[lldb] [llgs] Include process ID in stop responses

Include the process identifier in the `T` stop responses when
multiprocess extension is enabled (i.e. prepend it to the thread
identifier).  Use the exposed identifier to simplify the fork-and-follow
tests.

The LLDB client accounts for the possible PID since the multiprocess
extension support was added in b601c67.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.llvm.org/D127192

Author: Michał Górny

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

@@ -809,8 +809,12 @@ GDBRemoteCommunicationServerLLGS::SendStopReplyPacketForThread(
   // Print the signal number.
   response.PutHex8(signum & 0xff);
 
-  // Include the tid.
-  response.Printf("thread:%" PRIx64 ";", tid);
+  // Include the (pid and) tid.
+  response.PutCString("thread:");
+  if (bool(m_extensions_supported &
+           NativeProcessProtocol::Extension::multiprocess))
+    response.Format("p{0:x-}.", m_current_process->GetID());
+  response.Format("{0:x-};", tid);
 
   // Include the thread name if there is one.
   const std::string thread_name = thread->GetName();

lldb/test/API/tools/lldb-server/TestGdbRemoteFork.py

@@ -101,15 +101,12 @@ def fork_and_follow_test(self, variant):
         self.reset_test_sequence()
 
         # continue and expect fork
-        procinfo_regex = "[$]pid:([0-9a-f]+);.*"
-        fork_regex = "[$]T.*;{}:p([0-9a-f]+)[.]([0-9a-f]+).*".format(variant)
+        fork_regex = ("[$]T[0-9a-f]{{2}}thread:p([0-9a-f]+)[.][0-9a-f]+;.*"
+                      "{}:p([0-9a-f]+)[.]([0-9a-f]+).*".format(variant))
         self.test_sequence.add_log_lines([
-            "read packet: $qProcessInfo#00",
-            {"direction": "send", "regex": procinfo_regex,
-             "capture": {1: "parent_pid"}},
             "read packet: $c#00",
             {"direction": "send", "regex": fork_regex,
-             "capture": {1: "pid", 2: "tid"}},
+             "capture": {1: "parent_pid", 2: "pid", 3: "tid"}},
         ], True)
         ret = self.expect_gdbremote_sequence()
         parent_pid, pid, tid = (int(ret[x], 16) for x