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Re: [Qemu-devel] [PATCH v5 21/28] qapi: Require valid names

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v5 21/28] qapi: Require valid names
Date: Fri, 27 Mar 2015 14:15:29 -0600
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On 03/27/2015 02:48 AM, Markus Armbruster wrote:
> Eric Blake <address@hidden> writes:
>> Previous commits demonstrated that the generator overlooked various
>> bad naming situations:
>> - types, commands, and events need a valid name
>> - union and alternate branches cannot be marked optional
>> The set of valid names includes [a-zA-Z0-9._-] (where '.' is
>> useful only in downstream extensions).

>> +valid_characters = set(string.ascii_letters + string.digits + '.' + '-' + 
>> '_')
> strings.ascii_letters depends on the locale...



    The concatenation of the ascii_lowercase and ascii_uppercase
constants described below. This value is not locale-dependent.

You are thinking of string.letters, which IS locale-dependent.  I
intentionally used ascii_letters.

>> +def check_name(expr_info, source, name, allow_optional = False):
>> +    membername = name
>> +
>> +    if not isinstance(name, str):
>> +        raise QAPIExprError(expr_info,
>> +                            "%s requires a string name" % source)
>> +    if name == '**':
>> +        return
> Doesn't this permit '**' anywhere, not just as pseudo-type in command
> arguments and results?

Yes, on the grounds that check_type then filters it appropriately.  But
worthy of a comment (probably both in the commit message AND in the code
base).  Grounds for a v6 respin.

>> +    if name.startswith('*'):
>> +        membername = name[1:]
>> +        if not allow_optional:
>> +            raise QAPIExprError(expr_info,
>> +                                "%s does not allow optional name '%s'"
>> +                                % (source, name))
>> +    if not set(membername) <= valid_characters:
> ... so this check would break if we called locale.setlocale() in this
> program.  While I don't think we need to worry about it, I think you
> could just as well use something like
>     valid_name = re.compile(r"^[A-Za-z0-9-._]+$")
>     if not valid_name.match(membername):

regex is slightly slower than string matching _if the regex is
precompiled_, and MUCH slower than string matching if the regex is
compiled every time.  In turn, string matching is slower than
open-coding things, but has the benefit of being more compact and
maintainable (open-coded loops are the worst on that front). Here's
where I got my inspiration:


But I may just go with the regex after all (I don't know how efficient
python is about reusing a regex when a function is called multiple
times, rather than recompiling the regex every time.  Personal side
note: back in 2009 or so, I was able to make 'm4' significantly faster
in the context of 'autoconf' when I taught it to cache the compilation
of the 8 most-recently-encountered regex, rather than recompiling every
time; and then made 'autoconf' even faster when I taught it to do
actions that didn't require regex use from 'm4' in the first place.)

The nice thing, though, is that I factored things so that the
implementation of this one function can change without having to hunt
down all call-sites, if I keep the contract the same.

>>          discriminator_type = base_fields.get(discriminator)
>>          if not discriminator_type:
>>              raise QAPIExprError(expr_info,
> What happens when I try 'discriminator': '**'?

No clue.  Good thing for me to add somewhere in my series.  However, I
did manage to have this series at least think about a base type with
'*switch':'Enum', then use 'discriminator':'*switch', which got past the
generator (who knows what the C code would have done if have_switch was
false?), so I plugged that hole; but in the process of testing it, I
noticed that '*switch':'Enum' paired with 'discriminator':'switch'
complained that 'switch' was not a member of the base class (which is a
lie; it is present in the base class, but as an optional member).  Proof
that the generator is a bunch of hacks strung together :)

Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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