fix(serato): Fix resetting track colors on metadata reimport

When metadata for a file without Serato tags is reimported, the track
color is reset. This happens, because there is currently no way to
distinguish the case where no track color is set because the file does
not contain any Serato tags from the case where the file contains Serato
tags and the track color has been set to "no color" in Serato (i.e. the
stored color value is `0xFFFFFF`.

This fixes the issue by replacing the `RgbColor::optional_t` value with
a `std::optional<RgbColor::optional_t>` to separate the 3 cases:

1. If the outer optional is `nullopt` , there is no track color present in
  the tags.
2. If the inner optional is `nullopt`, there is a track color present in
  the tags, and that color is "no color".
3. If the inner option holds a value, there is a track color with that
   RGB color value present in the tags.

Fixes mixxxdj#11213.

src/test/seratotagstest.cpp

@@ -217,7 +217,9 @@ TEST_F(SeratoTagsTest, MarkersParseDumpRoundtrip) {
 
         const auto trackColor = seratoTags.getTrackColor();
         const auto cueInfos = seratoTags.getCueInfos();
-        seratoTags.setTrackColor(trackColor);
+        if (trackColor) {
+            seratoTags.setTrackColor(*trackColor);
+        }
         seratoTags.setCueInfos(cueInfos);
 
         const QByteArray outputData = seratoTags.dumpMarkers(filetype);
@@ -252,7 +254,9 @@ TEST_F(SeratoTagsTest, Markers2RoundTrip) {
         const auto trackColor = seratoTags.getTrackColor();
         const auto cueInfos = seratoTags.getCueInfos();
         seratoTags.setBpmLocked(bpmLocked);
-        seratoTags.setTrackColor(trackColor);
+        if (trackColor) {
+            seratoTags.setTrackColor(*trackColor);
+        }
         seratoTags.setCueInfos(cueInfos);
 
         const QByteArray outputData = seratoTags.dumpMarkers2(filetype);

src/track/serato/tags.cpp

@@ -427,19 +427,19 @@ void SeratoTags::setCueInfos(const QList<CueInfo>& cueInfos, double timingOffset
     m_seratoMarkers2.setCues(cueInfoList);
 }
 
-RgbColor::optional_t SeratoTags::getTrackColor() const {
+std::optional<RgbColor::optional_t> SeratoTags::getTrackColor() const {
     RgbColor::optional_t color = m_seratoMarkers.getTrackColor();
 
     if (!color) {
         // Markers_ is empty, but we may have a color in Markers2
         color = m_seratoMarkers2.getTrackColor();
     }
 
-    if (color) {
-        color = SeratoTags::storedToDisplayedTrackColor(*color);
+    if (!color) {
+        return std::nullopt;
     }
 
-    return color;
+    return std::optional<RgbColor::optional_t>{SeratoTags::storedToDisplayedTrackColor(*color)};
 }
 
 void SeratoTags::setTrackColor(RgbColor::optional_t color) {

src/track/serato/tags.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <optional>
+
 #include "audio/signalinfo.h"
 #include "track/beatsimporter.h"
 #include "track/cueinfoimporter.h"
@@ -99,7 +101,7 @@ class SeratoTags final {
         m_seratoBeatGrid.setBeats(pBeats, signalInfo, duration, timingOffset);
     }
 
-    RgbColor::optional_t getTrackColor() const;
+    std::optional<RgbColor::optional_t> getTrackColor() const;
     void setTrackColor(RgbColor::optional_t color);
 
     bool isBpmLocked() const;

src/track/track.cpp

@@ -206,10 +206,14 @@ void Track::importMetadata(
         const auto newKey = m_record.getGlobalKey();
 
         // Import track color from Serato tags if available
-        const auto newColor = m_record.getMetadata().getTrackInfo().getSeratoTags().getTrackColor();
-        const bool colorModified = compareAndSet(m_record.ptrColor(), newColor);
+        const std::optional<mixxx::RgbColor::optional_t> newColor =
+                m_record.getMetadata()
+                        .getTrackInfo()
+                        .getSeratoTags()
+                        .getTrackColor();
+        const bool colorModified = newColor && compareAndSet(m_record.ptrColor(), *newColor);
         modified |= colorModified;
-        DEBUG_ASSERT(!colorModified || m_record.getColor() == newColor);
+        DEBUG_ASSERT(!colorModified || m_record.getColor() == *newColor);
 
         if (!modified) {
             // Unmodified, nothing todo
@@ -228,7 +232,8 @@ void Track::importMetadata(
             emit replayGainUpdated(newReplayGain);
         }
         if (colorModified) {
-            emit colorUpdated(newColor);
+            DEBUG_ASSERT(newColor);
+            emit colorUpdated(*newColor);
         }
     }