[meta] Win32 Unicode support

[p5pRT]

Migrated from rt.perl.org#134286 (status was 'open')

Searchable as RT134286$

[p5pRT]

From @tonycoz

Created by @tonycoz

Perl typically uses so called "ANSI" APIs, due to compatibility
with other platforms, eg. calling unlink() is cross-platform, and
to a certain extent works in Unicode locales (using the encoded
bytes rather than characters), but the equivalents on Win32 don't
work.

chcp 65001 isn't a solution, since pretty much everything but
console output will still be in the system code page.

So this is a meta ticket covering tickets improving Perl's Win32
Unicode support.

Any changes need to be switchable (so a user with old ANSI
encoded filenames for example don't lose a bunch of work), and
need to not break other platforms.

I can see at least the following issues​:

- command line arguments

- filenames across many different operators

- process creation (system, exec, readpipe/qx(), pipe open())

- environment variables

- console (maybe)

- many similar changes in bundled modules

Discussion is welcome, patches that satsify the above
requirements more so.

Perl Info

Flags:
    category=core
    severity=low

Site configuration information for perl 5.28.1:

Configured by strawberry-perl at Sun Dec  2 14:25:09 2018.

Summary of my perl5 (revision 5 version 28 subversion 1) configuration:
   
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[p5pRT]

From bokutin@bokut.in

I've been waiting.

Python and Ruby are already possible.

I think it will not be helpful, I have built in the past by myself.
https://github.com/bokutin/strawberry-perl-USING_WIDE-revival

[p5pRT]

The RT System itself - Status changed from 'new' to 'open'

[p5pRT]

From @pali

I can see at least the following issues​:
...
- filenames across many different operators
- process creation (system, exec, readpipe/qx(), pipe open())
...
Discussion is welcome, patches that satsify the above
requirements more so.

Same problem is also on Linux systems. I already created ticket
https://rt.perl.org/Public/Bug/Display.html?id=130831
where is discussion how to handle. I proposed some solution (which could
work for both Windows and Linux systems) but seems other people do not
like it.

(PS​: Please CC me for future discussion as I do not know how can I add
myself to CC list on RT)

[tonycoz]

Platforms

This ticket is specifically about Win32, but ideally we should do the same for POSIX-like systems.

Aim

Provide support for accessing filenames, program arguments and the environment as strings of characters rather than as strings of bytes, and support Win32's unicode interfaces.

This would need to handle possibly mis-encoded filenames, environment entries and argv entries on both Win32 and non-Win32 systems.

This would be opt-in

The new behaviour would only occur under a new -C flag, possibly -Cs, to avoid breaking existing code written to work under the older rules.

User visible behaviour

With the new flag, as follows.

Win32

• argv[] would be populated with utf8 versions of the command-line parameters (-CA would be needed to decode it.)

• %ENV will be populated from the Unicode versions of the environment, and

• readdir() will return the UTF-8 version of the UTF-16 name.

POSIX-like systems

• If both -CA and -Cs are provided, @argv encoding will follow "Handling of mis-encoded names" (-CA currently just switches the UTF-8 flag on from a quick look)

• %ENV will be populated as if the environment was UTF-8 and will follow "Handling of mis-encoded names"

• readdir() will decode the returned names as UTF-8

The UTF-8 flag

No promises are made on the value of the flag, it may be set for strings representable as ISO 11859-1, or it may not.

Handling of mis-encoded names

Outside of Win32, the strings returned by POSIX readdir() and found in argv[] and the environment can contain non-UTF-8 byte sequences.

On Win32, filenames may contain lone surrogates, which cannot properly be converted to UTF-8.

When translating from bytes to characters non UTF-8 sequences such as overlongs or extended UTF-8 sequences (surrogates, code points above 0x10FFFF) will be treated as invalidly encoded bytes.

Invalidly encoded bytes will be encoded as code point 0x200000 + byte value.

