We expect to get valid fds to close in ~Connection in ~Server

src/connection.cpp

@@ -59,7 +59,14 @@ Connection::Connection(struct sockaddr_storage addr, int fd, int64_t id,
 
 Connection::~Connection() {
   handler_->on_connection_closed(*this);
-  assert(fd_ < 0 && "Connection fd was not closed before ~Connection");
+  if (fd_ >= 0) {
+    int e = ::close(fd_);
+    if (e == -1 && errno != EINTR) {
+      perror("close");
+      std::abort();
+    }
+    // EINTR ignored - fd is guaranteed closed on Linux
+  }
 }
 
 void Connection::close() {