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

[Marc-André Lureau]

Hi

On Tue, Jul 3, 2018 at 5:34 PM, Markus Armbruster <address@hidden> wrote:
> Marc-André Lureau <address@hidden> writes:
>
>> 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
>
> I'd recommend
>
>       +    if before == after:
>       +        return after
>
> If we pass crap, we die in gen_if()'s for ifc in ifcond.  The difference
> is now crap includes None.
>

makes sense, I didn't realize it no longers pass None.

>>>>> 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
>
> Does that mean I should expect v7 from you?

yes

>
>>> 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