dnfdaemon: Limit number of simultaneously active sessions

dnf5daemon-server/session_manager.cpp

@@ -26,11 +26,15 @@ along with libdnf.  If not, see <https://www.gnu.org/licenses/>.
 #include <sdbus-c++/sdbus-c++.h>
 
 #include <iostream>
+#include <numeric>
 #include <random>
 #include <sstream>
 #include <string>
 #include <thread>
 
+// TODO(mblaha): Make this constant configurable
+const int MAX_SESSIONS = 3;
+
 SessionManager::SessionManager() {
     connection = sdbus::createSystemBusConnection(dnfdaemon::DBUS_NAME);
     dbus_register();
@@ -98,6 +102,14 @@ sdbus::MethodReply SessionManager::open_session(sdbus::MethodCall & call) {
     if (!active) {
         throw sdbus::Error(dnfdaemon::ERROR, "Cannot open new session.");
     }
+    // limit number of simultaneously opened sessions
+    const int num_sessions = std::accumulate(
+        sessions.begin(), sessions.end(), 0, [](int sum, const auto & sender) { return sum + sender.second.size(); });
+    if (num_sessions >= MAX_SESSIONS) {
+        auto reply = call.createErrorReply(sdbus::Error(
+            dnfdaemon::ERROR, "Cannot open new session - maximal number of simultaneously opened sessions achieved."));
+        return reply;
+    }
 
     auto sender = call.getSender();
     dnfdaemon::KeyValueMap configuration;