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


A bit more discussion on fixing the behavior, after discussing on Reddit. 
Basically, it is two ways to fix it:

1. Just return to 2.6 behavior
2. Fully implement `internal_encoding` for StringIO

I believe **2 (make it use `internal_encoding`) is more useful**. One example where current behavior is not enough is substituting `StringIO` (for example, in tests) in code where `File` is usually used. For example, Hunspell's spellchecker data files use `SET <encoding>` tag as their first line. So, the code reading this file, could look this way:

```ruby
def read_aff(io)
  encoding = read_encoding(io)
  io.set_encoding(encoding, 'UTF-8') # read in whatever encoding it has, recode to internal encoding of the software
  # now, I assume
  io.read_line
  # will be equivalent to
  <read_bytes_from_io>.force_encoding(encoding).encode('UTF-8')
  # ...which is true for File and false for StringIO
end
```

However, **option 1 (just make it behave like 2.6)** is more **backward-compatible**. Otherwise (full fix) this simple code will change behavior under Rails (which set `default_internal` to `UTF-8`):

```ruby
str = 'foo'.force_encoding('Windows-1251')
io = StringIO.new(str)
io.read
```
In 2.6 it returns string in `Windows-1251` encoding, and some code may rely on it. If we'll make StringIO respect `default_internal`, it will start to return recoded to UTF-8 string.

So, maybe this report should be **split into two:**

1. At least fix the bug (by returning to the previous behavior) in 2.7.1
2. Make `StringIO` respect internal_encoding in 2.8/3.0

WDYT?

----------------------------------------
Bug #16497: StringIO#internal_encoding is broken (more severely in 2.7)
https://bugs.ruby-lang.org/issues/16497#change-83795

* Author: zverok (Victor Shepelev)
* Status: Assigned
* Priority: Normal
* Assignee: nobu (Nobuyoshi Nakada)
* Target version: 
* ruby -v: 
* Backport: 2.5: DONTNEED, 2.6: DONTNEED, 2.7: REQUIRED
----------------------------------------
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', 'Украна'.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>, "Украна", #<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>, "Украна", #<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>, "Украна", #<Encoding:UTF-8>]
```

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

```ruby
require 'stringio'
Encoding.default_internal = nil
str = 'Украна'.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 = 'Украна'.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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