Implemented atomic session replacement.

internal/playback/session_command.go

@@ -20,11 +20,13 @@ const (
 	CommandRemoveAudio
 	CommandEnableSync
 	CommandDisableSync
+	CommandSetSession
 )
 
 type SessionCommand struct {
-	Kind   SessionCommandKind
-	Config FeedConfig // Used only by SetVideo and SetAudio.
+	Kind    SessionCommandKind
+	Config  FeedConfig    // Used only by SetVideo and SetAudio.
+	Session SessionConfig // Used only by SetSession.
 }
 
 var (
@@ -99,6 +101,12 @@ func ApplySessionCommand(
 	case CommandDisableSync:
 		next.SyncRequested = false
 
+	case CommandSetSession:
+		next = command.Session
+		// Retry policy belongs to the running controller configuration, not to
+		// GUI or playlist session selections.
+		next.Retry = current.Retry
+
 	default:
 		return current, ErrUnknownSessionCommand
 	}

internal/playback/session_command_test.go

@@ -114,6 +114,27 @@ func TestApplySessionCommand(t *testing.T) {
 			command: SessionCommand{Kind: CommandDisableSync},
 			want:    func() SessionConfig { c := base; c.SyncRequested = false; return c }(),
 		},
+		{
+			name:    "set complete session atomically and preserve retry",
+			current: base,
+			command: SessionCommand{
+				Kind: CommandSetSession,
+				Session: SessionConfig{
+					Video:         newVideo,
+					Audio:         newAudio,
+					SyncRequested: false,
+					Retry: RetryPolicy{
+						MaxAttempts: 99,
+					},
+				},
+			},
+			want: SessionConfig{
+				Video:         newVideo,
+				Audio:         newAudio,
+				SyncRequested: false,
+				Retry:         base.Retry,
+			},
+		},
 	}
 
 	for _, tt := range tests {
@@ -157,6 +178,17 @@ func TestApplySessionCommandFailurePreservesState(t *testing.T) {
 			ErrAudioNotConfigured,
 		},
 		{"unknown command", base, SessionCommand{Kind: 255}, ErrUnknownSessionCommand},
+		{
+			"invalid complete session",
+			base,
+			SessionCommand{
+				Kind: CommandSetSession,
+				Session: SessionConfig{
+					Video: FeedConfig{UUID: "video", Active: true},
+				},
+			},
+			ErrFeedDomainRequired,
+		},
 	}
 
 	for _, tt := range tests {

internal/playback/session_controller_test.go

@@ -240,6 +240,39 @@ func TestSessionControllerUpdatesOnlyChangedIndependentSlot(t *testing.T) {
 	}
 }
 
+func TestSessionControllerReplacesSynchronizedPairWithOneCommand(t *testing.T) {
+	events := make(chan controllerEvent, 32)
+	controller := newRecordingController(t, events)
+	initial := validCommandSession()
+	commands := make(chan SessionCommand)
+	done := make(chan error, 1)
+	go func() { done <- controller.Run(context.Background(), initial, commands) }()
+	start := receiveControllerEvent(t, events)
+	if start.unit != UnitSync || start.action != "start" {
+		t.Fatalf("initial event = %+v, want sync start", start)
+	}
+
+	next := initial
+	next.Video = FeedConfig{Domain: "/next", UUID: "next-video", Active: true}
+	next.Audio = FeedConfig{Domain: "/next", UUID: "next-audio", Active: true}
+	commands <- SessionCommand{Kind: CommandSetSession, Session: next}
+	event := receiveControllerEvent(t, events)
+	wantPair := SyncPairConfig{Video: next.Video, Audio: next.Audio}
+	if event.unit != UnitSync || event.action != "command" || event.pair != wantPair {
+		t.Fatalf("replacement event = %+v, want one sync command for %#v", event, wantPair)
+	}
+	select {
+	case event := <-events:
+		t.Fatalf("atomic replacement emitted an extra event: %+v", event)
+	case <-time.After(20 * time.Millisecond):
+	}
+
+	close(commands)
+	if err := <-done; err != nil {
+		t.Fatalf("Run() error = %v", err)
+	}
+}
+
 func TestSessionControllerStopsIndependentSlotsBeforeStartingSync(t *testing.T) {
 	events := make(chan controllerEvent, 32)
 	controller := newRecordingController(t, events)