8338708: Don't create/destroy debug agent cmdQueueLock for each connection

Reviewed-by: amenkov, lmesnik

src/jdk.jdwp.agent/share/native/libjdwp/debugLoop.c

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 1998, 2022, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 1998, 2024, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -87,7 +87,9 @@ debugLoop_run(void)
     /* Initialize all statics */
     /* We may be starting a new connection after an error */
     cmdQueue = NULL;
-    cmdQueueLock = debugMonitorCreate("JDWP Command Queue Lock");
+    if (cmdQueueLock == NULL) {
+      cmdQueueLock = debugMonitorCreate("JDWP Command Queue Lock");
+    }
     transportError = JNI_FALSE;
 
     shouldListen = JNI_TRUE;
@@ -190,7 +192,6 @@ debugLoop_run(void)
      * be trying to send.
      */
     transport_close();
-    debugMonitorDestroy(cmdQueueLock);
 
     /* Reset for a new connection to this VM if it's still alive */
     if ( ! gdata->vmDead ) {

test/jdk/com/sun/jdi/ReattachStressTest.java

@@ -0,0 +1,126 @@
+/*
+ * Copyright (c) 2024, Oracle and/or its affiliates. All rights reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
+ * or visit www.oracle.com if you need additional information or have any
+ * questions.
+ */
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.util.Map;
+
+import jdk.test.lib.process.ProcessTools;
+
+import com.sun.jdi.Bootstrap;
+import com.sun.jdi.VirtualMachine;
+import com.sun.jdi.connect.AttachingConnector;
+import com.sun.jdi.connect.Connector;
+import com.sun.jdi.connect.IllegalConnectorArgumentsException;
+
+/**
+ * @test
+ * @bug 8338708
+ * @summary Stress test for reattaching to a debuggee
+ * @library /test/lib
+ * @modules jdk.jdi
+ * @run driver ProcessAttachTest
+ */
+
+class ReattachStressTestTarg {
+    public static void main(String args[]) throws Exception {
+        System.out.println("Debuggee started");
+        while (true) {
+            try {
+                Thread.sleep(100);
+            } catch (InterruptedException e) {
+            }
+        }
+    }
+}
+
+public class ReattachStressTest {
+    public static void main(String[] args) throws Exception {
+        System.out.println("Test 1: Debuggee start with suspend=n");
+        runTest("-agentlib:jdwp=transport=dt_socket,server=y,suspend=n");
+
+        System.out.println("Test 2: Debuggee start with suspend=y");
+        runTest("-agentlib:jdwp=transport=dt_socket,server=y,suspend=y");
+    }
+
+    private static void runTest(String jdwpArg) throws Exception {
+        ProcessBuilder pb = ProcessTools.createTestJavaProcessBuilder(
+                jdwpArg,
+                "ReattachStressTestTarg");
+        Process p = null;
+        try {
+            p = pb.start();
+
+            // Read the first character of output to make sure we've waited until the
+            // debuggee is ready. This will be the debug agent's "Listening..." message.
+            InputStream is = p.getInputStream();
+            is.read();
+
+            // Attach a debugger
+            tryDebug(p.pid(), is);
+        } finally {
+            p.destroyForcibly();
+        }
+    }
+
+    private static void tryDebug(long pid, InputStream is) throws IOException,
+            IllegalConnectorArgumentsException {
+        // Get the ProcessAttachingConnector, which can attach using the pid of the debuggee.
+        AttachingConnector ac = Bootstrap.virtualMachineManager().attachingConnectors()
+                .stream()
+                .filter(c -> c.name().equals("com.sun.jdi.ProcessAttach"))
+                .findFirst()
+                .orElseThrow(() -> new RuntimeException("Unable to locate ProcessAttachingConnector"));
+
+        // Set the connector's "pid" argument to the pid of the debuggee.
+        Map<String, Connector.Argument> args = ac.defaultArguments();
+        Connector.StringArgument arg = (Connector.StringArgument)args.get("pid");
+        arg.setValue("" + pid);
+
+        // Loop that will repeatedly attach and detach from the same debuggee.
+        for (int i = 0; i < 500; i++) {
+            System.out.println(i + ": Debugger is attaching to: " + pid + " ...");
+
+            // Attach to the debuggee.
+            VirtualMachine vm = ac.attach(args);
+
+            // Drain remaining "Listening..." output.  Otherwise too much
+            // output will buffer up and the debuggee may block until it is cleared.
+            while (is.available() > 0) {
+                is.read();
+            }
+
+            // We've attached. Do some things that will send JDWP commands.
+            System.out.println("Attached!");
+            System.out.println("JVM name: " + vm.name());
+            System.out.println("Num threads: " + vm.allThreads().size());
+
+            // We're all done with this debugger connection.
+            vm.dispose();
+
+            // Wait for first char of next "Listening..." output.
+            is.read();
+        }
+        System.out.println("Debugger done.");
+    }
+}