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Re: [Qemu-devel] [PATCH v8.5 1/4] qapi: Drop all_members parameter from


From: Markus Armbruster
Subject: Re: [Qemu-devel] [PATCH v8.5 1/4] qapi: Drop all_members parameter from check()
Date: Tue, 03 Nov 2015 15:02:49 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux)

Eric Blake <address@hidden> writes:

> On 11/03/2015 04:06 AM, Markus Armbruster wrote:
>> Eric Blake <address@hidden> writes:
>> 
>>> The implementation of QAPISchemaObjectTypeMember.check() always
>>> adds the member currently being checked to both the all_members
>>> and seen parameters.
>> 
>> QAPISchemaObjectTypeMember.check() does four things:
>> 
>> 1. Compute self.type
>> 
>>    Precondition: all types are defined.
>
> Correct, unchanged by this patch.
>
>> 
>> 2. Accumulate members
>> 
>>    all_members serves as accumulator.
>> 
>>    We'll see that its only actual use is the owning object type's
>>    check(), which uses it to compute self.members.
>
> This patch changes it to use seen.values(), which (once you use an
> OrderedDict() instead of plain {}) is identical to all_members.
>
>> 
>> 3. Check for collisions
>> 
>>    This works by accumulating names in seen.  Precondition: seen
>>    contains the names seen so far.
>> 
>>    Note that this part uses seen like a set.  See 4.
>
> Unchanged by this patch; but see also 2/4 and 3/4.
>
>> 
>> 4. Accumulate a map from names to members
>> 
>>    seen serves as accumulator.
>> 
>
> Unchanged by this patch.
>
>>    We'll see that its only actual user is the owning object type's
>>    variants.check(), which uses it to compute variants.tag_member from
>>    variants.tag_name.
>> 
>>>                      However, the three callers of this method
>>> pass in the following parameters:
>>>
>>> QAPISchemaObjectType.check():
>>>   - all_members contains all non-variant members seen to date,
>>>   for use in populating self.members
>>>   - seen contains all non-variant members seen to date, for
>>>   use in checking for collisions
>> 
>> Yes, and:
>> 
>> - we're calling it for m in self.local_members
>> - before the loop, all_members and seen are initialized to the inherited
>>   non-variant members
>> - after the loop, they therefore contain all non-variant members
>> 
>> This caller uses all four things done by QAPISchemaObjectType.check():
>> 
>> 1. Compute m.type
>
> Unchanged by this patch.
>
>> 2. Accumulate non-variant members
>
> Whether the accumulation is done via all_members (pre-patch) or by
> seen.values() (post-patch), this step is still done.
>
>> 3. Check for collisions among non-variant members
>>    Before the loop, seen contains the inherited members, which don't
>>    collide (self.base.check() ensures that).  The loop adds the local
>>    members one by one, checking for collisions.
>
> Unchanged by this patch.
>
>> 4. Accumulate a map from names to non-variant members
>>    Similar argument to 3.
>
> Unchanged by this patch.
>
>> 
>>> QAPISchemaObjectTypeVariant.check():
>> 
>> Do you mean QAPISchemaObjectVariants.check()?
>
> QAPISchemaObjectTypeVariants.check() calls
> QAPISchemaObjectTypeVariant.check() for each variant, but with a fresh
> copy of seen.  We'll later need to expand this copy of seen (patch 2/4),
> but for this patch its use is unchanged - we are appending a single
> value (the tag value) which is wrong, but no one cares that we appended
> it because it was a copy. Patch 3/4 fixes to not append to it.
>
>> 
>>>   - all_members is a throwaway empty list
>>>   - seen is a throwaway dictionary created as a copy by
>>>   QAPISchemaObjectVariants.check() (since the members of
>>>   one variant cannot collide with those from another), for
>>>   use in checking for collisions (technically, we no longer
>>>   need to check for collisions between tag values and QMP
>>>   key names, but that's a cleanup for another patch)
>>>
>>> QAPISchemaAlternateType.check():
>>>   - all_members is a throwaway empty list
>>>   - seen is a throwaway empty dict
>> 
>> I'm afraid you're omitting a few steps here, and I think you missed
>> QAPISchemaObjectVariants.check()'s self.tag_member.check().
>
> There is no self.tag_member.check(), any more; rather, my earlier
> patches have reworked things so that tag_member is checked by the owner
> (a flat union's base type, a simple union's local_members, or directly
> in QAPISchemaAlternateType prior to calling Variants.check()).  I guess
> that's a pitfall of seeing this patch without my rework of 5/17 to
> address your comments there.

