Introduce ProtocolFeatures

Introduces the ProtocolFeatures class which contains information that
affects how the CQL protocol should be serialized and deserialized.
Currently, it only supports the Scylla-specific SCYLLA_RATE_LIMIT_ERROR
extension.

Author: sylwiaszunejko

cassandra/protocol_features.py

@@ -0,0 +1,38 @@
+import logging
+
+log = logging.getLogger(__name__)
+
+
+RATE_LIMIT_ERROR_EXTENSION = "SCYLLA_RATE_LIMIT_ERROR"
+
+class ProtocolFeatures(object):
+    rate_limit_error = None
+
+    def __init__(self, rate_limit_error=None):
+        self.rate_limit_error = rate_limit_error
+
+    @staticmethod
+    def parse_from_supported(supported):
+        return ProtocolFeatures(rate_limit_error = ProtocolFeatures.maybe_parse_rate_limit_error(supported))
+
+    @staticmethod
+    def maybe_parse_rate_limit_error(supported):
+        vals = supported.get(RATE_LIMIT_ERROR_EXTENSION)
+        if vals is not None:
+            code_str = ProtocolFeatures.get_cql_extension_field(vals, "ERROR_CODE")
+            return int(code_str)
+
+    #  Looks up a field which starts with `key=` and returns the rest
+    @staticmethod
+    def get_cql_extension_field(vals, key):
+        for v in vals:
+            stripped_v = v.strip()
+            if stripped_v.startswith(key) and stripped_v[len(key)] == '=':
+                result = stripped_v[len(key) + 1:]
+                return result
+        return None
+
+    def add_startup_options(self, options):
+        if self.rate_limit_error is not None:
+            options[RATE_LIMIT_ERROR_EXTENSION] = ""
+