commands: if no explicit return type is specified, assume None

This is going to be a super common error for addon authors, so we might as well
handle it.

mitmproxy/command.py

@@ -58,7 +58,10 @@ class Command:
             if i.kind == i.VAR_POSITIONAL:
                 self.has_positional = True
         self.paramtypes = [v.annotation for v in sig.parameters.values()]
-        self.returntype = sig.return_annotation
+        if sig.return_annotation == inspect._empty:
+            self.returntype = None
+        else:
+            self.returntype = sig.return_annotation
 
     def paramnames(self) -> typing.Sequence[str]:
         v = [typename(i) for i in self.paramtypes]

test/mitmproxy/test_command.py

@@ -55,7 +55,20 @@ class TAddon:
         pass
 
 
+class TypeErrAddon:
+    @command.command("noret")
+    def noret(self):
+        pass
+
+
 class TestCommand:
+    def test_typecheck(self):
+        with taddons.context(loadcore=False) as tctx:
+            cm = command.CommandManager(tctx.master)
+            a = TypeErrAddon()
+            c = command.Command(cm, "noret", a.noret)
+            print(c.signature_help())
+
     def test_varargs(self):
         with taddons.context() as tctx:
             cm = command.CommandManager(tctx.master)