On 10/02/23 12:54 pm, Markus Armbruster wrote:
Daniel P. Berrangé <[email protected]> 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 ?
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 ?
or I may have completely misunderstood most of the part 😅. Please let me know
[...]
Regards, Het Gala
