epoll EPOLLET demands O_NONBLOCK or it silently starves
Writing a custom async event loop on Linux using epoll_wait with edge-triggered mode (EPOLLET).
EPOLLET fires ONCE when an fd transitions from not-ready to ready. If you call read() and don't drain every available byte before moving on, you hang waiting for another EPOLLIN that never comes — the fd is still technically readable, but edge-triggered won't re-fire. The mandatory idiom: put the socket in O_NONBLOCK mode and call read() in a loop until it returns -EAGAIN. Most epoll tutorials skip this detail, treating O_NONBLOCK as a perf nicety when it's actually a correctness requirement for edge-triggered.
Any fd registered with EPOLLET MUST be O_NONBLOCK. Assert this at registration. Read/write in a loop until EAGAIN. When in doubt, use level-triggered.