On Win32, lone surrogates will be encoded as code point 0x210000 + surrogate code point.

This will be reversed when calling the underlying file API.

Extended characters outside these ranges will result in a file not found error or perhaps EINVAL.

XS/API

New APIs will be provided:

PerlIO *PerlIO_open_sv(SV *path, const char *mode);
int PerlLIO_rename_sv(SV *oldname, SV *newname);
etc

either follow the current behaviour, or perform the filename conversions discussed above if perl was invoked with -Cs.

Rationale

Why not make the behaviour lexical

Filename are passed around between perl lexical scopes, consider filename received from File::Find, or passed to IO::File.

I use supers/overlongs in my filenames, this will break that

Don't use this option, it's intended to support displayable filenames transparently.

Why do all the fancy decoding?

POSIX filenames are byte strings, they may not be valid UTF-8, and Win32 has a similar issue with lone surrogates.

Simply storing the byte sequences with the UTF-8 flag on would produce invalid internal state for perl.

This is less of an issue for Win32's lone surrogates, but treating them as valid seems incorrect to me.

References

[PEP383] https://www.python.org/dev/peps/pep-0383/ - "PEP 383 -- Non-decodable Bytes in System Character Interfaces." This makes the faulty assemption that Win32 filenames are Unicode.

[Grinnz]

IMO there should be a mechanism to enable this behavior other than the commandline flag. The current -C options other than -CA have an equivalent operation via the open pragma (though sometimes obtusely). There's also precedent for such an operation having a global effect (unfortunately) as PerlIO layers can't be lexically scoped on the global standard handles.

[xenu]

I think it should be a completely separate CLI flag, that is, not a subflag of -C.

IMO there should be a mechanism to enable this behavior other than the commandline flag.

CLI flags can be set via PERL5OPT environmental variable and using the shebang. Also, I imagine that this feature will eventually become opt-out instead of opt-in.

Anyway, I generally agree with this proposal, however I don't understand why you don't want to use "Low Surrogates" and "High Surrogates" blocks for unpaired surrogates, as specified by WTF-8. What do we gain by keeping them illegal?

[xenu]

IMO there should be a mechanism to enable this behavior other than the commandline flag.

CLI flags can be set via PERL5OPT environmental variable and using the shebang. Also, I imagine that this feature will eventually become opt-out instead of opt-in.

On second thought, a global variable (in addition to the flag) probably would make sense but I'm worried that people will want to abuse it with local.

[pali]

Anyway, I generally agree with this proposal, however I don't understand why you don't want to use "Low Surrogates" and "High Surrogates" blocks for unpaired surrogates, as specified by WTF-8. What do we gain by keeping them illegal?

Unpaired "Low Surrogates" and "High Surrogates" are illegal in UNICODE. So I think we should avoid using it and also add posibility to detect between real unpaired surrogates which comes from other places (and are illegal) and from win32 filenames (which are legal).

[pali]

Win32

• argv[] would be populated with utf8 versions of the command-line parameters (-CA would be needed to decode it.)

I would suggest to have argv[] in UNICODE (not in UTF-8, not in UTF-16)

• readdir() will return the UTF-8 version of the UTF-16 name.

Same here, I would suggest to have it in UNICODE (not UTF-8, not UTF-16).

POSIX-like systems

• %ENV will be populated as if the environment was UTF-8 and will follow "Handling of mis-encoded names"
• readdir() will decode the returned names as UTF-8

And same here.

Why not make the behaviour lexical

Filename are passed around between perl lexical scopes, consider filename received from File::Find, or passed to IO::File.

Earlier I suggested solution for this issue: Add a new SV flag (or any other way to mark particular SV*) that indicates that its value is UNICODE version of filename. And lexical pragma could change behavior of Perl that for filenames it sets this new flag, to ensure that e.g. result from readdir() will be UNICODE also when stored into variable (SV*) and used outside of the lexical block.

[pali]

Related issues:
#15883
#17091
#9578