mem: multi-clients support for SharedMemoryServer

Record the client session with a map instead of a single unique_ptr so
our server can interact with multiple clients at once.

This will also avoid a race condition case where the client thought it
has closed previous connection and is trying to a new one while the
server hasn't clean up the previous entry and raise a fatal error.

Change-Id: Id08154fc4b54d2611629875b3f4e0d66c0e2ed92
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/61049
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Yu-hsin Wang <yuhsingw@google.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>

src/mem/shared_memory_server.hh

@@ -30,6 +30,7 @@
 
 #include <memory>
 #include <string>
+#include <unordered_map>
 
 #include "base/pollevent.hh"
 #include "params/SharedMemoryServer.hh"
@@ -86,7 +87,8 @@ class SharedMemoryServer : public SimObject
 
     int serverFd;
     std::unique_ptr<ListenSocketEvent> listenSocketEvent;
-    std::unique_ptr<ClientSocketEvent> clientSocketEvent;
+    std::unordered_map<int, std::unique_ptr<ClientSocketEvent>>
+        clientSocketEvents;
 };
 
 } // namespace memory