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    • Florian Fischer's avatar
      [IO] introduce emper::io a IO subsystem using io_uring · 460c2f05
      Florian Fischer authored
      Empers IO design is based on a proactor pattern where each worker
      can issue IO requests through its exclusive IoContext object which wraps an
      io_uring instance.
      
      IO completions are reaped at 4 places:
      1. After a submit to collect inline completions
      2. Before dispatching a new Fiber
      3. When no new IO can be submitted because the completion queue is full
      4. And by a global completer thread which gets notified about completions
         on worker IoContexts through registered eventfds
      
      All IO requests are modeled as Future objects which can be either
      instantiated and submitted manually, retrieved by POSIX-like non-blocking
      or implicitly used by posix-like blocking functions.
      
      User facing API is exported in the following headers:
      * emper/io.hpp (POSIX-like)
      * emper.h (POSIX-like)
      * emper/io/Future.hpp
      
      Catching short write/reads/sends and resubmitting the request without
      unblocking the Fiber is supported.
      
      Using AlarmFuture objects Fibers have a emper-native way to sleep for
      a given time.
      
      IO request timeouts with TimeoutWrapper class.
      Request Cancellation is supported with Future::cancel() or the
      CancelWrapper() Future class.
      
      A proactor design demands that buffers are committed to the kernel
      as long as the request is active. To guaranty memory safety Futures
      get canceled in their Destructor which will only return after the committed
      memory is free to use.
      
      Linking Futures to chains is supported using the Future::SetDependency()
      method. Future are submitted when their last Future gets submitted.
      A linked Request will start if the previous has finished.
      Error or partial completions will cancel the not started tail of a chain.
      
      TODO: Handle possible situations where the CQ of the global completer is full
      and no more sqe can be submitted to the SQ.
      460c2f05
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