[lldb][debugserver] Synchronize interrupt and resume signals (#131073)

This PR fixes a race condition in debugserver where the main thread
calls MachProcess::Interrupt, setting `m_sent_interrupt_signo` while the
exception monitoring thread is checking the value of the variable.

I was on the fence between introducing a new mutex and reusing the
existing exception mutex. With the notable exception of
MachProcess::Interrupt, all the other places where we were already
locking this mutex before accessing the variable. I renamed the mutex to
make it clear that it's now protecting more than the exception messages.

Jason, while investigating a real issue, had a suspicion there was race
condition related to interrupts and I was able to narrow it down by
building debugserver with TSan.

Author: JDevlieghere

lldb/tools/debugserver/source/MacOSX/MachProcess.h

@@ -427,16 +427,16 @@ class MachProcess {
       m_profile_data_mutex; // Multithreaded protection for profile info data
   std::vector<std::string>
       m_profile_data; // Profile data, must be protected by m_profile_data_mutex
-  PThreadEvent m_profile_events; // Used for the profile thread cancellable wait  
+  PThreadEvent m_profile_events; // Used for the profile thread cancellable wait
   DNBThreadResumeActions m_thread_actions; // The thread actions for the current
                                            // MachProcess::Resume() call
   MachException::Message::collection m_exception_messages; // A collection of
                                                            // exception messages
                                                            // caught when
                                                            // listening to the
                                                            // exception port
-  PThreadMutex m_exception_messages_mutex; // Multithreaded protection for
-                                           // m_exception_messages
+  PThreadMutex m_exception_and_signal_mutex; // Multithreaded protection for
+                                             // exceptions and signals.
 
   MachThreadList m_thread_list; // A list of threads that is maintained/updated
                                 // after each stop

lldb/tools/debugserver/source/MacOSX/MachProcess.mm

@@ -528,7 +528,7 @@ static bool FBSAddEventDataToOptions(NSMutableDictionary *options,
       m_profile_data_mutex(PTHREAD_MUTEX_RECURSIVE), m_profile_data(),
       m_profile_events(0, eMachProcessProfileCancel), m_thread_actions(),
       m_exception_messages(),
-      m_exception_messages_mutex(PTHREAD_MUTEX_RECURSIVE), m_thread_list(),
+      m_exception_and_signal_mutex(PTHREAD_MUTEX_RECURSIVE), m_thread_list(),
       m_activities(), m_state(eStateUnloaded),
       m_state_mutex(PTHREAD_MUTEX_RECURSIVE), m_events(0, kAllEventsMask),
       m_private_events(0, kAllEventsMask), m_breakpoints(), m_watchpoints(),
@@ -1338,8 +1338,11 @@ static bool mach_header_validity_test(uint32_t magic, uint32_t cputype) {
   m_stop_count = 0;
   m_thread_list.Clear();
   {
-    PTHREAD_MUTEX_LOCKER(locker, m_exception_messages_mutex);
+    PTHREAD_MUTEX_LOCKER(locker, m_exception_and_signal_mutex);
     m_exception_messages.clear();
+    m_sent_interrupt_signo = 0;
+    m_auto_resume_signo = 0;
+
   }
   m_activities.Clear();
   StopProfileThread();
@@ -1575,6 +1578,7 @@ static uint64_t bits(uint64_t value, uint32_t msbit, uint32_t lsbit) {
 bool MachProcess::Interrupt() {
   nub_state_t state = GetState();
   if (IsRunning(state)) {
+    PTHREAD_MUTEX_LOCKER(locker, m_exception_and_signal_mutex);
     if (m_sent_interrupt_signo == 0) {
       m_sent_interrupt_signo = SIGSTOP;
       if (Signal(m_sent_interrupt_signo)) {
@@ -1728,7 +1732,7 @@ static uint64_t bits(uint64_t value, uint32_t msbit, uint32_t lsbit) {
     m_thread_actions.Append(thread_action);
     m_thread_actions.SetDefaultThreadActionIfNeeded(eStateRunning, 0);
 
-    PTHREAD_MUTEX_LOCKER(locker, m_exception_messages_mutex);
+    PTHREAD_MUTEX_LOCKER(locker, m_exception_and_signal_mutex);
 
     ReplyToAllExceptions();
   }
@@ -1854,7 +1858,7 @@ static uint64_t bits(uint64_t value, uint32_t msbit, uint32_t lsbit) {
 }
 
 void MachProcess::ReplyToAllExceptions() {
-  PTHREAD_MUTEX_LOCKER(locker, m_exception_messages_mutex);
+  PTHREAD_MUTEX_LOCKER(locker, m_exception_and_signal_mutex);
   if (!m_exception_messages.empty()) {
     MachException::Message::iterator pos;
     MachException::Message::iterator begin = m_exception_messages.begin();
@@ -1888,7 +1892,7 @@ static uint64_t bits(uint64_t value, uint32_t msbit, uint32_t lsbit) {
   }
 }
 void MachProcess::PrivateResume() {
-  PTHREAD_MUTEX_LOCKER(locker, m_exception_messages_mutex);
+  PTHREAD_MUTEX_LOCKER(locker, m_exception_and_signal_mutex);
 
   m_auto_resume_signo = m_sent_interrupt_signo;
   if (m_auto_resume_signo)
@@ -2290,7 +2294,7 @@ static uint64_t bits(uint64_t value, uint32_t msbit, uint32_t lsbit) {
 // data has already been copied.
 void MachProcess::ExceptionMessageReceived(
     const MachException::Message &exceptionMessage) {
-  PTHREAD_MUTEX_LOCKER(locker, m_exception_messages_mutex);
+  PTHREAD_MUTEX_LOCKER(locker, m_exception_and_signal_mutex);
 
   if (m_exception_messages.empty())
     m_task.Suspend();
@@ -2304,7 +2308,7 @@ static uint64_t bits(uint64_t value, uint32_t msbit, uint32_t lsbit) {
 
 task_t MachProcess::ExceptionMessageBundleComplete() {
   // We have a complete bundle of exceptions for our child process.
-  PTHREAD_MUTEX_LOCKER(locker, m_exception_messages_mutex);
+  PTHREAD_MUTEX_LOCKER(locker, m_exception_and_signal_mutex);
   DNBLogThreadedIf(LOG_EXCEPTIONS, "%s: %llu exception messages.",
                    __PRETTY_FUNCTION__, (uint64_t)m_exception_messages.size());
   bool auto_resume = false;