Add Retry-After to M_LIMIT_EXCEEDED error responses

changelog.d/16136.feature

@@ -0,0 +1 @@
+Return a `Retry-After` with `M_LIMIT_EXCEEDED` error responses.

synapse/api/errors.py

@@ -16,6 +16,7 @@
 """Contains exceptions and error codes."""
 
 import logging
+import math
 import typing
 from enum import Enum
 from http import HTTPStatus
@@ -510,7 +511,12 @@ def __init__(
         retry_after_ms: Optional[int] = None,
         errcode: str = Codes.LIMIT_EXCEEDED,
     ):
-        super().__init__(code, msg, errcode)
+        headers = (
+            None
+            if retry_after_ms is None
+            else {"Retry-After": str(math.ceil(retry_after_ms / 1000))}
+        )
+        super().__init__(code, msg, errcode, headers=headers)
         self.retry_after_ms = retry_after_ms
 
     def error_dict(self, config: Optional["HomeServerConfig"]) -> "JsonDict":

tests/rest/client/test_login.py

@@ -189,12 +189,15 @@ def test_POST_ratelimiting_per_address(self) -> None:
             if i == 5:
                 self.assertEqual(channel.code, 429, msg=channel.result)
                 retry_after_ms = int(channel.json_body["retry_after_ms"])
+                retry_header = channel.headers.getRawHeaders("Retry-After")
             else:
                 self.assertEqual(channel.code, 200, msg=channel.result)
 
         # Since we're ratelimiting at 1 request/min, retry_after_ms should be lower
         # than 1min.
-        self.assertTrue(retry_after_ms < 6000)
+        self.assertLess(retry_after_ms, 6000)
+        assert retry_header
+        self.assertLessEqual(int(retry_header[0]), 6)
 
         self.reactor.advance(retry_after_ms / 1000.0 + 1.0)
 
@@ -234,12 +237,15 @@ def test_POST_ratelimiting_per_account(self) -> None:
             if i == 5:
                 self.assertEqual(channel.code, 429, msg=channel.result)
                 retry_after_ms = int(channel.json_body["retry_after_ms"])
+                retry_header = channel.headers.getRawHeaders("Retry-After")
             else:
                 self.assertEqual(channel.code, 200, msg=channel.result)
 
         # Since we're ratelimiting at 1 request/min, retry_after_ms should be lower
         # than 1min.
-        self.assertTrue(retry_after_ms < 6000)
+        self.assertLess(retry_after_ms, 6000)
+        assert retry_header
+        self.assertLessEqual(int(retry_header[0]), 6)
 
         self.reactor.advance(retry_after_ms / 1000.0)
 
@@ -279,12 +285,15 @@ def test_POST_ratelimiting_per_account_failed_attempts(self) -> None:
             if i == 5:
                 self.assertEqual(channel.code, 429, msg=channel.result)
                 retry_after_ms = int(channel.json_body["retry_after_ms"])
+                retry_header = channel.headers.getRawHeaders("Retry-After")
             else:
                 self.assertEqual(channel.code, 403, msg=channel.result)
 
         # Since we're ratelimiting at 1 request/min, retry_after_ms should be lower
         # than 1min.
-        self.assertTrue(retry_after_ms < 6000)
+        self.assertLess(retry_after_ms, 6000)
+        assert retry_header
+        self.assertLessEqual(int(retry_header[0]), 6)
 
         self.reactor.advance(retry_after_ms / 1000.0 + 1.0)