Reland "[lldb] [Process] Watch for fork/vfork notifications" for FreeBSD

The original commit was reverted because of the problems it introduced
on Linux.  However, FreeBSD should not be affected, so restore that part
and we will address Linux separately.

While at it, remove the dbreg hack as the underlying issue has been
fixed in the FreeBSD kernel and the problem is unlikely to happen
in real life use anyway.

Differential Revision: https://reviews.llvm.org/D98822

lldb/source/Plugins/Process/FreeBSD/NativeProcessFreeBSD.cpp

@@ -247,6 +247,19 @@ void NativeProcessFreeBSD::MonitorSIGTRAP(lldb::pid_t pid) {
     return;
   }
 
+  if (info.pl_flags & PL_FLAG_FORKED) {
+    MonitorClone(info.pl_child_pid);
+    return;
+  }
+
+  if (info.pl_flags & PL_FLAG_VFORK_DONE) {
+    Status error =
+        PtraceWrapper(PT_CONTINUE, pid, reinterpret_cast<void *>(1), 0);
+    if (error.Fail())
+      SetState(StateType::eStateInvalid);
+    return;
+  }
+
   if (info.pl_lwpid > 0) {
     for (const auto &t : m_threads) {
       if (t->GetID() == static_cast<lldb::tid_t>(info.pl_lwpid))
@@ -705,17 +718,17 @@ NativeProcessFreeBSD::GetFileLoadAddress(const llvm::StringRef &file_name,
 
 void NativeProcessFreeBSD::SigchldHandler() {
   Log *log(ProcessPOSIXLog::GetLogIfAllCategoriesSet(POSIX_LOG_PROCESS));
-  // Process all pending waitpid notifications.
   int status;
   ::pid_t wait_pid =
       llvm::sys::RetryAfterSignal(-1, waitpid, GetID(), &status, WNOHANG);
 
   if (wait_pid == 0)
-    return; // We are done.
+    return;
 
   if (wait_pid == -1) {
     Status error(errno, eErrorTypePOSIX);
     LLDB_LOG(log, "waitpid ({0}, &status, _) failed: {1}", GetID(), error);
+    return;
   }
 
   WaitStatus wait_status = WaitStatus::Decode(status);
@@ -885,7 +898,7 @@ Status NativeProcessFreeBSD::SetupTrace() {
       PtraceWrapper(PT_GET_EVENT_MASK, GetID(), &events, sizeof(events));
   if (status.Fail())
     return status;
-  events |= PTRACE_LWP;
+  events |= PTRACE_LWP | PTRACE_FORK | PTRACE_VFORK;
   status = PtraceWrapper(PT_SET_EVENT_MASK, GetID(), &events, sizeof(events));
   if (status.Fail())
     return status;
@@ -919,3 +932,39 @@ Status NativeProcessFreeBSD::ReinitializeThreads() {
 bool NativeProcessFreeBSD::SupportHardwareSingleStepping() const {
   return !m_arch.IsMIPS();
 }
+
+void NativeProcessFreeBSD::MonitorClone(::pid_t child_pid) {
+  Log *log(ProcessPOSIXLog::GetLogIfAllCategoriesSet(POSIX_LOG_PROCESS));
+  LLDB_LOG(log, "fork, child_pid={0}", child_pid);
+
+  int status;
+  ::pid_t wait_pid =
+      llvm::sys::RetryAfterSignal(-1, ::waitpid, child_pid, &status, 0);
+  if (wait_pid != child_pid) {
+    LLDB_LOG(log,
+             "waiting for pid {0} failed. Assuming the pid has "
+             "disappeared in the meantime",
+             child_pid);
+    return;
+  }
+  if (WIFEXITED(status)) {
+    LLDB_LOG(log,
+             "waiting for pid {0} returned an 'exited' event. Not "
+             "tracking it.",
+             child_pid);
+    return;
+  }
+
+  MainLoop unused_loop;
+  NativeProcessFreeBSD child_process{static_cast<::pid_t>(child_pid),
+                                     m_terminal_fd, m_delegate, m_arch,
+                                     unused_loop};
+  child_process.Detach();
+  Status pt_error =
+      PtraceWrapper(PT_CONTINUE, GetID(), reinterpret_cast<void *>(1), 0);
+  if (pt_error.Fail()) {
+    LLDB_LOG_ERROR(log, pt_error.ToError(),
+                   "unable to resume parent process {1}: {0}", GetID());
+    SetState(StateType::eStateInvalid);
+  }
+}

lldb/source/Plugins/Process/FreeBSD/NativeProcessFreeBSD.h

@@ -113,6 +113,7 @@ private:
   void MonitorSIGSTOP(lldb::pid_t pid);
   void MonitorSIGTRAP(lldb::pid_t pid);
   void MonitorSignal(lldb::pid_t pid, int signal);
+  void MonitorClone(::pid_t child_pid);
 
