Marton,
Below are my notes on blocking and non-blocking modes in protocols. Proposed
changes that I'd like to make are at the end.
Thanks,
Andriy
---General notes--
-blocking mode-
- transfer_function deals with polling and timeout
- block inside transfer_function for max rw_timeout.
- if rw_timeout expires the protocol designers have two options:
(1) transfer_funciton returns AVERROR(ETIMEDOUT). This will cause the
encoding/decoding to exit with timeout error.
(2) transfer_function returns AVERROR(EINTR). Immediately retry the
transfer_function after checking interrupt callback.
-non-blocking mode-
- retry_transfer_wrapper deals with retries
- if read/writes are not available transfer_function should immediately
return with AVERROR(EAGAIN)
- retry transfer_function with 5 x fast_retry (no sleep). Then, retry with
1ms sleep.
- If rw_timeout expires, exit encoding/decoding with AVERROR(EIO) (Probably
should be changed to AVERROR(ETIMEDOUT) to be consistent with blocking
mdoe.
But overall, the two modes seem similar to me:
- "Blocking" - blocks at the transfer_function
- "Non-blocking" - blocks in retry_transfer_function
----Potential advantage of non-blocking over blocking mode----
I'm trying to figure what would be the advantage of non-blocking over blocking
modes. The only reason I can think of: non-blocking mode gives more control to
the user rather than protocol designer. In non-blocking mode the user can force
an exit after rw_timeout expires. On the other hand, in blocking mode, the
protocol designer can force a retry by returing AVERROR(EINTR) after rw_timeout
expires instead of exiting the code (i.e. after AVERROR(ETIMEDOUT) is returned).
Any other reason someone would prefer non-blocking over blocking modes?
----Proposed changes----
(1) At the moment, non-blocking mode immediately exits after AVERROR(EAGAIN)
is returned without doing any retries. I'm quite sure there is a bug on
avio.c:379. It should be changed to: if (!(h->flags & AVIO_FLAG_NONBLOCK))
(2) The default value for rw_timeout = 0. This value implies an infinite
wait in both blocking/non-blocking mode. The documentation in
docs/protocols.texi should say this.
(3) Update avio.h/url.h to clarify difference between AVERROR(ETIMEDOUT) and
AVERROR(EINTR) in blocking mode. Clarify what is the potential advantage of
non-blocking mode over blocking mode.
(4) Add non-blocking option to the cli. Something like -avioflags
nonblocking. But add a note that this may not be supported by a specific
protocol
(5) If rw_timeout expires in non-blocking mode, return AVERROR(ETIMEDOUT)
instead of AVERROR(EIO) to be consistent with blocking mode.
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