fix: stuck MJPEG clients when one client fails to initialize

app/build.gradle

@@ -63,6 +63,8 @@ android {
                         '--add-exports', 'java.base/sun.nio.ch=ALL-UNNAMED',
                         '--add-opens', 'java.base/java.lang.reflect=ALL-UNNAMED',
                         '--add-opens', 'java.base/java.io=ALL-UNNAMED',
+                        '--add-opens', 'java.base/java.net=ALL-UNNAMED',
+                        '--add-opens', 'java.base/sun.net.www.protocol.http=ALL-UNNAMED',
                         '--add-exports', 'jdk.unsupported/sun.misc=ALL-UNNAMED',
                 ])
             }

app/src/main/java/io/appium/uiautomator2/server/mjpeg/MjpegScreenshotClient.java

@@ -102,8 +102,10 @@ void initialize() {
         }
 
         try {
-            while (!in.ready()) {
+            if (!in.ready()) {
                SystemClock.sleep(INPUT_NOT_READY_SLEEP_TIME_MS);
+               // Client initialization will be retried on the next frame
+               return;
             }
 
             Logger.info(String.format(

app/src/test/java/io/appium/uiautomator2/server/MjpegScreenshotTest.java

@@ -0,0 +1,75 @@
+package io.appium.uiautomator2.server.mjpeg;
+
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.After;
+import org.junit.Before;
+import static org.junit.Assert.*;
+
+import java.net.URL;
+import java.net.HttpURLConnection;
+import java.util.Collections;
+import java.nio.charset.StandardCharsets;
+import java.net.Socket;
+
+import static org.mockito.Mockito.spy;
+import org.powermock.api.mockito.PowerMockito;
+import org.powermock.core.classloader.annotations.PrepareForTest;
+import org.powermock.modules.junit4.PowerMockRunner;
+
+import io.appium.uiautomator2.server.mjpeg.MjpegScreenshotServer;
+import io.appium.uiautomator2.server.mjpeg.MjpegScreenshotStream;
+import io.appium.uiautomator2.server.ServerConfig;
+
+@RunWith(PowerMockRunner.class)
+@PrepareForTest(MjpegScreenshotStream.class)
+public class MjpegScreenshotTest {
+  private static MjpegScreenshotServer serverThread;
+  private static int streamingPort;
+
+  @Before
+  public void setUp() throws Exception {
+    // Create a MJPEG server with a mocked getScreenshot method
+    MjpegScreenshotStream mockScreenshotStreamSpy =
+        spy(new MjpegScreenshotStream(Collections.emptyList()));
+    byte[] mockScreenshotData = "screenshot data".getBytes(StandardCharsets.UTF_8);
+    PowerMockito.stub(PowerMockito.method(MjpegScreenshotStream.class, "getScreenshot"))
+        .toReturn(mockScreenshotData);
+    PowerMockito.whenNew(MjpegScreenshotStream.class)
+        .withAnyArguments()
+        .thenReturn(mockScreenshotStreamSpy);
+
+    streamingPort = ServerConfig.DEFAULT_MJPEG_SERVER_PORT;
+    serverThread = new MjpegScreenshotServer(streamingPort);
+    serverThread.start();
+    assertTrue(serverThread.isAlive());
+  }
+
+  @Test
+  public void shouldReceiveDataFromMjpegServer() throws Exception {
+    URL url = new URL(String.format("http://localhost:%d", streamingPort));
+    HttpURLConnection connection = (HttpURLConnection) url.openConnection();
+    connection.setRequestMethod("GET");
+    int responseCode = connection.getResponseCode();
+    assertEquals(HttpURLConnection.HTTP_OK, responseCode);
+  }
+
+  @Test
+  public void shouldNotBlockOnUnInitializedClient() throws Exception {
+    // Create a socket and connect to the streaming server: it will create an uninitialized client
+    Socket socket = new Socket("localhost", streamingPort);
+    socket.close();
+
+    // We should still be able to receive data
+    URL url = new URL(String.format("http://localhost:%d", streamingPort));
+    HttpURLConnection connection = (HttpURLConnection) url.openConnection();
+    connection.setRequestMethod("GET");
+    int responseCode = connection.getResponseCode();
+    assertEquals(HttpURLConnection.HTTP_OK, responseCode);
+  }
+
+  @After
+  public void tearDown() {
+    serverThread.interrupt();
+  }
+}