Add `Retry-After` to M_LIMIT_EXCEEDED error responses (#16136)

Implements MSC4041 behind an experimental configuration flag.

5 files changed, 73 insertions, 7 deletions

diff --git a/changelog.d/16136.feature b/changelog.d/16136.feature
new file mode 100644
index 0000000000..4ad98a88c3
--- /dev/null
+++ b/changelog.d/16136.feature
@@ -0,0 +1 @@
+Return a `Retry-After` with `M_LIMIT_EXCEEDED` error responses.
diff --git a/synapse/api/errors.py b/synapse/api/errors.py
index 7ffd72c42c..578e798773 100644
--- a/synapse/api/errors.py
+++ b/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
@@ -503,6 +504,8 @@ class InvalidCaptchaError(SynapseError):
 class LimitExceededError(SynapseError):
     """A client has sent too many requests and is being throttled."""
 
+    include_retry_after_header = False
+
     def __init__(
         self,
         code: int = 429,
@@ -510,7 +513,12 @@ class LimitExceededError(SynapseError):
         retry_after_ms: Optional[int] = None,
         errcode: str = Codes.LIMIT_EXCEEDED,
     ):
-        super().__init__(code, msg, errcode)
+        headers = (
+            {"Retry-After": str(math.ceil(retry_after_ms / 1000))}
+            if self.include_retry_after_header and retry_after_ms is not None
+            else None
+        )
+        super().__init__(code, msg, errcode, headers=headers)
         self.retry_after_ms = retry_after_ms
 
     def error_dict(self, config: Optional["HomeServerConfig"]) -> "JsonDict":
diff --git a/synapse/config/experimental.py b/synapse/config/experimental.py
index 84d6dd13af..cabe0d4397 100644
--- a/synapse/config/experimental.py
+++ b/synapse/config/experimental.py
@@ -18,6 +18,7 @@ from typing import TYPE_CHECKING, Any, Optional
 import attr
 import attr.validators
 
+from synapse.api.errors import LimitExceededError
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersions
 from synapse.config import ConfigError
 from synapse.config._base import Config, RootConfig
@@ -406,3 +407,11 @@ class ExperimentalConfig(Config):
         self.msc4010_push_rules_account_data = experimental.get(
             "msc4010_push_rules_account_data", False
         )
+
+        # MSC4041: Use HTTP header Retry-After to enable library-assisted retry handling
+        #
+        # This is a bit hacky, but the most reasonable way to *alway* include the
+        # headers.
+        LimitExceededError.include_retry_after_header = experimental.get(
+            "msc4041_enabled", False
+        )
diff --git a/tests/api/test_errors.py b/tests/api/test_errors.py
new file mode 100644
index 0000000000..319abfe63d
--- /dev/null
+++ b/tests/api/test_errors.py
@@ -0,0 +1,36 @@
+# Copyright 2023 The Matrix.org Foundation C.I.C.
+#
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+from synapse.api.errors import LimitExceededError
+
+from tests import unittest
+
+
+class ErrorsTestCase(unittest.TestCase):
+    # Create a sub-class to avoid mutating the class-level property.
+    class LimitExceededErrorHeaders(LimitExceededError):
+        include_retry_after_header = True
+
+    def test_limit_exceeded_header(self) -> None:
+        err = ErrorsTestCase.LimitExceededErrorHeaders(retry_after_ms=100)
+        self.assertEqual(err.error_dict(None).get("retry_after_ms"), 100)
+        assert err.headers is not None
+        self.assertEqual(err.headers.get("Retry-After"), "1")
+
+    def test_limit_exceeded_rounding(self) -> None:
+        err = ErrorsTestCase.LimitExceededErrorHeaders(retry_after_ms=3001)
+        self.assertEqual(err.error_dict(None).get("retry_after_ms"), 3001)
+        assert err.headers is not None
+        self.assertEqual(err.headers.get("Retry-After"), "4")
diff --git a/tests/rest/client/test_login.py b/tests/rest/client/test_login.py
index ffbc13bb8d..62c32cae5e 100644
--- a/tests/rest/client/test_login.py
+++ b/tests/rest/client/test_login.py
@@ -169,7 +169,8 @@ class LoginRestServletTestCase(unittest.HomeserverTestCase):
                 # which sets these values to 10000, but as we're overriding the entire
                 # rc_login dict here, we need to set this manually as well
                 "account": {"per_second": 10000, "burst_count": 10000},
-            }
+            },
+            "experimental_features": {"msc4041_enabled": True},
         }
     )
     def test_POST_ratelimiting_per_address(self) -> None:
@@ -189,12 +190,15 @@ class LoginRestServletTestCase(unittest.HomeserverTestCase):
             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)
 
@@ -217,7 +221,8 @@ class LoginRestServletTestCase(unittest.HomeserverTestCase):
                 # which sets these values to 10000, but as we're overriding the entire
                 # rc_login dict here, we need to set this manually as well
                 "address": {"per_second": 10000, "burst_count": 10000},
-            }
+            },
+            "experimental_features": {"msc4041_enabled": True},
         }
     )
     def test_POST_ratelimiting_per_account(self) -> None:
@@ -234,12 +239,15 @@ class LoginRestServletTestCase(unittest.HomeserverTestCase):
             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)
 
@@ -262,7 +270,8 @@ class LoginRestServletTestCase(unittest.HomeserverTestCase):
                 # rc_login dict here, we need to set this manually as well
                 "address": {"per_second": 10000, "burst_count": 10000},
                 "failed_attempts": {"per_second": 0.17, "burst_count": 5},
-            }
+            },
+            "experimental_features": {"msc4041_enabled": True},
         }
     )
     def test_POST_ratelimiting_per_account_failed_attempts(self) -> None:
@@ -279,12 +288,15 @@ class LoginRestServletTestCase(unittest.HomeserverTestCase):
             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)