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Re: [PATCH v3 05/17] qapi: pass QAPISchemaModule to visit_module instead

From: John Snow
Subject: Re: [PATCH v3 05/17] qapi: pass QAPISchemaModule to visit_module instead of str
Date: Wed, 20 Jan 2021 11:16:19 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0

On 1/20/21 11:02 AM, Eric Blake wrote:
On 1/20/21 6:07 AM, Markus Armbruster wrote:
John Snow <jsnow@redhat.com> writes:

Modify visit_module to pass the module itself instead of just its
name. This allows for future patches to centralize some
module-interrogation behavior within the QAPISchemaModule class itself,
cutting down on duplication between gen.py and schema.py.

We've been tempted to make similar changes before (don't worry, I'm not
building a case for "no" here).

When I wrote the initial version of QAPISchemaVisitor (commit 3f7dc21be,
2015), I aimed for a loose coupling of backends and the internal
representation.  Instead of

     def visit_foo(self, foo):

where @foo is a QAPISchemaFooBar, I wrote

     def visit_foo_bar(self, name, info, [curated attributes of @foo]):

In theory, this is nice: the information exposed to the backends is
obvious, and the backends can't accidentally mutate @foo.

In practice, it kind of failed right then and there:

     def visit_object_type(self, name, info, base, members, variants):

We avoid passing the QAPISchemaObjectType (loose coupling, cool!), only
to pass member information as List[QAPISchemaObjectTypeMember].

Morever, passing "curated atttibutes" has led to visit_commands() taking
a dozen arguments.  Meh.

This had made Eric and me wonder whether we should write off the
decoupling idea as misguided, and just pass the object instead of
"curated attributes", always.  Thoughts?

I'm open to the idea of passing just the larger object instead of the
curated list of relevant attributes.  It's a bit more coupling, but I
don't see any of our qapi code being reused outside its current scope
where the extra coupling will bite us.  But I'm not volunteering for the
refactoring work, because I'm not an expert on python typing hints.  If
consolidating parameters into the larger object makes for fewer
parameters and easier typing hints, I'm assuming the work can be done as
part of static typing; but if leaving things as they currently are is
manageable, that's also fine by me.

Yeah, it can definitely be left as-is for now. I've already gone through all the effort of typing out all of the interfaces, so it's not really a huge ordeal to just leave it as-is.

Passing the objects might be nicer for the future, though, as routing new information or features will involve less churn. (And the signatures will be a lot smaller.)

I suppose it does open us up to callers mutating the schema in the visitors, but they could already do that for the reasons that Markus points out. It's possible that the visitor dispatch could be modified to make deep copies of schema objects, but that adds overhead.

I can just revert this change for now and leave the topic for another day.


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