5 files changed, 15 insertions, 6 deletions
diff --git a/lldb/source/Host/common/Host.cpp b/lldb/source/Host/common/Host.cpp
index be206406e93..3ba9ab7f21f 100644
--- a/lldb/source/Host/common/Host.cpp
+++ b/lldb/source/Host/common/Host.cpp
@@ -99,7 +99,7 @@ struct MonitorInfo {
 
 static thread_result_t MonitorChildProcessThreadFunction(void *arg);
 
-HostThread Host::StartMonitoringChildProcess(
+llvm::Expected<HostThread> Host::StartMonitoringChildProcess(
     const Host::MonitorChildProcessCallback &callback, lldb::pid_t pid,
     bool monitor_signals) {
   MonitorInfo *info_ptr = new MonitorInfo();
@@ -112,7 +112,7 @@ HostThread Host::StartMonitoringChildProcess(
   ::snprintf(thread_name, sizeof(thread_name),
              "<lldb.host.wait4(pid=%" PRIu64 ")>", pid);
   return ThreadLauncher::LaunchThread(
-      thread_name, MonitorChildProcessThreadFunction, info_ptr, nullptr);
+      thread_name, MonitorChildProcessThreadFunction, info_ptr, 0);
 }
 
 #ifndef __linux__
diff --git a/lldb/source/Host/common/HostProcess.cpp b/lldb/source/Host/common/HostProcess.cpp
index d8f71af5e65..e180687551f 100644
--- a/lldb/source/Host/common/HostProcess.cpp
+++ b/lldb/source/Host/common/HostProcess.cpp
@@ -32,7 +32,7 @@ lldb::pid_t HostProcess::GetProcessId() const {
 
 bool HostProcess::IsRunning() const { return m_native_process->IsRunning(); }
 
-HostThread
+llvm::Expected<HostThread>
 HostProcess::StartMonitoring(const Host::MonitorChildProcessCallback &callback,
                              bool monitor_signals) {
   return m_native_process->StartMonitoring(callback, monitor_signals);
diff --git a/lldb/source/Host/common/MonitoringProcessLauncher.cpp b/lldb/source/Host/common/MonitoringProcessLauncher.cpp
index 2ee69353e19..55e9f69a089 100644
--- a/lldb/source/Host/common/MonitoringProcessLauncher.cpp
+++ b/lldb/source/Host/common/MonitoringProcessLauncher.cpp
@@ -53,8 +53,12 @@ MonitoringProcessLauncher::LaunchProcess(const ProcessLaunchInfo &launch_info,
     Log *log(lldb_private::GetLogIfAllCategoriesSet(LIBLLDB_LOG_PROCESS));
 
     assert(launch_info.GetMonitorProcessCallback());
-    process.StartMonitoring(launch_info.GetMonitorProcessCallback(),
-                            launch_info.GetMonitorSignals());
+    llvm::Expected<HostThread> maybe_thread =
+        process.StartMonitoring(launch_info.GetMonitorProcessCallback(),
+                                launch_info.GetMonitorSignals());
+    if (!maybe_thread)
+      error.SetErrorStringWithFormatv("failed to launch host thread: {}",
+                                      llvm::toString(maybe_thread.takeError()));
     if (log)
       log->PutCString("started monitoring child process.");
   } else {
diff --git a/lldb/source/Host/common/ProcessLaunchInfo.cpp b/lldb/source/Host/common/ProcessLaunchInfo.cpp
index ac8a41bf883..266b4676399 100644
--- a/lldb/source/Host/common/ProcessLaunchInfo.cpp
+++ b/lldb/source/Host/common/ProcessLaunchInfo.cpp
@@ -188,8 +188,13 @@ bool ProcessLaunchInfo::NoOpMonitorCallback(lldb::pid_t pid, bool exited, int si
 
 bool ProcessLaunchInfo::MonitorProcess() const {
   if (m_monitor_callback && ProcessIDIsValid()) {
+    llvm::Expected<HostThread> maybe_thread =
     Host::StartMonitoringChildProcess(m_monitor_callback, GetProcessID(),
                                       m_monitor_signals);
+    if (!maybe_thread)
+      LLDB_LOG(lldb_private::GetLogIfAllCategoriesSet(LIBLLDB_LOG_HOST),
+               "failed to launch host thread: {}",
+               llvm::toString(maybe_thread.takeError()));
     return true;
   }
   return false;
diff --git a/lldb/source/Host/posix/HostProcessPosix.cpp b/lldb/source/Host/posix/HostProcessPosix.cpp
index 725f59d7512..cc187d44246 100644
--- a/lldb/source/Host/posix/HostProcessPosix.cpp
+++ b/lldb/source/Host/posix/HostProcessPosix.cpp
@@ -87,7 +87,7 @@ bool HostProcessPosix::IsRunning() const {
   return error.Success();
 }
 
-HostThread HostProcessPosix::StartMonitoring(
+llvm::Expected<HostThread> HostProcessPosix::StartMonitoring(
     const Host::MonitorChildProcessCallback &callback, bool monitor_signals) {
   return Host::StartMonitoringChildProcess(callback, m_process,
                                            monitor_signals);