feat: add exponential and constant backoff function (#416)

- Implement the same backoff function as in the hcloud-go libary
- Preparation work to change the retry backoff function to use an
exponential backoff interval.
- Rename PollIntervalFunction to BackoffFunction, as it is not only used
for polling.

hcloud/__init__.py

@@ -1,6 +1,10 @@
 from __future__ import annotations
 
-from ._client import Client as Client  # noqa pylint: disable=C0414
+from ._client import (  # noqa pylint: disable=C0414
+    Client as Client,
+    constant_backoff_function as constant_backoff_function,
+    exponential_backoff_function as exponential_backoff_function,
+)
 from ._exceptions import (  # noqa pylint: disable=C0414
     APIException as APIException,
     HCloudException as HCloudException,

hcloud/_client.py

@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import time
+from random import uniform
 from typing import Protocol
 
 import requests
@@ -26,7 +27,7 @@
 from .volumes import VolumesClient
 
 
-class PollIntervalFunction(Protocol):
+class BackoffFunction(Protocol):
     def __call__(self, retries: int) -> float:
         """
         Return a interval in seconds to wait between each API call.
@@ -35,6 +36,47 @@ def __call__(self, retries: int) -> float:
         """
 
 
+def constant_backoff_function(interval: float) -> BackoffFunction:
+    """
+    Return a backoff function, implementing a constant backoff.
+
+    :param interval: Constant interval to return.
+    """
+
+    # pylint: disable=unused-argument
+    def func(retries: int) -> float:
+        return interval
+
+    return func
+
+
+def exponential_backoff_function(
+    *,
+    base: float,
+    multiplier: int,
+    cap: float,
+    jitter: bool = False,
+) -> BackoffFunction:
+    """
+    Return a backoff function, implementing a truncated exponential backoff with
+    optional full jitter.
+
+    :param base: Base for the exponential backoff algorithm.
+    :param multiplier: Multiplier for the exponential backoff algorithm.
+    :param cap: Value at which the interval is truncated.
+    :param jitter: Whether to add jitter.
+    """
+
+    def func(retries: int) -> float:
+        interval = base * multiplier**retries  # Exponential backoff
+        interval = min(cap, interval)  # Cap backoff
+        if jitter:
+            interval = uniform(base, interval)  # Add jitter
+        return interval
+
+    return func
+
+
 class Client:
     """Base Client for accessing the Hetzner Cloud API"""
 
@@ -48,7 +90,7 @@ def __init__(
         api_endpoint: str = "https://api.hetzner.cloud/v1",
         application_name: str | None = None,
         application_version: str | None = None,
-        poll_interval: int | float | PollIntervalFunction = 1.0,
+        poll_interval: int | float | BackoffFunction = 1.0,
         poll_max_retries: int = 120,
         timeout: float | tuple[float, float] | None = None,
     ):
@@ -73,7 +115,7 @@ def __init__(
         self._requests_timeout = timeout
 
         if isinstance(poll_interval, (int, float)):
-            self._poll_interval_func = lambda _: poll_interval  # Constant poll interval
+            self._poll_interval_func = constant_backoff_function(poll_interval)
         else:
             self._poll_interval_func = poll_interval
         self._poll_max_retries = poll_max_retries

tests/unit/test_client.py

@@ -6,7 +6,12 @@
 import pytest
 import requests
 
-from hcloud import APIException, Client
+from hcloud import (
+    APIException,
+    Client,
+    constant_backoff_function,
+    exponential_backoff_function,
+)
 
 
 class TestHetznerClient:
@@ -202,3 +207,24 @@ def test_request_limit_then_success(self, client, rate_limit_response):
             "POST", "http://url.com", params={"argument": "value"}, timeout=2
         )
         assert client._requests_session.request.call_count == 2
+
+
+def test_constant_backoff_function():
+    backoff = constant_backoff_function(interval=1.0)
+    max_retries = 5
+
+    for i in range(max_retries):
+        assert backoff(i) == 1.0
+
+
+def test_exponential_backoff_function():
+    backoff = exponential_backoff_function(
+        base=1.0,
+        multiplier=2,
+        cap=60.0,
+    )
+    max_retries = 5
+
+    results = [backoff(i) for i in range(max_retries)]
+    assert sum(results) == 31.0
+    assert results == [1.0, 2.0, 4.0, 8.0, 16.0]