Make effects of 582e574 conditionally compiled...

... It was only "mostly harmless" on Windows and Mac.

Now where is my towel?

src/AudioIO.cpp

@@ -1554,7 +1554,9 @@ bool AudioIO::StartPortAudioStream(double sampleRate,
    int  userData = 24;
    int* lpUserData = (captureFormat_saved == int24Sample) ? &userData : NULL;
 
+#ifndef USE_TIME_INFO
    mMidiTimeCorrection = 0;
+#endif
    mLastPaError = Pa_OpenStream( &mPortStreamV19,
                                  useCapture ? &captureParameters : NULL,
                                  usePlayback ? &playbackParameters : NULL,
@@ -1568,10 +1570,14 @@ bool AudioIO::StartPortAudioStream(double sampleRate,
    Px_SetInputVolume(mPortMixer, oldRecordVolume);
 #endif
    if (mPortStreamV19 != NULL && mLastPaError == paNoError) {
+
+#ifndef USE_TIME_INFO
       auto info = Pa_GetStreamInfo(mPortStreamV19);
       if (info)
          mMidiTimeCorrection =
             info->outputLatency - (MIDI_MINIMAL_LATENCY_MS / 1000.0);
+#endif
+
       #ifdef __WXMAC__
       if (mPortMixer) {
          if (Px_SupportsPlaythrough(mPortMixer)) {
@@ -4101,15 +4107,20 @@ PmTimestamp AudioIO::MidiTime()
    // sample count generated by the audio callback (in AudioTime()) might also
    // have the virtue of keeping the Midi output synched with audio, even though
    // pmlinuxalsa.c does not implement any synchronization of its own.
+
+#ifdef USE_TIME_INFO
+   auto offset = mAudioCallbackOutputDacTime - mAudioCallbackOutputCurrentTime;
+#else
    // We are now using mMidiTimeCorrection, computed once after opening the
    // portaudio stream, rather than the difference of dac and current
-   // times reported to the audio callback; because the answer is very nearly
-   // the same on Windows and macOs, doing no harm, whereas on Linux with ALSA,
+   // times reported to the audio callback; because on Linux with ALSA,
    // the dac and current times were not reliable, and that caused irregular
    // timing of Midi playback because latency was effectively zero.
+   auto offset = mMidiTimeCorrection;
+#endif
+
    auto clockChange = PaUtil_GetTime() - mAudioCallbackClockTime;
    // auto offset = mAudioCallbackOutputDacTime - mAudioCallbackOutputCurrentTime;
-   auto offset = mMidiTimeCorrection;
    return (PmTimestamp) ((unsigned long) (1000 * (
       AudioTime() + 1.0005 -
       mAudioFramesPerBuffer / mRate +
@@ -4349,11 +4360,10 @@ int audacityAudioCallback(const void *inputBuffer, void *outputBuffer,
    /* GSW: Save timeInfo in case MidiPlayback needs it */
    gAudioIO->mAudioCallbackClockTime = PaUtil_GetTime();
 
-   // PRL:  We no longer trust these numbers on Linux / ALSA:
-   /*
+#ifdef USE_TIME_INFO
    gAudioIO->mAudioCallbackOutputDacTime = timeInfo->outputBufferDacTime;
    gAudioIO->mAudioCallbackOutputCurrentTime = timeInfo->currentTime;
-    */
+#endif
 
    // printf("in callback, mAudioCallbackOutputDacTime %g\n", gAudioIO->mAudioCallbackOutputDacTime); //DBG
    gAudioIO->mAudioFramesPerBuffer = framesPerBuffer;

src/AudioIO.h

@@ -99,6 +99,13 @@ DECLARE_EXPORTED_EVENT_TYPE(AUDACITY_DLL_API, EVT_AUDIOIO_MONITOR, -1);
 // play.  PRL.
 #undef USE_MIDI_THREAD
 
+// Whether we trust all of the time info passed to audacityAudioCallback
+#ifdef __WXGTK__
+   #undef USE_TIME_INFO
+#else
+   #define USE_TIME_INFO
+#endif
+
 struct ScrubbingOptions;
 
 // To avoid growing the argument list of StartStream, add fields here
@@ -534,7 +541,11 @@ private:
    //   MIDI_PLAYBACK:
    PmStream        *mMidiStream;
    PmError          mLastPmError;
+
+#ifndef USE_TIME_INFO
    PaTime           mMidiTimeCorrection; // seconds
+#endif
+
    /// Latency of MIDI synthesizer
    long             mSynthLatency; // ms
 
@@ -543,17 +554,12 @@ private:
    /// PortAudio's clock time
    volatile double  mAudioCallbackClockTime;
 
-   /// PRL: no longer using the next two because they are not reliably
-   /// reported to the audio callback on Linux.
-   /// Compute the approximate difference of them by other means now:
-   /// use PaStreamInfo::outputLatency.
-
-   /*
+#ifdef USE_TIME_INFO
    /// PortAudio's currentTime -- its origin is unspecified!
    volatile double  mAudioCallbackOutputCurrentTime;
    /// PortAudio's outTime
    volatile double  mAudioCallbackOutputDacTime;
-    */
+#endif
 
    /// Number of frames output, including pauses
    volatile long    mNumFrames;