Sync Lock: When disabling explicit sync, make sure we pick a soft mas…

…ter if there are two playing sync decks

src/engine/sync/enginesync.cpp

@@ -196,8 +196,14 @@ Syncable* EngineSync::pickMaster(Syncable* enabling_syncable) {
         return m_pMasterSyncable;
     }
 
-    Syncable* first_stopped_deck = nullptr;
+    // First preference: some other sync deck that is not playing.
+    Syncable* first_other_playing_deck = nullptr;
+    // Second preference: whatever the first playing sync deck is, even if it's us.
     Syncable* first_playing_deck = nullptr;
+    // Third preference: the first stopped sync deck.
+    Syncable* first_stopped_deck = nullptr;
+    // Last resort: nullptr.
+
     int stopped_deck_count = 0;
     int playing_deck_count = 0;
 
@@ -219,6 +225,9 @@ Syncable* EngineSync::pickMaster(Syncable* enabling_syncable) {
             if (playing_deck_count == 0) {
                 first_playing_deck = pSyncable;
             }
+            if (!first_other_playing_deck && pSyncable != enabling_syncable) {
+                first_other_playing_deck = pSyncable;
+            }
             playing_deck_count++;
         } else {
             if (stopped_deck_count == 0) {
@@ -231,7 +240,7 @@ Syncable* EngineSync::pickMaster(Syncable* enabling_syncable) {
     if (playing_deck_count == 1) {
         return first_playing_deck;
     } else if (playing_deck_count > 1) {
-        return nullptr;
+        return first_other_playing_deck;
     }
 
     // No valid playing sync decks

src/test/enginesynctest.cpp

@@ -550,14 +550,14 @@ TEST_F(EngineSyncTest, SetExplicitMasterByLights) {
     ASSERT_TRUE(isExplicitMaster(m_sGroup1));
     ASSERT_TRUE(isFollower(m_sGroup2));
 
-    // Now set channel 1 to not-master, it will become a follower.
-    // master is handed over to Internal clock
+    // Now set channel 1 to not-master. The system will choose deck 2 as the next best
+    // option for soft master
     pButtonSyncMaster1->slotSet(0);
     ProcessBuffer();
 
     ASSERT_TRUE(isFollower(m_sInternalClockGroup));
     ASSERT_TRUE(isFollower(m_sGroup1));
-    ASSERT_TRUE(isFollower(m_sGroup2));
+    ASSERT_TRUE(isSoftMaster(m_sGroup2));
 }
 
 TEST_F(EngineSyncTest, SetExplicitMasterByLightsNoTracks) {