[ruby-core:104582] [Ruby master Bug#17664] Behavior of sockets changed in Ruby 3.0 to non-blocking

Issue #17664 has been updated by ioquatix (Samuel Williams).


I rewrote the `uring` implementation to track the number of pending operations. I'm kind of surprised that the SQ doesn't do this.

```
% time     seconds  usecs/call     calls    errors syscall
------ ----------- ----------- --------- --------- ------------------
 26.59    0.049621           4     10575           close
 24.85    0.046369           4     10335           write
 17.34    0.032363           3     10338         3 accept4
 13.77    0.025693           2     10727        79 read
 11.82    0.022051           2     10727         9 newfstatat
  5.31    0.009911           4      2079           mprotect
  0.11    0.000207           1       192           mmap
  0.09    0.000165           1        87           munmap
  0.05    0.000087           1        48           brk
  0.04    0.000083           1        83         1 io_uring_enter
```

Now for non-blocking sockets, we only see `io_uring_enter` count proportional to the number of `read` and `write` errors which makes more sense to me.

```
Running 30s test @ http://localhost:9090
  4 threads and 128 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency     2.87ms    1.36ms  90.28ms   98.60%
    Req/Sec    11.16k     1.40k   26.84k    70.73%
  1332172 requests in 30.04s, 58.44MB read
Requests/sec:  44341.92
Transfer/sec:      1.95MB
```

As you can imagine, with only 89 calls to `io_uring_enter`, whether or not it's done early or later has little impact on overall performance.

----------------------------------------
Bug #17664: Behavior of sockets changed in Ruby 3.0 to non-blocking
https://bugs.ruby-lang.org/issues/17664#change-92865

* Author: ciconia (Sharon Rosner)
* Status: Assigned
* Priority: Normal
* Assignee: ioquatix (Samuel Williams)
* ruby -v: 3.0.0
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
I'm not sure this is a bug, but apparently a change was introduced in Ruby 3.0 that makes sockets non-blocking by default. This change was apparently introduced as part of the work on the [FiberScheduler interface](https://github.com/ruby/ruby/blame/78f188524f551c97b1a7a44ae13514729f1a21c7/ext/socket/init.c#L411-L434). This change of behaviour is not discussed in the Ruby 3.0.0 release notes.

This change complicates the implementation of an io_uring-based fiber scheduler, since io_uring SQE's on fd's with `O_NONBLOCK` can return `EAGAIN` just like normal syscalls. Using io_uring with non-blocking fd's defeats the whole purpose of using io_uring in the first place.

A workaround I have put in place in the Polyphony [io_uring backend](https://github.com/digital-fabric/polyphony/blob/d3c9cf3ddc1f414387948fa40e5f6a24f68bf045/ext/polyphony/backend_io_uring.c#L28-L47) is to make sure `O_NONBLOCK` is not set before attempting I/O operations on any fd.



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