[ruby-core:120934] [Ruby master Bug#20919] IO#seek and IO#pos= do not clear the character buffer in some cases while transcoding

10 Feb 2025

      Issue #20919 has been updated by mjrzasa (Maciek Rząsa).

I believe the fix is ready for review https://github.com/ruby/ruby/pull/12714
Some CI jobs were failing (WebAssembly/Cygwin) but the failures seem not to be related to my changes and they're inconsistent (after rebasing Cygwin passed and WebAsm failed).

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Bug #20919: IO#seek and IO#pos= do not clear the character buffer in some cases while transcoding
https://bugs.ruby-lang.org/issues/20919#change-111825

* Author: javanthropus (Jeremy Bopp)
* Status: Open
* ruby -v: ruby 3.4.0dev (2024-11-28T12:38:16Z master 3af1a04741) +PRISM [x86_64-linux]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
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When transcoding characters, `IO#seek` and `IO#pos=` only clear the internal character buffer if `IO#getc` is called first:

```ruby
require 'tempfile'

Tempfile.open(encoding: 'utf-8:utf-16le') do |f|
  f.write('0123456789')
  f.rewind

  f.ungetc('a'.encode('utf-16le'))

  # Character buffer WILL NOT be cleared
  f.seek(2, :SET)

  f.getc       # => 'a'.encode('utf-16le'); should be '2'.encode('utf-16le')
end

Tempfile.open(encoding: 'utf-8:utf-16le') do |f|
  f.write('0123456789')
  f.rewind

  f.ungetc('a'.encode('utf-16le'))

  # Character buffer WILL NOT be cleared
  f.pos = 2

  f.getc       # => 'a'.encode('utf-16le'); should be '2'.encode('utf-16le')
end

Tempfile.open(encoding: 'utf-8:utf-16le') do |f|
  f.write('0123456789')
  f.rewind

  # Added a call to #getc here
  f.getc

  f.ungetc('a'.encode('utf-16le'))

  # Character buffer WILL be cleared now
  f.seek(2, :SET)
  # Same behavior for #pos=
  #f.pos = 2

  f.getc       # => '2'.encode('utf-16le')
end
```

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