexOf('} catch (e) {', start));
+      expect(body, contains('cancelled: _cancelRequested'),
+          reason: 'the exit handler must mark a cancelled job as cancelled');
+      expect(body, contains("_cancelRequested\n"),
+          reason: 'and use it for the message too');
+    });
+
+    test('startJob resets both flags so state cannot leak between jobs', () {
+      final start = source.indexOf('Future<void> startJob(');
+      final body = source.substring(start, source.indexOf('_process = await Process.start', start));
+      expect(body, contains('_completionEmitted = false;'));
+      expect(body, contains('_cancelRequested = false;'),
+          reason: 'a stale _cancelRequested would make the next job report '
+              'itself cancelled the moment it finished');
+      expect(body, contains('++_generation'),
+          reason: 'each job needs its own generation, or the guards never fire');
+    });
+  });
+}