Allow read-only methods to work against a frozen StringIO

[headius]

Add tests and fix behavior to allow several read-only StringIO methods to function when the StringIO is frozen.

• 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
• initialize_copy (allow original to be frozen)

See #120 for details.

[headius]

This is based on #121 and those commits will drop off when that PR is merged.

I would like input from @nobu @kou @hsbt @byroot since you all have related changes here. 🙇

[headius]

I can make the same change in the C extension (mostly switching a few StringIO() macro calls to check_strio() function calls, but @nobu @kou please advise me.

[byroot]

I would like input

It would make sense to me that StringIO.new(str.freeze) would try to behave like a read only file. But I'm not StringIO maintainer.

[kou]

See also: https://bugs.ruby-lang.org/issues/21151

[headius]

StringIO.new(str.freeze) would try to behave like a read only file

This isn't really the use case here. What I'm seeking is a way to freeze a StringIO so it can no longer be read from or written to, but still be able to access its current state like lineno and pos.

There's no reason I can think of that accessing current StringIO state should require that the StringIO be mutable, since we're just reading that state.