I'm assuming this patch is based on
[PATCH v8 06/17] qapi-types: Consolidate gen_struct() and gen_union()
which has

    def check(self, schema, members, seen):
        if self.tag_name:    # flat union
            self.tag_member = seen[self.tag_name]
        elif seen:           # simple union
            assert self.tag_member in seen.itervalues()
        else:                # alternate
--->        self.tag_member.check(schema, members, seen)

in QAPISchemaObjectTypeVariants.

>>> Therefore, in the one case where we care about all_members
>>> after seen has been populated, we know that it contains the
>>> same members as seen.values(); changing seen to be an
>>> OrderedDict() is sufficient to pick up this information with
>>> one less parameter being passed around.
>> 
>> I believe the first step should be dropping the obsolete check for
>> collision of tag value with non-variant members.  I believe this should
>> do:
>> 
>> @@ -1059,8 +1059,7 @@ class QAPISchemaObjectTypeVariants(object):
>>              self.tag_member.check(schema, members, seen)
>>          assert isinstance(self.tag_member.type, QAPISchemaEnumType)
>>          for v in self.variants:
>> -            vseen = dict(seen)
>> -            v.check(schema, self.tag_member.type, vseen)
>> +            v.check(schema, self.tag_member.type, {})
>
> Close, but not quite.  It should do:
>
> +          cases = {}
>            for v in self.variants:
>                vseen = dict(seen)
> -              v.check(schema, self.tag_member.type, vseen)
> +              v.check(schema, self.tag_member.type, vseen, cases)
>
> coupled with this in QAPISchemaObjectTypeVariant:
>
> -    def check(self, schema, tag_type, seen):
> -        QAPISchemaObjectTypeMember.check(self, schema, [], seen)
> +    def check(self, schema, tag_type, seen, cases):
> +        QAPISchemaObjectTypeMember.check(self, schema, [], cases)
>
> so that we are now checking collisions between tag values, rather than
> cases.  But that's what I did in patch 3/4.  And we still need seen
> passed to Variant.check(), because that's the checking added in 2/4.

A sanity check "no collisions between tag values" doesn't hurt, but I'd
simply rely on the tag type's .check().  The tag type is an enumeration
type, its check() ensures the enumeration values are distinct.  For
variants, checking the tag value is a is a value of the tag type
suffices to ensure they don't collide.

> Okay, you've convinced me - when I post v9, I'll reorder these four
> patches to put 3/4 first.
>
>>  
>>  
>>  class QAPISchemaObjectTypeVariant(QAPISchemaObjectTypeMember):
>> 
>> Then only one caller about 2-4., namely QAPISchemaObjectType.check().
>> Simplify radically: move 2-4. to the caller that cares, drop parameters
>> all_members and seen.
>
> Nope - because seen (well, a copy of seen) is still important to patch 2/4.
>
>> 
>> Still to do then: non-variant member collision checking.  Factor out
>> 3. into a helper function, use it for non-variant members.
>
> Factoring into a helper function is done in 4/4.  I can try and
> rearrange that earlier, too.
>
>> 
>> What do you think?
>> 
>
> Can you at least look at 2, 3, and 4 to see where I'm headed, and then I
> can rearrange things for the v9 spin?  We're probably talking a bit past
> each other, with the same end goal, but a muddle in the middle of how to
> get there.

Yes, we're almost certainly headed in the same direction.  But I got
thoroughly confused and lost in the details on the way, so I had to hack
things up myself to clear my head.  The result looks nice to me, so I'll
send it out in a jiffie in the hope you'll find it useful.



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