Support gRPC map

[billcchung]

Is your feature request related to a problem? Please describe it.

Current if we define proto with a map like

message UserPreferences {
  map<string, string> preference = 10;
}

the generated grpc.swift Will contain:

extension UserPreferences.PreferencesEntry: GRPCProtobufPayload {}

which resulted in an error:

'PreferencesEntry' is not a member type of 'UserPreferences'

in the proto3 spec map can be used.

Describe the solution you'd like

Hopefully we can have map supported.

Describe alternatives you've considered

In order to work around this we had to refactor the struct to use repeated:

message UserPreferences {
  repeated UserPreference preferences = 1;
}

message UserPreference {
  string key = 10;
  string value = 20;
}

but this isnt really ideal, hopefully we can have map supported.

Additional context

[glbrntt]

Thanks for the report @billcchung; I'm aware of this particular bug. It's related to how we support payloads other than Protobuf which has caused quite a lot of pain. As such I'm currently revisiting how we support other payloads: the work done in #886 and #889 remove the requirement to have GRPCProtobufPayload entirely and should just eliminate this class of bugs.

I need to make a few more PRs to finish off those changes but I'm hoping to release these changes (and resolve this bug) in the next couple of days.

[glbrntt]

In the meantime if you apply this patch to the code generator you should be able to use map in your proto:

--- a/Sources/protoc-gen-grpc-swift/Generator-Conformance.swift
+++ b/Sources/protoc-gen-grpc-swift/Generator-Conformance.swift
@@ -32,7 +32,7 @@ extension Generator {
     self.println("extension \(name): GRPCProtobufPayload {}")
 
     // Messages may be nested.
-    for message in message.messages {
+    for message in message.messages where !message.isMapEntry {
       self.printProtobufExtensions(for: message)
     }
   }

[billcchung]

thank you so much @glbrntt, we prefer to not apply some patches (but the suggestion is still appreciated!),
so I'll keep an eye on the release and once it's released we can revert our work around :D

[glbrntt]

This should now be fixed in https://github.com/grpc/grpc-swift/releases/tag/1.0.0-alpha.17 -- please let us know if you have any more problems.