Fix a race condition with tagged thread startup

There's a very rare race condition where a tagged thread doesn't seem to
exist when the proxy tries syncing startup with them, and so the proxy
thread hangs in startup.

This addresses it by avoiding looking at the `proxy_thread` variable
(which probably isn't thread safe) in the worker's startup, and
signalling the you-need-to-shutdown condition via a third option for the
(formerly boolean) `tagged_go`.

oxenmq/oxenmq.cpp

@@ -431,7 +431,7 @@ OxenMQ::~OxenMQ() {
             // up, so signal them so that they can end themselves.
             {
                 std::lock_guard lock{tagged_startup_mutex};
-                tagged_go = true;
+                tagged_go = tagged_go_mode::SHUTDOWN;
             }
             tagged_cv.notify_all();
             for (auto& [run, busy, queue] : tagged_workers)

oxenmq/oxenmq.h

@@ -788,7 +788,8 @@ private:
     /// then wait via this bool/c.v. to synchronize startup with the proxy thread.  This mutex isn't
     /// used after startup is complete.
     std::mutex tagged_startup_mutex;
-    bool tagged_go{false};
+    enum class tagged_go_mode { WAIT, GO, SHUTDOWN };
+    tagged_go_mode tagged_go = tagged_go_mode::WAIT;
     std::condition_variable tagged_cv;
 
 public:

oxenmq/proxy.cpp

@@ -455,7 +455,7 @@ void OxenMQ::proxy_loop_init() {
         OMQ_LOG(debug, "Waiting for tagged workers");
         {
             std::unique_lock lock{tagged_startup_mutex};
-            tagged_go = true;
+            tagged_go = tagged_go_mode::GO;
         }
         tagged_cv.notify_all();
         std::unordered_set<std::string_view> waiting_on;

oxenmq/worker.cpp

@@ -71,9 +71,9 @@ void OxenMQ::worker_thread(unsigned int index, std::optional<std::string> tagged
         // is running).
         {
             std::unique_lock lock{tagged_startup_mutex};
-            tagged_cv.wait(lock, [this] { return tagged_go; });
+            tagged_cv.wait(lock, [this] { return tagged_go != tagged_go_mode::WAIT; });
         }
-        if (!proxy_thread.joinable()) // OxenMQ destroyed without starting
+        if (tagged_go == tagged_go_mode::SHUTDOWN) // OxenMQ destroyed without starting
             return;
         tagged_socket.emplace(context, zmq::socket_type::dealer);
     }