[lldb] [gdb-remote] Fix process ID after following forked child

[mgorny]

Summary

Update the process ID after handling fork/vfork to ensure that
the process plugin reports the correct PID immediately.

Sponsored by: The FreeBSD Foundation

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

void ProcessGDBRemote::DidFork(lldb::pid_t child_pid, lldb::tid_t child_tid) {
   if (error.Fail()) {
     LLDB_LOG(log, "ProcessGDBRemote::DidFork() detach packet send failed: {0}",
              error.AsCString() ? error.AsCString() : "<unknown error>");
     return;
   }
 
   // Hardware breakpoints/watchpoints are not inherited implicitly,
   // so we need to readd them if we're following child.
-  if (GetFollowForkMode() == eFollowChild)
+  if (GetFollowForkMode() == eFollowChild) {
     DidForkSwitchHardwareTraps(true);
+    // Update our PID
+    SetID(child_pid);
+  }
 }
 
 void ProcessGDBRemote::DidVFork(lldb::pid_t child_pid, lldb::tid_t child_tid) {
   Log *log = GetLog(GDBRLog::Process);
 
   assert(!m_vfork_in_progress);
   m_vfork_in_progress = true;
 
void ProcessGDBRemote::DidVFork(lldb::pid_t child_pid, lldb::tid_t child_tid) {
   LLDB_LOG(log, "Detaching process {0}", detach_pid);
   Status error = m_gdb_comm.Detach(false, detach_pid);
   if (error.Fail()) {
       LLDB_LOG(log,
                "ProcessGDBRemote::DidFork() detach packet send failed: {0}",
                 error.AsCString() ? error.AsCString() : "<unknown error>");
       return;
   }
+
+  if (GetFollowForkMode() == eFollowChild) {
+    // Update our PID
+    SetID(child_pid);
+  }
 }
 
 void ProcessGDBRemote::DidVForkDone() {
   assert(m_vfork_in_progress);
   m_vfork_in_progress = false;
 
   // Reenable all software breakpoints that were enabled before vfork.
   if (m_gdb_comm.SupportsGDBStoppointPacket(eBreakpointSoftware))

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

 from __future__ import print_function
 import lldb
 import unittest
 from lldbsuite.test.lldbtest import *
 from lldbsuite.test.decorators import *
 from lldbsuite.test.gdbclientutils import *
 from lldbsuite.test.lldbgdbclient import GDBRemoteTestBase
 
 
 class TestMultiprocess(GDBRemoteTestBase):
 
-    def base_test(self, variant):
+    def base_test(self, variant, follow_child=False):
         class MyResponder(MockGDBServerResponder):
             def __init__(self):
                 super().__init__()
                 self.first = True
                 self.detached = None
                 self.property = "{}-events+".format(variant)
 
             def qSupported(self, client_supported):
def base_test(self, variant, follow_child=False):
                 return "OK"
 
         self.server.responder = MyResponder()
         target = self.dbg.CreateTarget('')
         if self.TraceOn():
           self.runCmd("log enable gdb-remote packets")
           self.addTearDownHook(
                 lambda: self.runCmd("log disable gdb-remote packets"))
+        if follow_child:
+            self.runCmd("settings set target.process.follow-fork-mode child")
         process = self.connect(target)
+        self.assertEqual(process.GetProcessID(), 1024)
         process.Continue()
-        self.assertRegex(self.server.responder.detached, r"D;0*401")
+        self.assertRegex(self.server.responder.detached,
+                         r"D;0*400" if follow_child else r"D;0*401")
+        self.assertEqual(process.GetProcessID(),
+                         1025 if follow_child else 1024)
 
     def test_fork(self):
         self.base_test("fork")
 
     def test_vfork(self):
         self.base_test("vfork")
+
+    def test_fork_follow_child(self):
+        self.base_test("fork", follow_child=True)
+
+    def test_vfork_follow_child(self):
+        self.base_test("vfork", follow_child=True)