[lldb] [gdb-remote] Always send PID when detaching w/ multiprocess

Always send PID in the detach packet when multiprocess extensions are
enabled.  This is required by qemu's GDB server, as plain 'D' packet
results in an error and the emulated system is not resumed.

Differential Revision: https://reviews.llvm.org/D110033

lldb/source/Plugins/Process/gdb-remote/GDBRemoteCommunicationClient.cpp

@@ -1543,14 +1543,16 @@ Status GDBRemoteCommunicationClient::Detach(bool keep_stopped,
     }
   }
 
-  if (pid != LLDB_INVALID_PROCESS_ID) {
-    if (!m_supports_multiprocess) {
-      error.SetErrorString(
-          "Multiprocess extension not supported by the server.");
-      return error;
-    }
+  if (m_supports_multiprocess) {
+    // Some servers (e.g. qemu) require specifying the PID even if only a single
+    // process is running.
+    if (pid == LLDB_INVALID_PROCESS_ID)
+      pid = GetCurrentProcessID();
     packet.PutChar(';');
     packet.PutHex64(pid);
+  } else if (pid != LLDB_INVALID_PROCESS_ID) {
+    error.SetErrorString("Multiprocess extension not supported by the server.");
+    return error;
   }
 
   StringExtractorGDBRemote response;

lldb/test/API/functionalities/gdb_remote_client/TestGDBRemoteClient.py

@@ -354,3 +354,46 @@ class TestGDBRemoteClient(GDBRemoteTestBase):
           "QEnvironmentHexEncoded:4e45454453454e43343d6623726f62",
           "QEnvironmentHexEncoded:455155414c533d666f6f3d626172",
         ])
+
+    def test_detach_no_multiprocess(self):
+        class MyResponder(MockGDBServerResponder):
+            def __init__(self):
+                super().__init__()
+                self.detached = None
+
+            def qfThreadInfo(self):
+                return "10200"
+
+            def D(self, packet):
+                self.detached = packet
+                return "OK"
+
+        self.server.responder = MyResponder()
+        target = self.dbg.CreateTarget('')
+        process = self.connect(target)
+        process.Detach()
+        self.assertEqual(self.server.responder.detached, "D")
+
+    def test_detach_pid(self):
+        class MyResponder(MockGDBServerResponder):
+            def __init__(self, test_case):
+                super().__init__()
+                self.test_case = test_case
+                self.detached = None
+
+            def qSupported(self, client_supported):
+                self.test_case.assertIn("multiprocess+", client_supported)
+                return "multiprocess+;" + super().qSupported(client_supported)
+
+            def qfThreadInfo(self):
+                return "mp400.10200"
+
+            def D(self, packet):
+                self.detached = packet
+                return "OK"
+
+        self.server.responder = MyResponder(self)
+        target = self.dbg.CreateTarget('')
+        process = self.connect(target)
+        process.Detach()
+        self.assertRegex(self.server.responder.detached, r"D;0*400")