Re: [PATCH v5 03/36] qapi-gen: Separate arg-parsing from generation

[Markus Armbruster <armbru@redhat.com>]

John Snow <jsnow@redhat.com> writes:

> On 10/6/20 7:51 AM, Markus Armbruster wrote:
>> John Snow <jsnow@redhat.com> writes:
>> 
>>> This is a minor re-work of the entrypoint script. It isolates a
>>> generate() method from the actual command-line mechanism.
>>>
>>> Signed-off-by: John Snow <jsnow@redhat.com>
>>> Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
>>> Reviewed-by: Cleber Rosa <crosa@redhat.com>
>>> Tested-by: Cleber Rosa <crosa@redhat.com>
>>> ---
>>>   scripts/qapi-gen.py | 85 +++++++++++++++++++++++++++++++++------------
>>>   1 file changed, 62 insertions(+), 23 deletions(-)
>>>
>>> diff --git a/scripts/qapi-gen.py b/scripts/qapi-gen.py
>>> index 541e8c1f55d..117b396a595 100644
>>> --- a/scripts/qapi-gen.py
>>> +++ b/scripts/qapi-gen.py
>>> @@ -1,30 +1,77 @@
>>>   #!/usr/bin/env python3
>>> -# QAPI generator
>>> -#
>>> +
>>>   # This work is licensed under the terms of the GNU GPL, version 2 or later.
>>>   # See the COPYING file in the top-level directory.
>>>   +"""
>>> +QAPI Generator
>>> +
>>> +This script is the main entry point for generating C code from the QAPI schema.
>>> +"""
>>>     import argparse
>>>   import re
>>>   import sys
>>>     from qapi.commands import gen_commands
>>> +from qapi.error import QAPIError
>>>   from qapi.events import gen_events
>>>   from qapi.introspect import gen_introspect
>>> -from qapi.schema import QAPIError, QAPISchema
>>> +from qapi.schema import QAPISchema
>>>   from qapi.types import gen_types
>>>   from qapi.visit import gen_visit
>> Unrelated cleanup.  Okay.
>> 
>>>     
>>> -def main(argv):
>>> +DEFAULT_OUTPUT_DIR = ''
>>> +DEFAULT_PREFIX = ''
>>> +
>>> +
>>> +def generate(schema_file: str,
>>> +             output_dir: str,
>>> +             prefix: str,
>>> +             unmask: bool = False,
>>> +             builtins: bool = False) -> None:
>>> +    """
>>> +    generate uses a given schema to produce C code in the target directory.
>>> +
>>> +    :param schema_file: The primary QAPI schema file.
>>> +    :param output_dir: The output directory to store generated code.
>>> +    :param prefix: Optional C-code prefix for symbol names.
>>> +    :param unmask: Expose non-ABI names through introspection?
>>> +    :param builtins: Generate code for built-in types?
>>> +
>>> +    :raise QAPIError: On failures.
>>> +    """
>>> +    match = re.match(r'([A-Za-z_.-][A-Za-z0-9_.-]*)?', prefix)
>>> +    if match.end() != len(prefix):
>>> +        msg = "funny character '{:s}' in prefix '{:s}'".format(
>>> +            prefix[match.end()], prefix)
>>> +        raise QAPIError('', None, msg)
>> Uh...
>>      $ python3 scripts/qapi-gen.py --prefix=@ x
>>      scripts/qapi-gen.py: : funny character '@' in prefix '@'
>> Unwanted " :".
>> This is due to a hack: you pass '' for info (*quack*).  Everything
>> else
>> passes QAPISourceInfo (I believe).
>>
>
> Quack indeed - why does our base error class require so much
> information from a specific part of the generation process?

Because it's not "a base error class", it's a base error class for the
QAPI schema compiler frontend.

> Ah, someone changes this in part 4 so that we have a more generic
> error class to use as a base when we are missing such information.

Evolving it to satisfy a need for a more widely usable error class is
okay.

> You are witnessing some more future-bleed.
>> Is it really a good idea to do this in generate?  It's not about
>> generating code, it's about validating a CLI option.
>> 
>
> One might also ask: Is it a good idea to only validate this on a
> frontend, and not in the implementation?

Yes, because that's where you can emit the better error message more
easily.

    $ python3 scripts/qapi-gen.py --prefix=@ x
    scripts/qapi-gen.py: 'funny character '@' in argument of --prefix

is better than

    $ python3 scripts/qapi-gen.py --prefix=@ x
    scripts/qapi-gen.py: funny character '@' in prefix '@'

In generate(), the knowledge where the offending prefix value comes from
is no longer available.

To emit this error message, you'd have to raise a sufficiently distinct
error in generate, catch it in main(), then put the error message
together somehow.  Bah.

Aside: there's a stray ' in the old error message.

