Protect random port selection against flaky docker port authorisation…

… failures

tests/integration/conftest.py

@@ -11,10 +11,26 @@
 import pytest
 
 
-def _get_free_port():
-    sock = socket.socket()
-    sock.bind(("", 0))
-    return sock.getsockname()[1]
+def _get_free_port(upper_bound=50_000):
+    """Returns a random free port, with an upper bound.
+
+    The upper bound is required as Docker does not have
+    permissions on high port numbers on some systems.
+
+    """
+    port = upper_bound + 1
+    attempts = 0
+    max_attempts = 10
+    while port > upper_bound and attempts < max_attempts:
+        sock = socket.socket()
+        sock.bind(("", 0))
+        port = sock.getsockname()[1]
+        attempts += 1
+
+    if attempts == max_attempts:
+        raise RuntimeError(f"Could not find a free port to use with the provided {upper_bound=}.")
+
+    return port
 
 
 def _get_random_name(length=6):