Add versioned daemon protocol envelope

.gitignore

@@ -0,0 +1,14 @@
+
+# Python
+__pycache__/
+*.py[cod]
+
+# Development backups
+*.pre-*
+*.bak
+*.orig
+*.rej
+
+# Accidental test-output files
+/-INFINITY
+/100

daemon/server.py

@@ -17,34 +17,76 @@ SOCKET_PATH = os.environ.get(
 server_socket = None
 registry = None
 
+PROTOCOL_VERSION = 1
+
+
+def protocol_response(payload):
+    """
+    Return a daemon response carrying the wire-protocol version.
+
+    protocol_version is daemon-owned metadata and cannot be overridden by
+    registry or plugin output.
+    """
+    response = dict(payload)
+    response["protocol_version"] = PROTOCOL_VERSION
+    return response
+
 
 def handle_request(req):
+    if not isinstance(req, dict):
+        return protocol_response({
+            "ok": False,
+            "error_code": "invalid_request",
+            "error": "request must be a JSON object",
+        })
+
+    requested_version = req.get("protocol_version")
+
+    # Backward compatibility for the pre-versioned proxy:
+    # absence is accepted during the rolling upgrade, but an explicitly
+    # incompatible version is rejected.
+    if (
+        requested_version is not None
+        and requested_version != PROTOCOL_VERSION
+    ):
+        return protocol_response({
+            "ok": False,
+            "error_code": "unsupported_protocol_version",
+            "error": (
+                "unsupported protocol version: "
+                f"{requested_version}; "
+                f"supported={PROTOCOL_VERSION}"
+            ),
+        })
+
     action = req.get("action")
 
     if action == "ping":
-        return {
+        return protocol_response({
             "ok": True,
             "daemon": "managed-containerd",
-            "protocol_version": 1,
+        })
-        }
 
     if action == "registry.list":
-        return {
+        return protocol_response({
             "ok": True,
             "resources": registry.list_resources(),
-        }
+        })
 
     resource = req.get("resource")
 
     if not resource:
-        return {
+        return protocol_response({
             "ok": False,
+            "error_code": "invalid_request",
             "error": "resource is required",
-        }
+        })
 
-    return registry.execute(
+    return protocol_response(
-        resource,
+        registry.execute(
-        action,
+            resource,
+            action,
+        )
     )
 
 
@@ -136,18 +178,20 @@ def serve():
                     ValueError,
                     KeyError,
                 ) as exc:
-                    response = {
+                    response = protocol_response({
                         "ok": False,
+                        "error_code": "request_error",
                         "error": str(exc),
-                    }
+                    })
 
                 except Exception as exc:
                     traceback.print_exc()
 
-                    response = {
+                    response = protocol_response({
                         "ok": False,
+                        "error_code": "internal_error",
                         "error": str(exc),
-                    }
+                    })
 
                 conn.sendall(
                     json.dumps(