msggen: Add VersioningCheck

This is a visitor that ensures every new field has at least an `added`
field, and that we don't change the `added` or `deprecated` annotation
after the fact.

contrib/msggen/msggen/__main__.py

@@ -73,6 +73,9 @@ def run(rootdir: Path):
     p.apply(service)
     OptionalPatch().apply(service)
 
+    # Run the checks here, we should eventually split that out to a
+    # separate subcommand
+    VersioningCheck().check(service)
     generator_chain = GeneratorChain()
 
     add_handler_gen_grpc(generator_chain, meta)

contrib/msggen/msggen/checks.py

@@ -0,0 +1,36 @@
+from abc import ABC
+from msggen import model
+
+
+class Check(ABC):
+    """A check is a visitor that throws exceptions on inconsistencies.
+
+    """
+    def visit(self, field: model.Field) -> None:
+        pass
+
+    def check(self, service: model.Service) -> None:
+        def recurse(f: model.Field):
+            # First recurse if we have further type definitions
+            if isinstance(f, model.ArrayField):
+                self.visit(f.itemtype)
+                recurse(f.itemtype)
+            elif isinstance(f, model.CompositeField):
+                for c in f.fields:
+                    self.visit(c)
+                    recurse(c)
+            # Now visit ourselves
+            self.visit(f)
+        for m in service.methods:
+            recurse(m.request)
+            recurse(m.response)
+
+
+class VersioningCheck(Check):
+    """Check that all schemas have the `added` and `deprecated` annotations.
+    """
+    def visit(self, f: model.Field) -> None:
+        if not hasattr(f, "added"):
+            raise ValueError(f"Field {f.path} is missing the `added` annotation")
+        if not hasattr(f, "deprecated"):
+            raise ValueError(f"Field {f.path} is missing the `deprecated` annotation")