Add AsioContextStore::stop()

This is meant to help with the following scenario: main thread
creates secondary threads, each secondary thread asks the context
store for its own context, then goes into a "forever" scheduling
loop. Main thread waits for the secondary threads to finish, and
owns the context store. It would be nice to be able to have yet
another thread (say, a SIGINT handler) be able to call .stop()
on all the contexts managed by the context store, thus being able
to end the loops in the secondary threads and allow the main
thread to exit.

Signed-off-by: Razvan Cojocaru <razvan.cojocaru@openvpn.com>

openvpn/asio/asiocontext.hpp

@@ -36,12 +36,25 @@ class AsioContextStore
     {
         openvpn_io::io_context *ioc = new openvpn_io::io_context(concurrency_hint);
         {
-            std::lock_guard<std::mutex> lock(mutex);
+            std::lock_guard lock(mutex);
             contexts.emplace_back(ioc);
         }
         return *ioc;
     }
 
+    /**
+     *  This is to be used only as a last resort. The proper way to end an
+     *  io_context-driven thread is to simply stop scheduling work on the reactor
+     *  and exit gracefully. DO NOT USE THIS IF THERE'S AN ALTERNATIVE!
+     */
+    void stop()
+    {
+        std::lock_guard lock(mutex);
+
+        for (auto &context : contexts)
+            context->stop();
+    }
+
   private:
     std::mutex mutex;
     std::vector<std::unique_ptr<openvpn_io::io_context>> contexts;