[App] Reserve APP_SERVER_PORT in cloud port allocation (#16782)

Co-authored-by: thomas chaton <[email protected]>

Author: 2 people

src/lightning_app/utilities/network.py

@@ -68,7 +68,7 @@ def find_free_network_port() -> int:
 def _find_free_network_port_cloudspace():
     """Finds a free port in the exposed range when running in a cloudspace."""
     for port in range(
-        constants.APP_SERVER_PORT,
+        constants.APP_SERVER_PORT + 1,  # constants.APP_SERVER_PORT is reserved for the app server
         constants.APP_SERVER_PORT + constants.LIGHTNING_CLOUDSPACE_EXPOSED_PORT_COUNT,
     ):
         if port in _reserved_ports:

tests/tests_app/utilities/test_network.py

@@ -2,6 +2,7 @@
 
 import pytest
 
+from lightning_app.core import constants
 from lightning_app.utilities.network import find_free_network_port, LightningClient
 
 
@@ -40,6 +41,9 @@ def test_find_free_network_port_cloudspace(_, patch_constants):
     # Check that all ports are unique
     assert len(ports) == num_ports
 
+    # Shouldn't use the APP_SERVER_PORT
+    assert constants.APP_SERVER_PORT not in ports
+
 
 def test_lightning_client_retry_enabled():