[ruby-core:96758] [Ruby master Bug#16497] StringIO#internal_encoding is broken (more severely in 2.7)

Issue #16497 has been reported by zverok (Victor Shepelev).

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Bug #16497: StringIO#internal_encoding is broken (more severely in 2.7)
https://bugs.ruby-lang.org/issues/16497

* Author: zverok (Victor Shepelev)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: 
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN
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To the best of my understanding from [Encoding](https://docs.ruby-lang.org/en/master/Encoding.html) docs, the following is true:

* external encoding (explicitly specified or taken from `Encoding.default_external`) specifies how the IO understands input and stores it internally
* internal encoding (explicitly specified or taken from `Encoding.default_internal`) specifies how the IO converts what it reads.

Demonstration with regular files:

```ruby
# prepare data
File.write('test.txt', 'Україна'.encode('KOI8-U'), encoding: 'KOI8-U') #=> 7

def test(io)
  str = io.read
  [io.external_encoding, io.internal_encoding, str, str.encoding]
end

# read it:
test(File.open('test.txt', 'r:KOI8-U'))
# => [#<Encoding:KOI8-U>, nil, "\xF5\xCB\xD2\xC1\xA7\xCE\xC1", #<Encoding:KOI8-U>]

# We can specify internal encoding when opening the file:
test(File.open('test.txt', 'r:KOI8-U:UTF-8'))
# => [#<Encoding:KOI8-U>, #<Encoding:UTF-8>, "Україна", #<Encoding:UTF-8>]

# ...or when it is already opened
test(File.open('test.txt').tap { |f| f.set_encoding('KOI8-U', 'UTF-8') })
# => [#<Encoding:KOI8-U>, #<Encoding:UTF-8>, "Україна", #<Encoding:UTF-8>]

# ...or with Encoding.default_internal
Encoding.default_internal = 'UTF-8'
test(File.open('test.txt', 'r:KOI8-U'))
# => [#<Encoding:KOI8-U>, #<Encoding:UTF-8>, "Україна", #<Encoding:UTF-8>]
```

But with StringIO, **internal encoding can't be set** in Ruby **2.6**:

```ruby
require 'stringio'
Encoding.default_internal = nil
str = 'Україна'.encode('KOI8-U')

# Simplest form:
test(StringIO.new(str))
# => [#<Encoding:KOI8-U>, nil, "\xF5\xCB\xD2\xC1\xA7\xCE\xC1", #<Encoding:KOI8-U>]

# Try to set via mode
test(StringIO.new(str, 'r:KOI8-U:UTF-8'))
# => [#<Encoding:KOI8-U>, nil, "\xF5\xCB\xD2\xC1\xA7\xCE\xC1", #<Encoding:KOI8-U>]

# Try to set via set_encoding:
test(StringIO.new(str, 'r:KOI8-U:UTF-8').tap { |f| f.set_encoding('KOI8-U', 'UTF-8') })
# => [#<Encoding:KOI8-U>, nil, "\xF5\xCB\xD2\xC1\xA7\xCE\xC1", #<Encoding:KOI8-U>]

# Try to set via Enoding.default_internal:
Encoding.default_internal = 'UTF-8'
test(StringIO.new(str))
# => [#<Encoding:KOI8-U>, nil, "\xF5\xCB\xD2\xC1\xA7\xCE\xC1", #<Encoding:KOI8-U>]
```

So, in 2.6, any attempt to do something with StringIO's internal encoding are **just ignored**.

In **2.7**, though, matters became much worse:
```ruby
require 'stringio'
Encoding.default_internal = nil
str = 'Україна'.encode('KOI8-U')

# Behaves same as 2.6
test(StringIO.new(str))
# => [#<Encoding:KOI8-U>, nil, "\xF5\xCB\xD2\xC1\xA7\xCE\xC1", #<Encoding:KOI8-U>]

# Try to set via mode: WEIRD behavior starts
test(StringIO.new(str, 'r:KOI8-U:UTF-8'))
# => [#<Encoding:UTF-8>, nil, "\xF5\xCB\xD2\xC1\xA7\xCE\xC1", #<Encoding:UTF-8>]

# Try to set via set_encoding: still just ignored
test(StringIO.new(str, 'r:KOI8-U:UTF-8').tap { |f| f.set_encoding('KOI8-U', 'UTF-8') })
# => [#<Encoding:KOI8-U>, nil, "\xF5\xCB\xD2\xC1\xA7\xCE\xC1", #<Encoding:KOI8-U>]

# Try to set via Enoding.default_internal: WEIRD behavior again
Encoding.default_internal = 'UTF-8'
test(StringIO.new(str))
# => [#<Encoding:UTF-8>, nil, "\xF5\xCB\xD2\xC1\xA7\xCE\xC1", #<Encoding:UTF-8>]
```

So, **2.7** not just ignores attempts to set **internal** encoding, but erroneously sets it to **external** one, so strings are not recoded, but their encoding is forced to change.

I believe it is severe bug (more severe than 2.6's "just ignoring").

[This Reddit thread](https://www.reddit.com/r/ruby/comments/emd6q4/is_this_a_stringio_bug_in_ruby_270/) shows how it breaks existing code:

* the author uses `StringIO` to work with `ASCII-8BIT` strings;
* the code is performed in Rails environment (which sets `internal_encoding` to `UTF-8` by default);
* under **2.7**, `StringIO#read` returns `ASCII-8BIT` content in Strings saying their encoding is `UTF-8`.




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