Re: [Qemu-devel] [PATCH v6 07/15] qapi: add #if/#endif helpers

[Marc-André Lureau]

Hi

On Tue, Jul 3, 2018 at 3:37 PM, Markus Armbruster <address@hidden> wrote:
> Marc-André Lureau <address@hidden> writes:
>
>> Hi
>>
>> On Tue, Jul 3, 2018 at 1:53 PM, Markus Armbruster <address@hidden> wrote:
>>> Marc-André Lureau <address@hidden> writes:
>>>
>>>> Add helpers to wrap generated code with #if/#endif lines.
>>>>
>>>> Add QAPIGenCSnippet class to write C snippet code, make QAPIGenC
>>>> inherit from it, for full C files with copyright headers etc.
>>>
>>> It's called QAPIGenCCode now.  Can touch up when I apply, along with
>>> another instance below.
>>>
>>> Leaves the reader wondering *why* you need to splice QAPIGenCCode
>>> between QAPIGen and QAPIGenC.  Becomes clear only in PATCH 10.
>>> Providing the class now reduces code churn, but you should explain why.
>>> Perhaps:
>>>
>>>   A later patch wants to use QAPIGen for generating C snippets rather
>>>   than full C files with copyright headers etc.  Splice in class
>>>   QAPIGenCCode between QAPIGen and QAPIGenC.
>>
>> sure, thanks
>>
>>>
>>>> Add a 'with' statement context manager that will be used to wrap
>>>> generator visitor methods.  The manager will check if code was
>>>> generated before adding #if/#endif lines on QAPIGenCSnippet
>>>> objects. Used in the following patches.
>>>>
>>>> Signed-off-by: Marc-André Lureau <address@hidden>
>>>> ---
>>>>  scripts/qapi/common.py | 101 +++++++++++++++++++++++++++++++++++++++--
>>>>  1 file changed, 97 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/scripts/qapi/common.py b/scripts/qapi/common.py
>>>> index 1b56065a80..44eaf25850 100644
>>>> --- a/scripts/qapi/common.py
>>>> +++ b/scripts/qapi/common.py
>>>> @@ -12,6 +12,7 @@
>>>>  # See the COPYING file in the top-level directory.
>>>>
>>>>  from __future__ import print_function
>>>> +from contextlib import contextmanager
>>>>  import errno
>>>>  import os
>>>>  import re
>>>> @@ -1974,6 +1975,40 @@ def guardend(name):
>>>>                   name=guardname(name))
>>>>
>>>>
>>>> +def gen_if(ifcond):
>>>> +    ret = ''
>>>> +    for ifc in ifcond:
>>>> +        ret += mcgen('''
>>>> +#if %(cond)s
>>>> +''', cond=ifc)
>>>> +    return ret
>>>> +
>>>> +
>>>> +def gen_endif(ifcond):
>>>> +    ret = ''
>>>> +    for ifc in reversed(ifcond):
>>>> +        ret += mcgen('''
>>>> +#endif /* %(cond)s */
>>>> +''', cond=ifc)
>>>> +    return ret
>>>> +
>>>> +
>>>> +def wrap_ifcond(ifcond, before, after):
>>>
>>> Looks like a helper method.  Name it _wrap_ifcond?
>>
>> Not fond of functions with _. Iirc, it was suggested by a linter to
>> make it a top-level function. I don't mind if you change it on commit.
>
> It's just how Python defines a module's public names.  I don't like the
> proliferation of leading underscores either, but I feel it's best to go
> with the flow here.

ok

>
>>>> +    if ifcond is None or before == after:
>>>> +        return after
>>>
>>> The before == after part suppresses empty conditionals.  Suggest
>>>
>>>            return after            # suppress empty #if ... #endif
>>>
>>> The ifcond is None part is suppresses "non-conditionals".  How can this
>>> happen?  If it can't, let's drop this part.
>>
>> because the function can be called without additional checks on ifcond
>> is None. I don't see what would be the benefit in moving this to the
>> caller responsibility.
>
> Why stop at None?  Why not empty string?  Empty containers other than
> []?  False?  Numeric zero?
>
> To answer my own question: because a precise function signatures reduce
> complexity.  "The first argument is a list of strings, no ifs, no buts"
> is simpler than "the first argument may be None, or a list of I don't
> remember what exactly, but if you screw it up, the function hopefully
> throws up before it does real damage" ;)

So you prefer if not ifcond or before == after ? ok

>
>>> Another non-conditional is [].  See below.
>>>
>>>> +
>>>> +    assert after.startswith(before)
>>>> +    out = before
>>>> +    added = after[len(before):]
>>>> +    if added[0] == '\n':
>>>> +        out += '\n'
>>>> +        added = added[1:]
>>>
>>> The conditional adjusts vertical space around #if.  This might need
>>> tweaking, but let's not worry about that now.
>>>
>>>> +    out += gen_if(ifcond)
>>>> +    out += added
>>>> +    out += gen_endif(ifcond)
>>>
>>> There's no similar adjustment for #endif.  Again, let's not worry about
>>> that now.
>>>
>>>> +    return out
>>>> +
>>>> +
>>>
>>> Since gen_if([]) and gen_endif([]) return '', wrap_ifcond([], before,
>>> after) returns after.  Okay.
>>>
>>>>  def gen_enum_lookup(name, values, prefix=None):
>>>>      ret = mcgen('''
>>>>
>>>> @@ -2064,6 +2099,7 @@ class QAPIGen(object):
>>>>      def __init__(self):
>>>>          self._preamble = ''
>>>>          self._body = ''
>>>> +        self._start_if = None
>>>>
>>>>      def preamble_add(self, text):
>>>>          self._preamble += text
>>>> @@ -2071,6 +2107,25 @@ class QAPIGen(object):
>>>>      def add(self, text):
>>>>          self._body += text
>>>>
>>>> +    def start_if(self, ifcond):
>>>> +        assert self._start_if is None
>>>> +
>>>
>>> Let's drop this blank line.
>>
>> I prefer to have preconditions separated from the code, but ok
>
> I sympathize, but not if both are one-liners.

