Fix #207346: Restrict first track to timing events

Currently, exported MIDI files will have data events in the first MIDI
track. While this is not forbidden by the standard, the industry
convention is to only put timing related events in the first track, in
which case it is called a tempo track. MuseScore should follow that
convention because a lot of the available software assumes this
convention. This has been discussed and agreed upon in:

https://musescore.org/en/node/207346

audio/exports/exportmidi.cpp

@@ -33,11 +33,10 @@ namespace Ms {
 //   writeHeader
 //---------------------------------------------------------
 
-void ExportMidi::writeHeader()
+void ExportMidi::writeHeader(MidiTrack& tempoTrack)
       {
       if (mf.tracks().isEmpty())
             return;
-      MidiTrack &track  = mf.tracks().front();
 #if 0 // TODO
       MeasureBase* measure  = cs->first();
 
@@ -86,7 +85,7 @@ void ExportMidi::writeHeader()
       //--------------------------------------------
 
       int staffIdx = 0;
-      for (auto& track1: mf.tracks()) {
+      for (auto& track: mf.tracks()) {
             Staff* staff  = cs->staff(staffIdx);
 
             QByteArray partName = staff->partName().toUtf8();
@@ -101,7 +100,7 @@ void ExportMidi::writeHeader()
             ev.setEData(data);
             ev.setLen(len);
 
-            track1.insert(0, ev);
+            track.insert(0, ev);
 
 
             ++staffIdx;
@@ -149,7 +148,7 @@ void ExportMidi::writeHeader()
                   ev.setMetaType(META_TIME_SIGNATURE);
                   ev.setEData(data);
                   ev.setLen(4);
-                  track.insert(pauseMap.addPauseTicks(is->first + tickOffset), ev);
+                  tempoTrack.insert(pauseMap.addPauseTicks(is->first + tickOffset), ev);
                   }
             }
 
@@ -159,7 +158,7 @@ void ExportMidi::writeHeader()
       //---------------------------------------------------
 
       staffIdx = 0;
-      for (auto& track1: mf.tracks()) {
+      for (auto& track: mf.tracks()) {
             Staff* staff  = cs->staff(staffIdx);
             KeyList* keys = staff->keyList();
 
@@ -183,7 +182,7 @@ void ExportMidi::writeHeader()
                         data[1]   = 0;  // major
                         ev.setEData(data);
                         int tick = ik->first + tickOffset;
-                        track1.insert(pauseMap.addPauseTicks(tick), ev);
+                        track.insert(pauseMap.addPauseTicks(tick), ev);
                         if (tick == 0)
                               initialKeySigFound = true;
                         }
@@ -200,7 +199,7 @@ void ExportMidi::writeHeader()
                   data[0]   = key;
                   data[1]   = 0;  // major
                   ev.setEData(data);
-                  track1.insert(0, ev);
+                  track.insert(0, ev);
                   }
 
             ++staffIdx;
@@ -228,7 +227,7 @@ void ExportMidi::writeHeader()
             data[1]   = tempo >> 8;
             data[2]   = tempo;
             ev.setEData(data);
-            track.insert(it->first, ev);
+            tempoTrack.insert(it->first, ev);
             }
       }
 
@@ -249,6 +248,8 @@ bool ExportMidi::write(QIODevice* device, bool midiExpandRepeats, bool exportRPN
       mf.setDivision(MScore::division);
       mf.setFormat(1);
       QList<MidiTrack>& tracks = mf.tracks();
+      MidiTrack tempoTrack;
+      tempoTrack.setOutChannel(0);
 
       for (int i = 0; i < cs->nstaves(); ++i)
             tracks.append(MidiTrack());
@@ -257,7 +258,7 @@ bool ExportMidi::write(QIODevice* device, bool midiExpandRepeats, bool exportRPN
       cs->renderMidi(&events, false, midiExpandRepeats, synthState);
 
       pauseMap.calculate(cs);
-      writeHeader();
+      writeHeader(tempoTrack);
 
       int staffIdx = 0;
       for (auto &track: tracks) {
@@ -370,6 +371,7 @@ bool ExportMidi::write(QIODevice* device, bool midiExpandRepeats, bool exportRPN
                   }
             ++staffIdx;
             }
+      tracks.prepend(tempoTrack);
       return !mf.write(device);
       }
 

audio/exports/exportmidi.h

@@ -47,7 +47,7 @@ class ExportMidi {
 
       PauseMap pauseMap;
 
-      void writeHeader();
+      void writeHeader(MidiTrack& tempoTrack);
 
    public:
       MidiFile mf;