   Status PopulateMemoryRegionCache();
   void SigchldHandler();

lldb/test/Shell/Subprocess/Inputs/fork.cpp

@@ -0,0 +1,50 @@
+#include <sys/types.h>
+#include <sys/wait.h>
+#include <assert.h>
+#if defined(TEST_CLONE)
+#include <sched.h>
+#endif
+#include <stdint.h>
+#include <stdio.h>
+#include <unistd.h>
+
+int g_val = 0;
+
+void parent_func() {
+  g_val = 1;
+  printf("function run in parent\n");
+}
+
+int child_func(void *unused) {
+  // we need to avoid memory modifications for vfork(), yet we want
+  // to be able to test watchpoints, so do the next best thing
+  // and restore the original value
+  g_val = 2;
+  g_val = 0;
+  return 0;
+}
+
+int main() {
+  alignas(uintmax_t) char stack[4096];
+
+#if defined(TEST_CLONE)
+  pid_t pid = clone(child_func, &stack[sizeof(stack)], 0, NULL);
+#elif defined(TEST_FORK)
+  pid_t pid = TEST_FORK();
+  if (pid == 0)
+    _exit(child_func(NULL));
+#endif
+  assert(pid != -1);
+
+  parent_func();
+  int status, wait_flags = 0;
+#if defined(TEST_CLONE)
+  wait_flags = __WALL;
+#endif
+  pid_t waited = waitpid(pid, &status, wait_flags);
+  assert(waited == pid);
+  assert(WIFEXITED(status));
+  printf("child exited: %d\n", WEXITSTATUS(status));
+
+  return 0;
+}

lldb/test/Shell/Subprocess/fork-follow-parent-wp.test

@@ -0,0 +1,13 @@
+# REQUIRES: native && dbregs-set
+# UNSUPPORTED: system-windows
+# RUN: %clangxx_host -g %p/Inputs/fork.cpp -DTEST_FORK=fork -o %t
+# RUN: %lldb -b -s %s %t | FileCheck %s
+process launch -s
+watchpoint set variable -w write g_val
+# CHECK: Watchpoint created:
+continue
+# CHECK-NOT: function run in parent
+# CHECK: stop reason = watchpoint
+continue
+# CHECK: function run in parent
+# CHECK: child exited: 0

lldb/test/Shell/Subprocess/fork-follow-parent.test

@@ -0,0 +1,11 @@
+# REQUIRES: native
+# UNSUPPORTED: system-windows
+# RUN: %clangxx_host %p/Inputs/fork.cpp -DTEST_FORK=fork -o %t
+# RUN: %lldb -b -s %s %t | FileCheck %s
+b parent_func
+process launch
+# CHECK-NOT: function run in parent
+# CHECK: stop reason = breakpoint
+continue
+# CHECK: function run in parent
+# CHECK: child exited: 0

lldb/test/Shell/Subprocess/lit.local.cfg

@@ -0,0 +1,2 @@
+if 'lldb-repro' in config.available_features:
+  config.unsupported = True

lldb/test/Shell/Subprocess/vfork-follow-parent-wp.test

@@ -0,0 +1,14 @@
+# REQUIRES: native && dbregs-set
+# UNSUPPORTED: system-windows
+# UNSUPPORTED: system-darwin
+# RUN: %clangxx_host -g %p/Inputs/fork.cpp -DTEST_FORK=vfork -o %t
+# RUN: %lldb -b -s %s %t | FileCheck %s
+process launch -s
+watchpoint set variable -w write g_val
+# CHECK: Watchpoint created:
+continue
+# CHECK-NOT: function run in parent
+# CHECK: stop reason = watchpoint
+continue
+# CHECK: function run in parent
+# CHECK: child exited: 0

lldb/test/Shell/Subprocess/vfork-follow-parent.test

@@ -0,0 +1,11 @@
+# REQUIRES: native
+# UNSUPPORTED: system-windows
+# RUN: %clangxx_host %p/Inputs/fork.cpp -DTEST_FORK=vfork -o %t
+# RUN: %lldb -b -s %s %t | FileCheck %s
+b parent_func
+process launch
+# CHECK-NOT: function run in parent
+# CHECK: stop reason = breakpoint
+continue
+# CHECK: function run in parent
+# CHECK: child exited: 0