[ruby-core:117778] [Ruby master Bug#18995] IO#set_encoding sometimes set an IO's internal encoding to the default external encoding

["javanthropus (Jeremy Bopp) via ruby-core" <ruby-core@ml.ruby-lang.org>]

Issue #18995 has been updated by javanthropus (Jeremy Bopp).


@jeremyevans, did you ever take a look at this issue when I referenced it in #18899?  The behavior is unchanged in Ruby 3.3.

The script above prints the following:
```
external encoding: #<Encoding:UTF-8>          internal encoding: #<Encoding:ISO-8859-2>   
external encoding: #<Encoding:UTF-8>          internal encoding: #<Encoding:ISO-8859-1>   
external encoding: #<Encoding:UTF-8>          internal encoding: #<Encoding:ISO-8859-3>
```

I expected it to print this:
```
external encoding: #<Encoding:UTF-8>          internal encoding: #<Encoding:ISO-8859-2>
external encoding: #<Encoding:UTF-8>          internal encoding: #<Encoding:ISO-8859-2>
external encoding: #<Encoding:UTF-8>          internal encoding: #<Encoding:ISO-8859-4>
```

----------------------------------------
Bug #18995: IO#set_encoding sometimes set an IO's internal encoding to the default external encoding
https://bugs.ruby-lang.org/issues/18995#change-108185

* Author: javanthropus (Jeremy Bopp)
* Status: Open
* ruby -v: ruby 3.1.2p20 (2022-04-12 revision 4491bb740a) [x86_64-linux]
* Backport: 2.7: UNKNOWN, 3.0: UNKNOWN, 3.1: UNKNOWN
----------------------------------------
This script demonstrates the behavior:

```ruby
def show(io)
  printf(
    "external encoding: %-25p  internal encoding: %-25p\n",
    io.external_encoding,
    io.internal_encoding
  )
end

Encoding.default_external = 'iso-8859-1'
Encoding.default_internal = 'iso-8859-2'

File.open('/dev/null') do |f|
  f.set_encoding('utf-8', nil)
  show(f)                             # f.internal_encoding is iso-8859-2, as expected

  f.set_encoding('utf-8', 'invalid')
  show(f)                             # f.internal_encoding is now iso-8859-1!

  Encoding.default_external = 'iso-8859-3'
  Encoding.default_internal = 'iso-8859-4'
  show(f)                             # f.internal_encoding is now iso-8859-3!
end
```

In the 1st case, we see that the IO's internal encoding is set to the current setting of Encoding.default_internal. In the 2nd case, the IO's internal encoding is set to Encoding.default_external instead. The 3rd case is more interesting because it shows that the IO's internal encoding is actually following the current setting of Encoding.default_external. It didn't just copy it when #set_encoding was called. It changes whenever Encoding.default_external changes.

What should the correct behavior be?



-- 
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