ok

>
>>>
>>>> +        self._start_if = (ifcond, self._body, self._preamble)
>>>
>>> It's now obvious that you can't nest .start_if() ... .endif().  Good.
>>>
>>>> +
>>>> +    def _wrap_ifcond(self):
>>>> +        pass
>>>> +
>>>> +    def end_if(self):
>>>> +        assert self._start_if
>>>> +
>>>
>>> Let's drop this blank line.
>>>
>>>> +        self._wrap_ifcond()
>>>> +        self._start_if = None
>>>> +
>>>> +    def get_content(self, fname=None):
>>>> +        assert self._start_if is None
>>>> +        return (self._top(fname) + self._preamble + self._body
>>>> +                + self._bottom(fname))
>>>> +
>>>>      def _top(self, fname):
>>>>          return ''
>>>>
>>>
>>> Note for later: all this code has no effect unless ._wrap_ifcond() is
>>> overridden.
>>>
>>>> @@ -2091,8 +2146,7 @@ class QAPIGen(object):
>>>>              f = open(fd, 'r+', encoding='utf-8')
>>>>          else:
>>>>              f = os.fdopen(fd, 'r+')
>>>> -        text = (self._top(fname) + self._preamble + self._body
>>>> -                + self._bottom(fname))
>>>> +        text = self.get_content(fname)
>>>>          oldtext = f.read(len(text) + 1)
>>>>          if text != oldtext:
>>>>              f.seek(0)
>>>> @@ -2101,10 +2155,49 @@ class QAPIGen(object):
>>>>          f.close()
>>>>
>>>>
>>>> -class QAPIGenC(QAPIGen):
>>>> address@hidden
>>>> +def ifcontext(ifcond, *args):
>>>> +    """A 'with' statement context manager to wrap with start_if()/end_if()
>>>>
>>>> -    def __init__(self, blurb, pydoc):
>>>> +    *args: variable length argument list of QAPIGen
>>>
>>> Perhaps: any number of QAPIGen objects
>>>
>>
>> sure
>>
>>>> +
>>>> +    Example::
>>>> +
>>>> +        with ifcontext(ifcond, self._genh, self._genc):
>>>> +            modify self._genh and self._genc ...
>>>> +
>>>> +    Is equivalent to calling::
>>>> +
>>>> +        self._genh.start_if(ifcond)
>>>> +        self._genc.start_if(ifcond)
>>>> +        modify self._genh and self._genc ...
>>>> +        self._genh.end_if()
>>>> +        self._genc.end_if()
>>>> +
>>>
>>> Can we drop this blank line?
>>>
>>
>> I guess we can, I haven't tested the rst formatting this rigorously.
>> Hopefully the linter does it.
>>
>>>> +    """
>>>> +    for arg in args:
>>>> +        arg.start_if(ifcond)
>>>> +    yield
>>>> +    for arg in args:
>>>> +        arg.end_if()
>>>> +
>>>> +
>>>> +class QAPIGenCCode(QAPIGen):
>>>> +
>>>> +    def __init__(self):
>>>>          QAPIGen.__init__(self)
>>>> +
>>>> +    def _wrap_ifcond(self):
>>>> +        self._body = wrap_ifcond(self._start_if[0],
>>>> +                                 self._start_if[1], self._body)
>>>> +        self._preamble = wrap_ifcond(self._start_if[0],
>>>> +                                     self._start_if[2], self._preamble)
>>>
>>> Can you explain why you put the machinery for conditionals in QAPIGen
>>> and not QAPIGenCCode?
>>
>> Iirc, this has to do with the fact that QAPIGenDoc is used for
>> visiting, and thus QAPIGen start_if()/end_if() could be handled there,
>
> Can you point me to calls of .start_if(), .end_if(), .get_content() for
> anything but QAPIGenCCode and its subtypes?
>
>> while we only want to generate #ifdef wrapping in QAPIGenCCode. I
>> guess I could move more of start_if()/end_if() data to QAPIGenCCode
>> (make them virtual / 'pass' in QAPIGen) Is that what you would like to
>> see?
>
> If there are no such calls, we should simply move the whole shebang to
> QAPIGenCCode.

done

>
> If there are such calls, the common supertype QAPIGen has to provide the
> methods.
>
> If we expect subtypes other than QAPIGenCCode to implement conditional
> code generation the same way, except for a different ._wrap_ifcond(),
> then the patch is okay as is.
>
> If we don't, the transformation you described looks like an improvement
> to me, because it keeps the actual code closer together.
>
> What's your take on it?
>
> I think I could make the transformation when I apply.
>
>>>
>>>> +
>>>> +
>>>> +class QAPIGenC(QAPIGenCCode):
>>>> +
>>>> +    def __init__(self, blurb, pydoc):
>>>> +        QAPIGenCCode.__init__(self)
>>>>          self._blurb = blurb
>>>>          self._copyright = '\n * '.join(re.findall(r'^Copyright .*', pydoc,
>>>>                                                    re.MULTILINE))
>>>



-- 
Marc-André Lureau