[java] Make Preferences support for legacy dashboards opt-in

This removes one of the triggers of #8215 (but not the underlying cause).

The code to set new topics discovered under /Preferences as persistent was
added back in 2015 in 87fc49c. Dashboards that want to want data for new keys
to be saved should be explicitly marking the topics as persistant.

Having a listener that updates new topics in the background can result in
a closed NetworkTablesInstance being accessed (resulting in a SIGSEGV) when
Preferences.setNetworkTableInstance() is called soon after new topics are
added.

It's possible this race condition is why PreferencesTest.removeAllTest() was
flaky.

Changes:
- Add enableLegacyDashboardSupport()
- Only create a listener in setNetworkTableInstance() if
  enableLegacyDashboardSupport() was called

Author: kcooney

wpilibj/src/main/java/edu/wpi/first/wpilibj/Preferences.java

@@ -45,6 +45,7 @@ public final class Preferences {
   private static StringPublisher m_typePublisher;
   private static MultiSubscriber m_tableSubscriber;
   private static NetworkTableListener m_listener;
+  private static boolean m_supportLegacyDashboards;
 
   /** Creates a preference class. */
   private Preferences() {}
@@ -78,11 +79,40 @@ public static synchronized void setNetworkTableInstance(NetworkTableInstance ins
     }
     m_tableSubscriber = new MultiSubscriber(inst, new String[] {m_table.getPath() + "/"});
 
-    // Listener to set all Preferences values to persistent
-    // (for backwards compatibility with old dashboards).
     if (m_listener != null) {
       m_listener.close();
+      m_listener = null;
     }
+    if (m_supportLegacyDashboards) {
+      createTopicPersistingListener();
+    }
+  }
+
+  /**
+   * Enables support for dashboards that create Preferences topics without marking them as
+   * persistent.
+   */
+  public static synchronized void enableLegacyDashboardSupport() {
+    if (!m_supportLegacyDashboards) {
+      m_supportLegacyDashboards = true;
+      createTopicPersistingListener();
+    }
+  }
+
+  static synchronized void disableLegacyDashboardSupport() {
+    if (m_supportLegacyDashboards) {
+      m_supportLegacyDashboards = false;
+      m_listener.close();
+      m_listener = null;
+    }
+  }
+
+  /**
+   * Creates a listener that update all Preference topics to be persistent.
+   *
+   * <p>This is done for backwards compatibility with old dashboards.
+   */
+  private static synchronized void createTopicPersistingListener() {
     m_listener =
         NetworkTableListener.createListener(
             m_tableSubscriber,

wpilibj/src/test/java/edu/wpi/first/wpilibj/PreferencesTest.java

@@ -12,15 +12,21 @@
 import static org.junit.jupiter.api.parallel.ExecutionMode.SAME_THREAD;
 
 import edu.wpi.first.networktables.NetworkTable;
+import edu.wpi.first.networktables.NetworkTableEntry;
+import edu.wpi.first.networktables.NetworkTableEvent;
 import edu.wpi.first.networktables.NetworkTableInstance;
+import edu.wpi.first.networktables.NetworkTableListener;
 import edu.wpi.first.networktables.Topic;
 import java.io.IOException;
 import java.io.InputStream;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.EnumSet;
 import java.util.List;
+import java.util.concurrent.Semaphore;
+import java.util.concurrent.TimeUnit;
 import java.util.stream.Stream;
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.BeforeEach;
@@ -69,6 +75,8 @@ void setup(@TempDir Path tempDir) {
 
   @AfterEach
   void cleanup() {
+    Preferences.disableLegacyDashboardSupport();
+    m_inst.waitForListenerQueue(0.1);
     m_inst.close();
   }
 
@@ -121,6 +129,45 @@ void defaultValueTest() {
         () -> assertFalse(Preferences.getBoolean("checkedValueBoolean", true)));
   }
 
+  @Test
+  void enableLegacyDashboardSupportTest() throws InterruptedException {
+    // Publish a value, wait until we are sure the listener would have fired, and verify that the
+    // topic is not marked as persistant.
+    Semaphore semaphore = new Semaphore(0);
+    NetworkTableEntry entry = m_table.getEntry("legacyDashboardValueLong");
+    try (NetworkTableListener listener =
+        NetworkTableListener.createListener(
+            entry,
+            EnumSet.of(NetworkTableEvent.Kind.kImmediate, NetworkTableEvent.Kind.kValueAll),
+            event -> {
+              if (event.valueData != null) {
+                semaphore.release();
+              }
+            })) {
+      // Set a value, wait for listeners to fire, and do that again. This ensures any listener for
+      // thus entry would have been called at least once.
+      for (int value = 1; value < 3; value++) {
+        entry.setInteger(value);
+        if (!semaphore.tryAcquire(100, TimeUnit.MILLISECONDS)) {
+          fail("timed out waiting for event listener");
+        }
+      }
+
+      assertFalse(entry.isPersistent());
+
+      // When legacy dashboard support is enabled, the topic should be marked as persistant.
+      Preferences.enableLegacyDashboardSupport();
+      for (int value = 3; value < 5; value++) {
+        entry.setInteger(value);
+        if (!semaphore.tryAcquire(100, TimeUnit.MILLISECONDS)) {
+          fail("timed out waiting for event listener");
+        }
+      }
+
+      assertTrue(entry.isPersistent());
+    }
+  }
+
   @Nested
   class PutGetTests {
     @Test