Test both walk cancellation checks on their own (closes #81)

walkOneDir's check was hiding the worker's: a worker that walked a
queued directory on a cancelled scan still emitted nothing, because
walkOneDir stopped at its first entry. The worker test now queues a
missing directory, whose read fails and sends a warning before
walkOneDir's check is reached. A new test calls walkOneDir directly on
a cancelled scan and checks it returns no subdirectory to descend into.

Model: opus-5-5

cancel_test.go

@@ -586,20 +586,49 @@ func TestSendEventAbandonsBlockedSend(t *testing.T) {
 	awaitReturn(t, done, "sendEvent")
 }
 
-// TestWalkWorkersDropQueuedDirs checks that cancelled walk workers keep
-// reading jobs and drop the directories rather than stopping their
-// read: the range over jobs has to run out for the pool to tear down
-// and close its event stream.
-func TestWalkWorkersDropQueuedDirs(t *testing.T) {
+// TestWalkOneDirStopsWhenCancelled checks that a cancelled scan stops
+// reading a directory instead of going through the rest of its
+// entries. A walk that kept going would return the subdirectory below
+// to descend into. Unlike a file event, that return is not a send the
+// cancellation can abandon, so the test catches the regression every
+// time.
+func TestWalkOneDirStopsWhenCancelled(t *testing.T) {
 	t.Parallel()
 
 	dir := t.TempDir()
-	writeEmptyFiles(t, dir, walkCancelFilesPerDir)
+
+	err := os.Mkdir(filepath.Join(dir, "sub"), 0o750)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Unbuffered and unread: on a cancelled scan every send gives up.
+	events := make(chan walkEvent)
+
+	subs := walkOneDir(cancelledContext(t), dirJob{path: dir}, false, events)
+	if len(subs) != 0 {
+		t.Errorf("cancelled walkOneDir returned %+v to descend into, "+
+			"want none", subs)
+	}
+}
+
+// TestWalkWorkersDropQueuedDirs checks that cancelled walk workers keep
+// reading jobs and drop the directories rather than stopping their
+// read: the range over jobs has to run out for the pool to tear down
+// and close its event stream. The queued directory does not exist, so
+// a worker that walked it anyway would send a warning before
+// walkOneDir's own cancellation check could stop it. On a cancelled
+// scan that send delivers or gives up at random, so with 64 jobs
+// queued the regression has a one in 2^64 chance of passing.
+func TestWalkWorkersDropQueuedDirs(t *testing.T) {
+	t.Parallel()
+
+	missing := filepath.Join(t.TempDir(), "missing")
 
 	jobs, _, events := startWalkWorkers(cancelledContext(t), 2, false)
 
-	for range 4 {
-		jobs <- dirJob{path: dir}
+	for range 64 {
+		jobs <- dirJob{path: missing}
 	}
 
 	close(jobs)