> The idea here was to create a function that could be used in a script
> (for tests, debugging interfaces, other python packages) to do all of 
> the same things that the CLI tool did, just sans the actual CLI.

YAGNI.

> Wouldn't make sense to allow garbage to flow in from one interface but
> not the other; so the check is here.

"@prefix is sane" is a precondition of generate().

When there's a real risk of preconditions getting violated, or readers
getting confused about preconditions, check them with assert.

>>> +
>>> +    schema = QAPISchema(schema_file)
>>> +    gen_types(schema, output_dir, prefix, builtins)
>>> +    gen_visit(schema, output_dir, prefix, builtins)
>>> +    gen_commands(schema, output_dir, prefix)
>>> +    gen_events(schema, output_dir, prefix)
>>> +    gen_introspect(schema, output_dir, prefix, unmask)
>>> +
>>> +
>>> +def main() -> int:
>>> +    """
>>> +    gapi-gen shell script entrypoint.
>>> +    Expects arguments via sys.argv, see --help for details.
>>> +
>>> +    :return: int, 0 on success, 1 on failure.
>>> +    """
>>>       parser = argparse.ArgumentParser(
>>>           description='Generate code from a QAPI schema')
>>>       parser.add_argument('-b', '--builtins', action='store_true',
>>>                           help="generate code for built-in types")
>>> -    parser.add_argument('-o', '--output-dir', action='store', default='',
>>> +    parser.add_argument('-o', '--output-dir', action='store',
>>> +                        default=DEFAULT_OUTPUT_DIR,
>>>                           help="write output to directory OUTPUT_DIR")
>>> -    parser.add_argument('-p', '--prefix', action='store', default='',
>>> +    parser.add_argument('-p', '--prefix', action='store',
>>> +                        default=DEFAULT_PREFIX,
>>>                           help="prefix for symbols")
>> I don't like the changes to default=, because:
>> 1. They are only losely related to the patch's purpose.
>> 
>
> Subjective, but OK.
>
>> 2. They split the definition of the CLI: most of it is here, except for
>> defaults, which are defined elsewhere.
>> 
>
> All of it is in main.py, though! If you were to, say, move generate()
> elsewhere, it'd look pretty compact as just the CLI frontend, no?

Same statement is more compact than same screenful is more compact than
same file :)

>> 3. The defaults will not change, and nothing else uses the constants.
>> 
>
> But, fine. Cleber had the same comment but I wasn't fully on-board,
> but two folks saying the same thing ...
>
>>>       parser.add_argument('-u', '--unmask-non-abi-names', action='store_true',
>>>                           dest='unmask',
>>> @@ -32,25 +79,17 @@ def main(argv):
>>>       parser.add_argument('schema', action='store')
>>>       args = parser.parse_args()
>>>   -    match = re.match(r'([A-Za-z_.-][A-Za-z0-9_.-]*)?',
>>> args.prefix)
>>> -    if match.end() != len(args.prefix):
>>> -        print("%s: 'funny character '%s' in argument of --prefix"
>>> -              % (sys.argv[0], args.prefix[match.end()]),
>>> -              file=sys.stderr)
>>> -        sys.exit(1)
>>> -
>>>       try:
>>> -        schema = QAPISchema(args.schema)
>>> +        generate(args.schema,
>>> +                 output_dir=args.output_dir,
>>> +                 prefix=args.prefix,
>>> +                 unmask=args.unmask,
>>> +                 builtins=args.builtins)
>>>       except QAPIError as err:
>>> -        print(err, file=sys.stderr)
>>> -        exit(1)
>>> -
>>> -    gen_types(schema, args.output_dir, args.prefix, args.builtins)
>>> -    gen_visit(schema, args.output_dir, args.prefix, args.builtins)
>>> -    gen_commands(schema, args.output_dir, args.prefix)
>>> -    gen_events(schema, args.output_dir, args.prefix)
>>> -    gen_introspect(schema, args.output_dir, args.prefix, args.unmask)
>>> +        print(f"{sys.argv[0]}: {str(err)}", file=sys.stderr)
>>> +        return 1
>>> +    return 0
>> Subtle change: you move the gen_FOO() into the try ... except.
>> Okay;
>> they don't raise QAPIError, but perhaps worth a mention in the commit
>> message.
>> 
>
> Forbidden future knowledge; I intend them to.

I don't mind the move.

>>>     
>>>   if __name__ == '__main__':
>>> -    main(sys.argv)
>>> +    sys.exit(main())
>> "Python was designed to be easy to understand and fun to use."
>> Ha ha ha.
>> 
>
> I mean, I'm having fun, aren't you?

So many kinds of fun!  The fun I'm having with this patch hunk is
mocking "easy and fun" Python for requiring such an elaborate menuett
just to express "can run as program".

This emperor has no clothes, either.  And that's funny, isn't it?