[FIXED] Early closed connection may linger in the server

If the connection is marked as closed while sending the INFO, the
connection would not be removed from the internal map, which would
cause it to be shown in the monitoring list of opened connections.

Resolves #1384

Signed-off-by: Ivan Kozlovic <ivan@synadia.com>

server/client_test.go

@@ -2038,3 +2038,34 @@ func TestCloseConnectionLogsReason(t *testing.T) {
 		t.Fatal("Log does not contain closed reason")
 	}
 }
+
+func TestCloseConnectionVeryEarly(t *testing.T) {
+	o := DefaultOptions()
+	s := RunServer(o)
+	defer s.Shutdown()
+
+	// The issue was with a connection that would break right when
+	// server was sending the INFO. Creating a bare TCP connection
+	// and closing it right away won't help reproduce the problem.
+	// So testing in 2 steps.
+
+	// Get a normal TCP connection to the server.
+	c, err := net.Dial("tcp", fmt.Sprintf("127.0.0.1:%d", o.Port))
+	if err != nil {
+		s.mu.Unlock()
+		t.Fatalf("Unable to create tcp connection")
+	}
+	// Now close it.
+	c.Close()
+
+	// Wait that num clients falls to 0.
+	checkClientsCount(t, s, 0)
+
+	// Call again with this closed connection. Alternatively, we
+	// would have to call with a fake connection that implements
+	// net.Conn but returns an error on Write.
+	s.createClient(c)
+
+	// This connection should not have been added to the server.
+	checkClientsCount(t, s, 0)
+}