On Fri, Feb 17, 2023 at 04:48:59PM +0530, Het Gala wrote: > > On 14/02/23 3:46 pm, Markus Armbruster wrote: > > Het Gala <het.g...@nutanix.com> writes: > > > > > On 10/02/23 12:54 pm, Markus Armbruster wrote: > > > > Daniel P. Berrangé <berra...@redhat.com> writes: > > > > > > > > [...] > > > > > > > > > > +## > > > > > > +# @MigrateAddress: > > > > > > +# > > > > > > +# The options available for communication transport mechanisms for > > > > > > migration > > > > > > +# > > > > > > +# Since 8.0 > > > > > > +## > > > > > > +{ 'union' : 'MigrateAddress', > > > > > > + 'base' : { 'transport' : 'MigrateTransport'}, > > > > > > + 'discriminator' : 'transport', > > > > > > + 'data' : { > > > > > > + 'socket' : 'MigrateSocketAddr', > > > > > > + 'exec' : 'MigrateExecAddr', > > > > > > + 'rdma': 'MigrateRdmaAddr' } } > > > > > Ideally this would be > > > > > > > > > > 'data' : { > > > > > 'socket' : 'SocketAddress', > > > > > 'exec' : 'MigrateCommand', > > > > > 'rdma': 'InetSocketAddress' } } > > > > > > > > > > though the first SocketAddress isn't possible unless it is easy to > > > > > lift the QAPI limitation. > > > > Context: SocketAddress is a QAPI union, and "the QAPI limitation" is > > > > > > > > scripts/qapi-gen.py: In file included from > > > > ../qapi/qapi-schema.json:79: > > > > ../qapi/migration.json: In union 'MigrateAddress': > > > > ../qapi/migration.json:1505: branch 'socket' cannot use union > > > > type 'SocketAddress' > > > > > > > > Emitted by schema.py like this: > > > > > > > > if (not isinstance(v.type, QAPISchemaObjectType) > > > > or v.type.variants): > > > > raise QAPISemError( > > > > self.info, > > > > "%s cannot use %s" > > > > % (v.describe(self.info), v.type.describe())) > > > > > > > > This enforces docs/devel/qapi-code-gen.rst's clause > > > > > > > > The BRANCH's value defines the branch's properties, in particular > > > > its > > > > type. The type must a struct type. [...] > > > > > > > > Next paragraph: > > > > > > > > In the Client JSON Protocol, a union is represented by an object > > > > with > > > > the common members (from the base type) and the selected branch's > > > > members. The two sets of member names must be disjoint. > > > > > > > > So, we're splicing in the members of the branch's JSON object. For that > > > > to even make sense, the branch type needs to map to a JSON object. This > > > > is fundamental. It's the first part of the condition in the code > > > > snippet above. > > > > > > > > We have two kinds of QAPI types that map to a JSON object: struct and > > > > union. The second part of the condition restricts to struct. Unless > > > > I'm missing something (imperfect memory...), this is *not* fundamental, > > > > just a matter of implementing it. But I'd have to try to be sure. > > > > > > > > > > > > Instead of simply allowing unions in addition to structs here, I'd like > > > > to go one step further, and fuse the two into "objects". Let me > > > > explain. > > > > > > > > If we abstract from syntax, structs have become almost a special kind of > > > > union. Unions have a set of common members and sets of variant members, > > > > and a special common member (the tag) selects the set of variant > > > > members. Structs are unions with zero variants and no tag. > > > > > > > > The generator code actually represents both structs and unions as a > > > > common QAPISchemaObjectType already. QAPI/QMP introspection does the > > > > same: it uses a single meta type 'object' for both. > > > > > > > > > > > > There is another spot where only structs are allowed: a struct or > > > > union's base type. That restriction will be awkward to lift, as I made > > > > the mistake of baking the assumption "object type has at most one tag > > > > member" into QAPI/QMP introspection . > > > Hi Markus, thankyou for explaning in such detail. I tried to understand > > > of what you explained. > > > > > > So IIUC, you mentioned the QAPI generator treats both structs and unions > > > same, but basically in the schema.py checks is where it tries to > > > distinguish between the two ? and because of the fact that > > > docs/devel/qapi-code-gen.rst states that for a union, it's branches must > > > be 'struct', and that's the reason it gives an error ? > > Permit me a brief digression into history. > > > > The initial QAPI design language provided product types (structs) and > > sum types (unions containing exactly one of several types, and a tag > > member that tells which one). The two are orthogonal. > > > > These unions turned out rather awkward. > > > > The unions we have today are more general. They have common members, > > and one of them is the tag member, of enumeration type. For each tag > > value, they have variant members. Both the common members and each tag > > value's variant members are given as struct types. > > > > What if the tag's enumeration type is empty, i.e. has no values? We get > > a union with no variant members, only common ones. Isn't that a struct? > > > > Not quite. To get a struct, we also have to drop the tag member. It > > has no possible values anyway. > > > > You see, struct types are almost a special case of today's union types. > > To overcome "almost", we can introduce the notion of "object type": > > > > * An object type has common members, one of them can be a tag member, of > > enumeration type, not empty. For each tag value, it additionally has > > variant members. > > > > * A union type is an object type with a tag member and variant members. > > > > * A struct type is an object type without tag member and variant > > members. > > > > The QAPI generator code already made the jump to this object type > > notion. It transform the special cases into the general case at first > > opportunity, in QAPISchema._def_struct_type() and ._def_union_type(). > > > > *Except* we haven't implemented support for variant members in a few > > places where they cannot occur now, e.g. as a tag value's variant. This > > is the restriction you ran into. > > > > I'd like to make the jump to object type in the QAPI schema language, > > too. But that's not a prerequisite to lifting the restriction. > > > > > If that's the case, can we improve on our checks and allow union as a > > > part of branch of a union ? or something else ? > > I believe we can implement the missing parts and relax the checks. But > > to be sure, we need to try. > > > > > or I may have completely misunderstood most of the part 😅. Please let me > > > know > > More questions? > > Completely understood everything. Thankyou for the wonderful explanation. > Looking forward to implement the missing parts in QAPI schema language.
I cc'd you on a patch that implements this missing feature a couple of days ago, and its on Markus' review todo list. So we should be able to decide how to move forward sometime next week. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|