feat: implement retry policy (#430)

Implement a retry handler to retry requests based on an internal retry
policy.

The retry policy will evolve over time to cover more use cases, but for
now we want to start simple, and maybe someday allow users to tweak the
retry policy.

Related to hetznercloud/hcloud-go#470

BEGIN_COMMIT_OVERRIDE
feat: retry requests when the api gateway errors (#430)
feat: retry requests when the network timed outs (#430)
feat: retry requests when the rate limit was reached (#430)
feat: retry requests when the api returns a conflict error (#430)
END_COMMIT_OVERRIDE

---------

Co-authored-by: Julian Tölle <[email protected]>

hcloud/_client.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import time
+from http import HTTPStatus
 from random import uniform
 from typing import Protocol
 
@@ -78,7 +79,33 @@ def func(retries: int) -> float:
 
 
 class Client:
-    """Base Client for accessing the Hetzner Cloud API"""
+    """
+    Client for the Hetzner Cloud API.
+
+    The Hetzner Cloud API reference is available at https://docs.hetzner.cloud.
+
+    **Retry mechanism**
+
+    The :attr:`Client.request` method will retry failed requests that match certain criteria. The
+    default retry interval is defined by an exponential backoff algorithm truncated to 60s
+    with jitter. The default maximal number of retries is 5.
+
+    The following rules define when a request can be retried:
+
+    - When the client returned a network timeout error.
+    - When the API returned an HTTP error, with the status code:
+
+        - ``502`` Bad Gateway
+        - ``504`` Gateway Timeout
+
+    - When the API returned an application error, with the code:
+
+        - ``conflict``
+        - ``rate_limit_exceeded``
+
+    Changes to the retry policy might occur between releases, and will not be considered
+    breaking changes.
+    """
 
     _version = __version__
     __user_agent_prefix = "hcloud-python"
@@ -256,50 +283,71 @@ def request(  # type: ignore[no-untyped-def]
 
         retries = 0
         while True:
-            response = self._requests_session.request(
-                method=method,
-                url=url,
-                headers=headers,
-                **kwargs,
-            )
-
-            correlation_id = response.headers.get("X-Correlation-Id")
-            payload = {}
             try:
-                if len(response.content) > 0:
-                    payload = response.json()
-            except (TypeError, ValueError) as exc:
-                raise APIException(
-                    code=response.status_code,
-                    message=response.reason,
-                    details={"content": response.content},
-                    correlation_id=correlation_id,
-                ) from exc
-
-            if not response.ok:
-                if not payload or "error" not in payload:
-                    raise APIException(
-                        code=response.status_code,
-                        message=response.reason,
-                        details={"content": response.content},
-                        correlation_id=correlation_id,
-                    )
-
-                error: dict = payload["error"]
-
-                if (
-                    error["code"] == "rate_limit_exceeded"
-                    and retries < self._retry_max_retries
-                ):
+                response = self._requests_session.request(
+                    method=method,
+                    url=url,
+                    headers=headers,
+                    **kwargs,
+                )
+                return self._read_response(response)
+            except APIException as exception:
+                if retries < self._retry_max_retries and self._retry_policy(exception):
                     time.sleep(self._retry_interval(retries))
                     retries += 1
                     continue
-
+                raise
+            except requests.exceptions.Timeout:
+                if retries < self._retry_max_retries:
+                    time.sleep(self._retry_interval(retries))
+                    retries += 1
+                    continue
+                raise
+
+    def _read_response(self, response: requests.Response) -> dict:
+        correlation_id = response.headers.get("X-Correlation-Id")
+        payload = {}
+        try:
+            if len(response.content) > 0:
+                payload = response.json()
+        except (TypeError, ValueError) as exc:
+            raise APIException(
+                code=response.status_code,
+                message=response.reason,
+                details={"content": response.content},
+                correlation_id=correlation_id,
+            ) from exc
+
+        if not response.ok:
+            if not payload or "error" not in payload:
                 raise APIException(
-                    code=error["code"],
-                    message=error["message"],
-                    details=error.get("details"),
+                    code=response.status_code,
+                    message=response.reason,
+                    details={"content": response.content},
                     correlation_id=correlation_id,
                 )
 
-            return payload
+            error: dict = payload["error"]
+            raise APIException(
+                code=error["code"],
+                message=error["message"],
+                details=error.get("details"),
+                correlation_id=correlation_id,
+            )
+
+        return payload
+
+    def _retry_policy(self, exception: APIException) -> bool:
+        if isinstance(exception.code, str):
+            return exception.code in (
+                "rate_limit_exceeded",
+                "conflict",
+            )
+
+        if isinstance(exception.code, int):
+            return exception.code in (
+                HTTPStatus.BAD_GATEWAY,
+                HTTPStatus.GATEWAY_TIMEOUT,
+            )
+
+        return False

tests/unit/test_client.py

@@ -109,12 +109,12 @@ def test_request_fails(self, client, fail_response):
 
     def test_request_fails_correlation_id(self, client, response):
         response.headers["X-Correlation-Id"] = "67ed842dc8bc8673"
-        response.status_code = 409
+        response.status_code = 422
         response._content = json.dumps(
             {
                 "error": {
-                    "code": "conflict",
-                    "message": "some conflict",
+                    "code": "service_error",
+                    "message": "Something crashed",
                 }
             }
         ).encode("utf-8")
@@ -125,11 +125,11 @@ def test_request_fails_correlation_id(self, client, response):
                 "POST", "http://url.com", params={"argument": "value"}, timeout=2
             )
         error = exception_info.value
-        assert error.code == "conflict"
-        assert error.message == "some conflict"
+        assert error.code == "service_error"
+        assert error.message == "Something crashed"
         assert error.details is None
         assert error.correlation_id == "67ed842dc8bc8673"
-        assert str(error) == "some conflict (conflict, 67ed842dc8bc8673)"
+        assert str(error) == "Something crashed (service_error, 67ed842dc8bc8673)"
 
     def test_request_500(self, client, fail_response):
         fail_response.status_code = 500
@@ -208,6 +208,42 @@ def test_request_limit_then_success(self, client, rate_limit_response):
         )
         assert client._requests_session.request.call_count == 2
 
+    @pytest.mark.parametrize(
+        ("exception", "expected"),
+        [
+            (
+                APIException(code="rate_limit_exceeded", message="Error", details=None),
+                True,
+            ),
+            (
+                APIException(code="conflict", message="Error", details=None),
+                True,
+            ),
+            (
+                APIException(code=409, message="Conflict", details=None),
+                False,
+            ),
+            (
+                APIException(code=429, message="Too Many Requests", details=None),
+                False,
+            ),
+            (
+                APIException(code=502, message="Bad Gateway", details=None),
+                True,
+            ),
+            (
+                APIException(code=503, message="Service Unavailable", details=None),
+                False,
+            ),
+            (
+                APIException(code=504, message="Gateway Timeout", details=None),
+                True,
+            ),
+        ],
+    )
+    def test_retry_policy(self, client, exception, expected):
+        assert client._retry_policy(exception) == expected
+
 
 def test_constant_backoff_function():
     backoff = constant_backoff_function(interval=1.0)