[ruby-core:122950] [Ruby Bug#21151] IO and StringIO raise FrozenError even for read-only methods

Issue #21151 has been updated by headius (Charles Nutter).


Edit: previously I referred to non-writable state, but in actuality any state of `StringIO` or `IO` should be safe to read even if the object itself is frozen. There's no need for either type to be mutable just to read `sync` or `external_encoding` for example.

----------------------------------------
Bug #21151: IO and StringIO raise FrozenError even for read-only methods
https://bugs.ruby-lang.org/issues/21151#change-114254

* Author: headius (Charles Nutter)
* Status: Feedback
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN, 3.4: UNKNOWN
----------------------------------------
Problem
--------

While fixing a small [regression](https://github.com/ruby/stringio/issues/119) from my recent StringIO [fixes](https://github.com/ruby/stringio/issues/116) I discovered that a large number of read-only methods will fail if the StringIO is frozen. A few examples:

```
$ ruby -rstringio -e 's = StringIO.new; p s.lineno; s.freeze; p s.lineno' 
0
-e:1:in 'StringIO#lineno': can't modify frozen StringIO: #<StringIO:0x0000000103711ea8> (FrozenError)
	from -e:1:in '<main>'
$ ruby -rstringio -e 's = StringIO.new; p s.closed?; s.freeze; p s.closed?'
false
-e:1:in 'StringIO#closed?': can't modify frozen StringIO: #<StringIO:0x00000001008c1e68> (FrozenError)
	from -e:1:in '<main>'
$ ruby -rstringio -e 's = StringIO.new; p s.eof?; s.freeze; p s.eof?'
true
-e:1:in 'StringIO#eof?': can't modify frozen StringIO: #<StringIO:0x000000011c171e80> (FrozenError)
	from -e:1:in '<main>'
$ ruby -rstringio -e 's = StringIO.new; p s.pos; s.freeze; p s.pos'
0
-e:1:in 'StringIO#pos': can't modify frozen StringIO: #<StringIO:0x0000000105551df0> (FrozenError)
	from -e:1:in '<main>'
```

@kou pointed out that IO also raises for at least `lineno` and suggested I re-open the issue here. I still believe these read-only operations should be allowed on frozen IO or StringIO.

Cause in StringIO
-----------------

This is because the `StringIO` macro calls `get_strio` which calls `rb_io_taint_check` which calls `rb_check_frozen`. The `StringIO` macro is used in almost every method to access the rb_stringio_t data structure.

A list of methods I believe should be operable when the StringIO is frozen (in stringio.c definition order):

* string (returns underlying String but does not mutate anything)
* lineno
* pos
* closed?/closed_read?/closed_write?
* eof/eof?
* sync
* pid (a dummy method but it writes nothing)
* fileno (dummy)
* pread (by definition does not modify state)
* isatty/tty?
* size/length
* external_encoding
* internal_encoding

 In addition, `initialize_copy` probably should not require the original StringIO to be writable:

```
$ ruby -rstringio -e 's = StringIO.new("foo"); s.freeze; p s.dup'                                                                                           
-e:1:in 'StringIO#initialize_copy': can't modify frozen StringIO: #<StringIO:0x0000000102eb1df8> (FrozenError)
	from -e:1:in 'Kernel#initialize_dup'
	from -e:1:in 'Kernel#dup'
	from -e:1:in '<main>'
```

The data from the original StringIO is unmodified by `initialize_copy`, other than the reference-counting `ptr->count` (which should not be subject to frozen checks).

Cause in IO
-----------

The `GetOpenFile` macro calls `RB_IO_POINTER` macro which calls `rb_io_taint_check` which calls `rb_check_frozen`.

Methods in IO that should work on a frozen IO include those from the StringIO list above that currently raise. For example `lineno` uses `GetOpenFile` and raises, but `fileno` does not and does not raise.

There's clearly some inconsistency here we can clean up.

Origin
------

I believe most of the StringIO cases are caused by this change from 2011 that added frozen checks to StringIO (the class and the macro): https://github.com/ruby/ruby/commit/d8d9bac5c8b071135e50ad3f21c8a9b6a9c06e54

In IO, this behavior dates way back to 2000 by @matz himself: https://github.com/ruby/ruby/commit/087c83d7ceed6893afff93066937fb570ae4a115

Notes
-----

Originally filed as https://github.com/ruby/stringio/issues/120.

I have started to fix the StringIO cases for JRuby in https://github.com/ruby/stringio/pull/122. I could probably fix the C version of StringIO as well, but I'm a little more unsure about how to fix IO.



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