[Java] Safely call onError on Subjects

src/SignalR/clients/java/signalr/core/src/main/java/com/microsoft/signalr/HubConnection.java

@@ -1413,14 +1413,14 @@ public void handleHandshake(ByteBuffer payload) {
                     handshakeResponse = HandshakeProtocol.parseHandshakeResponse(handshakeResponseString);
                 } catch (RuntimeException ex) {
                     RuntimeException exception = new RuntimeException("An invalid handshake response was received from the server.", ex);
-                    handshakeResponseSubject.onError(exception);
+                    errorHandshake(exception);
                     throw exception;
                 }
                 if (handshakeResponse.getHandshakeError() != null) {
                     String errorMessage = "Error in handshake " + handshakeResponse.getHandshakeError();
                     logger.error(errorMessage);
                     RuntimeException exception = new RuntimeException(errorMessage);
-                    handshakeResponseSubject.onError(exception);
+                    errorHandshake(exception);
                     throw exception;
                 }
                 handshakeReceived = true;
@@ -1431,12 +1431,7 @@ public void handleHandshake(ByteBuffer payload) {
         public void timeoutHandshakeResponse(long timeout, TimeUnit unit) {
             handshakeTimeout = Executors.newSingleThreadScheduledExecutor();
             handshakeTimeout.schedule(() -> {
-                // If onError is called on a completed subject the global error handler is called
-                if (!(handshakeResponseSubject.hasComplete() || handshakeResponseSubject.hasThrowable()))
-                {
-                    handshakeResponseSubject.onError(
-                        new TimeoutException("Timed out waiting for the server to respond to the handshake message."));
-                }
+                errorHandshake(new TimeoutException("Timed out waiting for the server to respond to the handshake message."));
             }, timeout, unit);
         }
 
@@ -1476,6 +1471,18 @@ public List<Type> getParameterTypes(String methodName) {
 
             return handlers.get(0).getTypes();
         }
+
+        private void errorHandshake(Exception error) {
+            lock.lock();
+            try {
+                // If onError is called on a completed subject the global error handler is called
+                if (!(handshakeResponseSubject.hasComplete() || handshakeResponseSubject.hasThrowable())) {
+                    handshakeResponseSubject.onError(error);
+                }
+            } finally {
+                lock.unlock();
+            }
+        }
     }
 
     // We don't have reconnect yet, but this helps align the Java client with the .NET client

src/SignalR/clients/java/signalr/test/src/main/java/com/microsoft/signalr/HubConnectionTest.java

@@ -17,6 +17,7 @@
 import java.util.concurrent.atomic.AtomicReference;
 
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
 
 import ch.qos.logback.classic.spi.ILoggingEvent;
 import io.reactivex.rxjava3.core.Completable;
@@ -29,6 +30,7 @@
 import io.reactivex.rxjava3.subjects.ReplaySubject;
 import io.reactivex.rxjava3.subjects.SingleSubject;
 
+@ExtendWith({RxJavaUnhandledExceptionsExtensions.class})
 class HubConnectionTest {
     private static final String RECORD_SEPARATOR = "\u001e";
     private static final Type booleanType = (new TypeReference<Boolean>() { }).getType();

src/SignalR/clients/java/signalr/test/src/main/java/com/microsoft/signalr/RxJavaUnhandledExceptionsExtensions.java

@@ -0,0 +1,41 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+package com.microsoft.signalr;
+
+import java.io.PrintWriter;
+import java.io.StringWriter;
+import java.util.concurrent.BlockingQueue;
+import java.util.concurrent.LinkedBlockingQueue;
+
+import org.junit.jupiter.api.extension.AfterAllCallback;
+import org.junit.jupiter.api.extension.BeforeAllCallback;
+import org.junit.jupiter.api.extension.ExtensionContext;
+
+import io.reactivex.rxjava3.plugins.RxJavaPlugins;
+
+// Use by adding "@ExtendWith({RxJavaUnhandledExceptionsExtensions.class})" to a test class
+class RxJavaUnhandledExceptionsExtensions implements BeforeAllCallback, AfterAllCallback {
+    private final BlockingQueue<Throwable> errors = new LinkedBlockingQueue<Throwable>();
+
+    @Override
+    public void beforeAll(final ExtensionContext context) {
+        RxJavaPlugins.setErrorHandler(error -> {
+            errors.put(error);
+        });
+    }
+
+    @Override
+    public void afterAll(final ExtensionContext context) {
+        if (errors.size() != 0) {
+            String RxErrors = "";
+            for (final Throwable throwable : errors) {
+                StringWriter stringWriter = new StringWriter();
+                PrintWriter printWriter = new PrintWriter(stringWriter);
+                throwable.printStackTrace(printWriter);
+                RxErrors += String.format("%s\n", stringWriter.toString());
+            }
+            throw new RuntimeException(RxErrors);
+        }
+    }
+}