add unit tests for heartbeat.

src/py/flwr/client/grpc_rere_client/connection.py

@@ -131,8 +131,6 @@ def ping() -> None:
         # Get Node
         if node is None:
             log(ERROR, "Node instance missing")
-            if not ping_stop_event.is_set():
-                ping_stop_event.wait(PING_CALL_TIMEOUT)
             return
 
         # Construct the ping request

src/py/flwr/client/heartbeat.py

@@ -20,13 +20,41 @@
 
 import grpc
 
+from flwr.common.constant import PING_CALL_TIMEOUT
+from flwr.common.retry_invoker import RetryInvoker, RetryState, exponential
+
 
 def _ping_loop(ping_fn: Callable[[], None], stop_event: threading.Event) -> None:
-    while not stop_event.is_set():
-        try:
+    def wait_fn(wait_time: float) -> None:
+        if not stop_event.is_set():
+            stop_event.wait(wait_time)
+
+    def on_backoff(state: RetryState) -> None:
+        err = state.exception
+        if not isinstance(err, grpc.RpcError):
+            return
+        status_code = err.code()
+        # If ping call timeout is triggered
+        if status_code == grpc.StatusCode.DEADLINE_EXCEEDED:
+            # Avoid long wait time.
+            if state.actual_wait is None:
+                return
+            state.actual_wait = max(state.actual_wait - PING_CALL_TIMEOUT, 0.0)
+
+    def wrapped_ping() -> None:
+        if not stop_event.is_set():
             ping_fn()
-        except grpc.RpcError:
-            pass
+
+    retrier = RetryInvoker(
+        exponential,
+        grpc.RpcError,
+        max_tries=None,
+        max_time=None,
+        on_backoff=on_backoff,
+        wait_function=wait_fn,
+    )
+    while not stop_event.is_set():
+        retrier.invoke(wrapped_ping)
 
 
 def start_ping_loop(

src/py/flwr/client/heartbeat_test.py

@@ -0,0 +1,59 @@
+# Copyright 2024 Flower Labs GmbH. All Rights Reserved.
+#
+# 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.
+# ==============================================================================
+"""Unit tests for heartbeat utility functions."""
+
+
+import threading
+import time
+import unittest
+from unittest.mock import MagicMock
+
+from .heartbeat import start_ping_loop
+
+
+class TestStartPingLoopWithFailures(unittest.TestCase):
+    """Test heartbeat utility functions."""
+
+    def test_ping_loop_terminates(self) -> None:
+        """Test if the ping loop thread terminates when flagged."""
+        # Prepare
+        ping_fn = MagicMock()
+        stop_event = threading.Event()
+
+        # Execute
+        thread = start_ping_loop(ping_fn, stop_event)
+        time.sleep(1)
+        stop_event.set()
+        thread.join(timeout=1)
+
+        # Assert
+        self.assertTrue(ping_fn.called)
+        self.assertFalse(thread.is_alive())
+
+    def test_ping_loop_with_failures_terminates(self) -> None:
+        """Test if the ping loop thread with failures terminates when flagged."""
+        # Prepare
+        ping_fn = MagicMock(side_effect=RuntimeError())
+        stop_event = threading.Event()
+
+        # Execute
+        thread = start_ping_loop(ping_fn, stop_event)
+        time.sleep(1)
+        stop_event.set()
+        thread.join(timeout=1)
+
+        # Assert
+        self.assertTrue(ping_fn.called)
+        self.assertFalse(thread.is_alive())

src/py/flwr/client/rest_client/connection.py

@@ -148,8 +148,6 @@ def ping() -> None:
         # Get Node
         if node is None:
             log(ERROR, "Node instance missing")
-            if not ping_stop_event.is_set():
-                ping_stop_event.wait(PING_CALL_TIMEOUT)
             return
 
         # Construct the ping request
@@ -170,26 +168,20 @@ def ping() -> None:
 
         # Check status code and headers
         if res.status_code != 200:
-            if not ping_stop_event.is_set():
-                ping_stop_event.wait(PING_CALL_TIMEOUT)
             return
         if "content-type" not in res.headers:
             log(
                 WARN,
                 "[Node] POST /%s: missing header `Content-Type`",
                 PATH_PULL_TASK_INS,
             )
-            if not ping_stop_event.is_set():
-                ping_stop_event.wait(PING_CALL_TIMEOUT)
             return
         if res.headers["content-type"] != "application/protobuf":
             log(
                 WARN,
                 "[Node] POST /%s: header `Content-Type` has wrong value",
                 PATH_PULL_TASK_INS,
             )
-            if not ping_stop_event.is_set():
-                ping_stop_event.wait(PING_CALL_TIMEOUT)
             return
 
         # Deserialize ProtoBuf from bytes