[ruby-core:107762] [Ruby master Bug#18476] Call to require stuck forever after receiving EAGAIN on writev when running with zeus

Issue #18476 has been updated by JelteF (Jelte Fennema).


I looked into this a bit more. And I'm pretty sure this is related to the introduction of the fiber scheduler.

----------------------------------------
Bug #18476: Call to require stuck forever after receiving EAGAIN on writev when running with zeus
https://bugs.ruby-lang.org/issues/18476#change-96687

* Author: JelteF (Jelte Fennema)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.0.3p157 (2021-11-24 revision 3fb7d2cadc) [x86_64-linux]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
# Environment

I'm using Ubuntu 18.04 running on Windows using WSL2:
```
$ uname -a
Linux myhostname 5.10.60.1-microsoft-standard-WSL2 #1 SMP Wed Aug 25 23:20:18 UTC 2021 x86_64 x86_64 x86_64 GNU/Linux
$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.6 LTS
Release:    18.04
Codename:   bionic
```

# Problem description
When using `zeus` to run a ruby project in an effort to cache dependency loading it gets stuck when calling `require 'rubocop'` on Ruby 3.0 and 3.1. Everything works fine on Ruby 2.6.

When looking at strace output of the stuck process it shows a `writev` call that receives an `EAGAIN` right before the process gets to its stuck state:
```
[pid 24872] writev(14, [{iov_base="/home/jelte/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/rubocop-1.23.0/lib/rubocop/cop/lint/number_conversion.rb", iov_len=115}, {iov_base="\n", iov_len=1}], 2) = 116
[pid 24872] writev(14, [{iov_base="/home/jelte/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/rubocop-1.23.0/lib/rubocop/cop/lint/numbered_parameter_assignment.rb", iov_len=127}, {iov_base="\n", iov_len=1}], 2) = 128
[pid 24872] writev(14, [{iov_base="/home/jelte/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/rubocop-1.23.0/lib/rubocop/cop/lint/or_assignment_to_constant.rb", iov_len=123}, {iov_base="\n", iov_len=1}], 2) = 124
[pid 24872] writev(14, [{iov_base="/home/jelte/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/rubocop-1.23.0/lib/rubocop/cop/lint/ordered_magic_comments.rb", iov_len=120}, {iov_base="\n", iov_len=1}], 2) = 121
[pid 24872] writev(14, [{iov_base="/home/jelte/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/rubocop-1.23.0/lib/rubocop/cop/lint/out_of_range_regexp_ref.rb", iov_len=121}, {iov_base="\n", iov_len=1}], 2) = 122
[pid 24872] writev(14, [{iov_base="/home/jelte/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/rubocop-1.23.0/lib/rubocop/cop/lint/parentheses_as_grouped_expression"..., iov_len=131}, {iov_base="\n", iov_len=1}], 2) = 132
[pid 24872] writev(14, [{iov_base="/home/jelte/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/rubocop-1.23.0/lib/rubocop/cop/lint/percent_string_array.rb", iov_len=118}, {iov_base="\n", iov_len=1}], 2) = 119
[pid 24872] writev(14, [{iov_base="/home/jelte/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/rubocop-1.23.0/lib/rubocop/cop/lint/percent_symbol_array.rb", iov_len=118}, {iov_base="\n", iov_len=1}], 2) = 119
[pid 24872] writev(14, [{iov_base="/home/jelte/.rbenv/versions/3.1.0/lib/ruby/gems/3.1.0/gems/rubocop-1.23.0/lib/rubocop/cop/lint/raise_exception.rb", iov_len=113}, {iov_base="\n", iov_len=1}], 2) = -1 EAGAIN (Resource temporarily unavailable)
[pid 24872] getpid()                    = 24872
[pid 24872] ppoll([{fd=14, events=POLLOUT}, {fd=7, events=POLLIN}], 2, NULL, NULL, 8 <unfinished ...>
[pid 24859] <... futex resumed> )       = -1 ETIMEDOUT (Connection timed out)
[pid 24859] select(0, NULL, NULL, NULL, {tv_sec=0, tv_usec=20}) = 0 (Timeout)
[pid 24859] futex(0x605f38, FUTEX_WAIT, 0, {tv_sec=60, tv_nsec=0}
```

The exact file at which the process receives the EAGAIN and is stuck is consistent across runs with the same Ruby version (at least on my machine). For 3.1.0 it's `raise_exception.rb` and for 3.0.3 it's `redundant_dir_glob_sort.rb`

For reference this is the link to the repo of `zeus`: https://github.com/burke/zeus


# How to reproduce
See this example repo for minimal steps on how to reproduce this: https://github.com/JelteF/ruby-zeus-bug

# Expectation
For Ruby to not get stuck after receiving `EAGAIN` on the `writev` call.



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