Fix bad phase seek when starting from preroll.

We were interpreting all <0 playposition as "invalid", which is not correct.
fixes https://bugs.launchpad.net/mixxx/+bug/1930143

src/engine/controls/bpmcontrol.cpp

@@ -814,7 +814,9 @@ double BpmControl::getBeatMatchPosition(
         if (!pOtherEngineBuffer || pOtherEngineBuffer->getSpeed() == 0.0) {
             // "this" track is playing, or just starting
             // only match phase if the sync target is playing as well
-            // else use the previous phase of "this" track before the seek
+            // else use the previous phase of "this" track before the seek.
+            // This means when the DJ does a quantized seek, we presere
+            // the exact beat distance.
             pOtherEngineBuffer = getEngineBuffer();
         }
     } else if (!pOtherEngineBuffer) {

src/engine/enginebuffer.cpp

@@ -1353,14 +1353,10 @@ void EngineBuffer::slotEjectTrack(double v) {
 }
 
 double EngineBuffer::getExactPlayPos() {
-    double visualPlayPos = getVisualPlayPos();
-    if (visualPlayPos > 0) {
-        return getVisualPlayPos() * getTrackSamples();
-    } else {
-        // Track was just loaded and the first buffer was not processed yet.
-        // assume it is at 0:00
+    if (!m_visualPlayPos->isValid()) {
         return 0.0;
     }
+    return m_visualPlayPos->getEnginePlayPos() * getTrackSamples();
 }
 
 double EngineBuffer::getVisualPlayPos() {

src/test/enginesynctest.cpp

@@ -1361,6 +1361,26 @@ TEST_F(EngineSyncTest, ZeroBPMRateAdjustIgnored) {
             ControlObject::getControl(ConfigKey(m_sGroup2, "rate"))->get());
 }
 
+TEST_F(EngineSyncTest, BeatDistanceBeforeStart) {
+    // https://bugs.launchpad.net/mixxx/+bug/1930143
+    // If the start position is before zero, we should still initialize the beat distance
+    // correctly.  Unfortunately, this currently doesn't work.
+
+    mixxx::BeatsPointer pBeats1 = BeatFactory::makeBeatGrid(
+            m_pTrack1->getSampleRate(), 128, 0);
+    m_pTrack1->trySetBeats(pBeats1);
+    ControlObject::getControl(ConfigKey(m_sGroup1, "quantize"))->set(1.0);
+    ControlObject::set(ConfigKey(m_sGroup1, "playposition"), -.05);
+    ProcessBuffer();
+    ControlObject::getControl(ConfigKey(m_sGroup1, "play"))->set(1.0);
+    ProcessBuffer();
+    // This fraction is one buffer beyond the seek position -- indicating that
+    // we seeked correctly.
+    EXPECT_NEAR(0.49143461829176116,
+            ControlObject::getControl(ConfigKey(m_sGroup1, "beat_distance"))->get(),
+            kMaxBeatDistanceEpsilon);
+}
+
 TEST_F(EngineSyncTest, ZeroLatencyRateChangeNoQuant) {
     // Confirm that a rate change in an explicit master is instantly communicated
     // to followers.

src/waveform/visualplayposition.h

@@ -60,6 +60,9 @@ class VisualPlayPosition : public QObject {
     static void setCallbackEntryToDacSecs(double secs, const PerformanceTimer& time);
 
     void setInvalid() { m_valid = false; };
+    bool isValid() const {
+        return m_valid;
+    }
 
   private slots:
     void slotAudioBufferSizeChanged(double sizeMs);