mavsdk_server: fix crash on stop/destruction (mavlink#2417)

This fixes a segfault which happens when the mavsdk_server is stopped
and destroyed before every having discovered any autopilot.

What happened is that:
1. ConnectionInitiator would be cancelled.
2. ConnectionInitiator would be destructed.
3. And only now the connect thread would wake up from sleeping and read
   the _should_exit flag which at this point has been destroyed.
   At this point the connect function would try to access
   Mavsdk::systems() which is also destructed by that point and cause a
   segfault.

The fix is to wrap the ConnectionInitiator class in a shared ptr and
keep that one alive in the connect function to avoid destruction until
it has returned.

src/mavsdk_server/src/connection_initiator.h

@@ -1,25 +1,26 @@
 #pragma once
 
+#include <atomic>
 #include <chrono>
 #include <future>
+#include <memory>
 #include <mutex>
 #include <thread>
 #include <string>
 
 #include "connection_result.h"
 #include "log.h"
+#include "mavsdk.h"
 
 namespace mavsdk {
 namespace mavsdk_server {
 
-template<typename Mavsdk> class ConnectionInitiator {
+class ConnectionInitiator : public std::enable_shared_from_this<ConnectionInitiator> {
 public:
-    ConnectionInitiator() {}
-    ~ConnectionInitiator() {}
-
-    bool connect(Mavsdk& mavsdk, const std::string& connection_url)
+    bool connect(mavsdk::Mavsdk& mavsdk, const std::string& connection_url)
     {
-        LogInfo() << "Waiting to discover system on " << connection_url << "...";
+        // Keep this class alive while this function is running.
+        auto self = shared_from_this();
 
         if (!add_any_connection(mavsdk, connection_url)) {
             return false;
@@ -49,7 +50,7 @@ template<typename Mavsdk> class ConnectionInitiator {
     void cancel() { _should_exit = true; }
 
 private:
-    bool add_any_connection(Mavsdk& mavsdk, const std::string& connection_url)
+    bool add_any_connection(mavsdk::Mavsdk& mavsdk, const std::string& connection_url)
     {
         mavsdk::ConnectionResult connection_result = mavsdk.add_any_connection(connection_url);
 

src/mavsdk_server/src/mavsdk_server.cpp

@@ -16,7 +16,8 @@ class MavsdkServer::Impl {
 
     bool connect(const std::string& connection_url)
     {
-        return _connection_initiator.connect(_mavsdk, connection_url);
+        _connection_initiator = std::make_shared<ConnectionInitiator>();
+        return _connection_initiator->connect(_mavsdk, connection_url);
     }
 
     int startGrpcServer(const int port)
@@ -31,7 +32,7 @@ class MavsdkServer::Impl {
 
     void stop()
     {
-        _connection_initiator.cancel();
+        _connection_initiator->cancel();
 
         if (_server != nullptr) {
             _server->stop();
@@ -47,7 +48,7 @@ class MavsdkServer::Impl {
 
 private:
     mavsdk::Mavsdk _mavsdk;
-    ConnectionInitiator<mavsdk::Mavsdk> _connection_initiator;
+    std::shared_ptr<ConnectionInitiator> _connection_initiator;
     std::unique_ptr<GrpcServer> _server;
     int _grpc_port;
 };