[ruby-core:117474] [Ruby master Bug#20416] IO#read doesn't preserve buffer encoding if `maxlen = nil`

Issue #20416 has been updated by byroot (Jean Boussier).


I git blamed this behavior all the way down to 2007, by @matz: https://github.com/ruby/ruby/commit/0ca7036682dad1caea4e7a4fe8fd908d94c878e8 and it seems deliberate

```
* io.c (read_all): should associate default external encoding.
* io.c (io_read): should NOT associate default external encoding.
```

----------------------------------------
Bug #20416: IO#read doesn't preserve buffer encoding if `maxlen = nil`
https://bugs.ruby-lang.org/issues/20416#change-107862

* Author: andrykonchin (Andrew Konchin)
* Status: Open
* ruby -v: 3.3.0
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
`IO#read(nil, buffer)` doesn't change buffer's encoding, but it does when `maxlen` parameter is specified:

```ruby
buffer = "".encode(Encoding::ISO_8859_1)
p File.open("test.rb").read(1000, buffer).encoding # => #<Encoding:ISO-8859-1>

buffer = "".encode(Encoding::ISO_8859_1)
p File.open("test.rb").read(nil, buffer).encoding  # => #<Encoding:UTF-8>
```

I would expect changing buffer encoding in all the cases.



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