Several read-only methods fail if StringIO is frozen

[headius]

Problem

While fixing a small regression from my recent StringIO fixes 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>'

Cause

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 be writable:

$ cx 3.4 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).

Origin

I believe most of these cases are caused by this change from 2011 that added frozen checks to StringIO (the class and the macro).
ruby/ruby@d8d9bac

Fix

Assuming we agree these read-only methods should work with a frozen, I'll modify them in the JRuby extension to do the equivalent of check_strio which only confirms the ptr has been initialized. Perhaps a StringIOReadOnly macro would make sense for CRuby.

[kou]

Good catch!

But it seems that IO has the same behavior:

$  ruby -e 'File.open("README.md") {|f| f.freeze; f.lineno}'
-e:1:in 'IO#lineno': can't modify frozen File: #<File:README.md> (FrozenError)
	from -e:1:in 'block in <main>'
	from -e:1:in 'IO.open'
	from -e:1:in '<main>'

How about discussing this in https://bugs.ruby-lang.org/ not here? StringIO behavior should be aligned with IO behavior.

[headius]

@kou I will open an issue on ruby-lang! Thanks!

[headius]

Filed https://bugs.ruby-lang.org/issues/21151.

[headius]

@kou Given that this will take more discussion, so we can align IO and StringIO, could we get #119/#121 released? I would really appreciate it! 👍

[kou]

OK. I'll release a new version with #121.

[kou]

Done.