[wpiutil] SafeThread: Provide start/stop hooks (#4880)

Co-authored-by: David Vo <auscompgeek@users.noreply.github.com>

wpiutil/src/main/native/cpp/SafeThread.cpp

@@ -4,8 +4,38 @@
 
 #include "wpi/SafeThread.h"
 
+#include <atomic>
+
 using namespace wpi;
 
+// thread start/stop notifications for bindings that need to set up
+// per-thread state
+
+static void* DefaultOnThreadStart() {
+  return nullptr;
+}
+static void DefaultOnThreadEnd(void*) {}
+
+using OnThreadStartFn = void* (*)();
+using OnThreadEndFn = void (*)(void*);
+static std::atomic<int> gSafeThreadRefcount;
+static std::atomic<OnThreadStartFn> gOnSafeThreadStart{DefaultOnThreadStart};
+static std::atomic<OnThreadEndFn> gOnSafeThreadEnd{DefaultOnThreadEnd};
+
+namespace wpi::impl {
+void SetSafeThreadNotifiers(OnThreadStartFn OnStart, OnThreadEndFn OnEnd) {
+  if (gSafeThreadRefcount != 0) {
+    throw std::runtime_error(
+        "cannot set notifier while safe threads are running");
+  }
+  // Note: there's a race here, but if you're not calling this function on
+  // the main thread before you start anything else, you're using this function
+  // incorrectly
+  gOnSafeThreadStart = OnStart ? OnStart : DefaultOnThreadStart;
+  gOnSafeThreadEnd = OnEnd ? OnEnd : DefaultOnThreadEnd;
+}
+}  // namespace wpi::impl
+
 void SafeThread::Stop() {
   m_active = false;
   m_cond.notify_all();
@@ -43,7 +73,13 @@ void detail::SafeThreadOwnerBase::Start(std::shared_ptr<SafeThreadBase> thr) {
   if (auto thr = m_thread.lock()) {
     return;
   }
-  m_stdThread = std::thread([=] { thr->Main(); });
+  m_stdThread = std::thread([=] {
+    gSafeThreadRefcount++;
+    void* opaque = (gOnSafeThreadStart.load())();
+    thr->Main();
+    (gOnSafeThreadEnd.load())(opaque);
+    gSafeThreadRefcount--;
+  });
   thr->m_threadId = m_stdThread.get_id();
   m_thread = thr;
 }