Hi, I have found a bug as described in the title. The serve command hangs when a client attempts to open multiple connections and decides to send requests out-of-order, which actually seems be the behavior of real web browsers when dealing with multiple calls to the window.fetch(...) JS function. I can reproduce it with both the latest Firefox and Chromium.

This "httptest" repository is a simple reproducer - just clone it, serve it, and open the served index.html in a browser. It just runs the fetch() function on 10 files filled with zeroes. When everything loads, a paragraph should pop up saying Loaded all.. In the current main it stays on Loading... indefinitely.

I have a fix ready using epoll to make the TCP accept and the actual serving asynchronous relatively to each other, but that limits the serve command to Linux, so it's probably not usable as-is? I can open a PR nevertheless, if desirable.