Issue #18244 has been updated by Eregon (Benoit Daloze).


I believe the real issue here is to `$stderr.sync = false`, why do you do that?
That's like explicitly breaking what you are asking.

If it's for performance because you are writing a lot to stderr, then why are you writing so much to stderr?

This is not only about exceptions, with `$stderr.sync = false` the output will be interleaved in unclear ways anyway.
Maybe you want something like `$stderr = $stdout` so both are actually the same and use a single buffer?
Both `$stderr` and `$stdout` unbuffered would also work.

----------------------------------------
Bug #18244: Unexpected errors output order
https://bugs.ruby-lang.org/issues/18244#change-94081

* Author: inversion (Yura Babak)
* Status: Feedback
* Priority: Normal
* ruby -v: Ruby v3.0.2.107 (2021-07-07) [x64-mingw32]
* Backport: 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
Having this code:
```ruby
# $stderr.sync = false
warn 'warn'
$stderr.puts 'err'
fail
```
When I run it like this `ruby test.rb 2>err.log`, output is ok:
```
warn
err
test.rb:3:in `<main>': unhandled exception
```

But when I uncomment the first line ― the output is different:
```
warn
test.rb:3:in `<main>': unhandled exception
err
```
Why buffered output to the `$stderr` redirected to file has the **wrong order**?
If we do not redirect to the file ― the order is ok (exception is the last line of the output).

Checked platforms:
`Windows10: Ruby v3.0.2.107 (2021-07-07) [x64-mingw32]`
`Linux: Ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-linux]`



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