Use syncronization to address #38

Author: brunchboy

CHANGELOG.md

@@ -15,6 +15,17 @@ This change log follows the conventions of
   normal `0xF0`. Thanks to [@eclab](https://github.com/eclab) for
   identifying this in [Issue
   37](https://github.com/DerekCook/CoreMidi4J/issues/37).
+- Synchronization is used to protect against non-thread-safe behavior
+  by
+  [`MidiSystem.getMidiDeviceInfo()`](https://docs.oracle.com/javase/7/docs/api/javax/sound/midi/MidiSystem.html?is-external=true#getMidiDeviceInfo())
+  when using
+  [`CoreMidiDeviceProvider.addNotificationListener()`](https://deepsymmetry.org/coremidi4j/apidocs/uk/co/xfactorylibrarians/coremidi4j/CoreMidiDeviceProvider.html#addNotificationListener(uk.co.xfactorylibrarians.coremidi4j.CoreMidiNotification))
+  on non-Mac platforms. (To be protected you must always use
+  [`CoreMidiDeviceProvider.getMidiDeviceInfo()`](https://deepsymmetry.org/coremidi4j/apidocs/uk/co/xfactorylibrarians/coremidi4j/CoreMidiDeviceProvider.html#getMidiDeviceInfo())
+  instead of the one provided by `javax.sound.midi.MidiSystem`.)
+  Thanks to [Mailüfterl s.r.o.](https://github.com/mailuefterl-sro)
+  for identifying this in [Issue
+  38](https://github.com/DerekCook/CoreMidi4J/issues/38).
 
 
 ## [1.4] - 2020-02-09

CoreMIDI4J/src/uk/co/xfactorylibrarians/coremidi4j/CoreMidiDeviceProvider.java

@@ -417,7 +417,7 @@ private static Set<List<String>> snapshotCurrentEnvironment() {
 
     Set<List<String>> results = new HashSet<>();
 
-    for (MidiDevice.Info info : MidiSystem.getMidiDeviceInfo()) {
+    for (MidiDevice.Info info : getSystemMidiDeviceInfo()) {
 
       List<String> summary = new LinkedList<>();
       summary.add(info.getName());
@@ -486,6 +486,13 @@ private static void watchForChanges() {
    * is not running macOS, then ensure that our watcher daemon thread is running in order to
    * be able to deliver these notifications, since we don't have CoreMIDI to initiate them.</p>
    *
+   * <p>If you are using this capability on a non-macOS system, you must be sure to only call
+   * {@link #getMidiDeviceInfo()} on this class, and never call {@link MidiSystem#getMidiDeviceInfo()}
+   * directly, because the latter is not thread-safe, and you can run into exceptions trying to
+   * get information about MIDI devices if your call happens to occur at the same time that our
+   * watcher daemon thread is gathering information. The version provided by this class uses
+   * synchronization to avoid such conflicts.</p>
+   *
    * <p>Instances of {@link CoreMidiDeviceProvider} register themselves when they are constructed,
    * so they can keep their lists of working CoreMIDI-backed devices up to date. We only keep the
    * most recent one, since the Java MIDI subsystem will create many, and we only want to update
@@ -684,23 +691,40 @@ public static String getLibraryVersion() {
   }
 
   /**
-   * Obtains an array of information objects representing the set of all working MIDI devices available on the system.
-   * This is a replacement for javax.sound.midi.MidiSystem.getMidiDeviceInfo(), and only returns fully-functional
-   * MIDI devices. If you call it on a non-Mac system, it simply delegates to the javax.sound.midi implementation.
+   * Call the JDK's raw MIDI device information collection method, while holding a lock to make sure that
+   * this is never happening on more than one thread, which evidently can cause problems on some operating
+   * systems.
+   *
+   * @return an array of <code>MidiDevice.Info</code> objects, one
+   * for each installed MIDI device.  If no such devices are installed,
+   * an array of length 0 is returned.
+   */
+  private static synchronized MidiDevice.Info[] getSystemMidiDeviceInfo() {
+    return MidiSystem.getMidiDeviceInfo();
+  }
+
+  /**
+   * <p>Obtains an array of information objects representing the set of all working MIDI devices available on the system.
+   * This is a replacement for {@link MidiSystem#getMidiDeviceInfo()}, and only returns fully-functional
+   * MIDI devices. If you call it on a non-Mac system, it simply delegates to the {@code javax.sound.midi} implementation.
    * On a Mac, it calls that function, but filters out the broken devices, returning only the replacement versions
    * that CoreMidi4J provides. So by using this method rather than the standard one, you can give your users a
-   * menu of MIDI devices which are guaranteed to properly support MIDI System Exclusive messages.
+   * menu of MIDI devices which are guaranteed to properly support MIDI System Exclusive messages.</p>
+   *
+   * <p>A returned information object can then be used to obtain the corresponding device object,
+   * by invoking {@link MidiSystem#getMidiDevice(MidiDevice.Info)}.</p>
    *
-   * A returned information object can then be used to obtain the corresponding device object,
-   * by invoking javax.sound.midi.MidiSystem.getMidiDevice().
+   * <p>As mentioned in {@link #addNotificationListener(CoreMidiNotification)}, if you use that method, you must
+   * only ever call this version of {@code getMidiDeviceInfo()}, never {@link MidiSystem#getMidiDeviceInfo()},
+   * due to thread-safety issues in the latter.</p>
    *
-   * @return an array of MidiDevice.Info objects, one for each installed and fully-functional MIDI device.
+   * @return an array of {@link MidiDevice.Info} objects, one for each installed and fully-functional MIDI device.
    *         If no such devices are installed, an array of length 0 is returned.
    */
 
   public static MidiDevice.Info[] getMidiDeviceInfo() {
 
-    MidiDevice.Info[] allInfo = MidiSystem.getMidiDeviceInfo();
+    MidiDevice.Info[] allInfo = getSystemMidiDeviceInfo();
 
     try {