test(flannel): add regression test for per-host interface default

- Add a focused test that renders the sample inventory's flannel_iface
  expression against fake ansible facts
- Assert a non-eth0 host (enp1s0, ens3) resolves its own interface and that
  the expression defaults from ansible_facts.default_ipv4.interface
- Wire it as a local pre-commit hook (default-interface-test)

.github/scripts/test-default-interface.py

@@ -0,0 +1,90 @@
+#!/usr/bin/env python3
+"""Assert the sample inventory resolves the flannel interface per host.
+
+flannel_iface defaults to the host's default IPv4 interface rather than a
+hardcoded eth0. This test extracts the flannel_iface expression from the
+sample inventory and proves that a host whose primary interface is not named
+eth0 (e.g. enp1s0, ens3) resolves the interface from ansible facts.
+"""
+
+from __future__ import print_function
+
+import os
+import re
+import subprocess
+
+from jinja2 import Environment, StrictUndefined
+
+
+def repo_root():
+    return subprocess.check_output(
+        ["git", "rev-parse", "--show-toplevel"], text=True
+    ).strip()
+
+
+def fail(message):
+    raise SystemExit("default-interface test failed: " + message)
+
+
+class FakeAnsibleFacts(object):
+    """Stand-in for the per-host ``ansible_facts`` dict."""
+
+    def __init__(self, default_iface, iface_ip):
+        self._default = {"interface": default_iface, "address": iface_ip}
+        self._ifaces = {
+            default_iface: {"ipv4": {"address": iface_ip}},
+        }
+
+    @property
+    def default_ipv4(self):
+        return self._default
+
+    def __getitem__(self, key):
+        return self._ifaces[key]
+
+
+def read_all_yml(root):
+    path = os.path.join(root, "inventory", "sample", "group_vars", "all.yml")
+    with open(path, "r") as handle:
+        return handle.read()
+
+
+def extract_value(content, key):
+    # Match a quoted value assigned to the key, e.g. flannel_iface: "...".
+    match = re.search(r"^%s:\s*\"(.+)\"\s*$" % re.escape(key), content, re.M)
+    if not match:
+        fail("could not find %s in the sample inventory" % key)
+    return match.group(1)
+
+
+def resolve(env, expression, facts):
+    template = env.from_string(expression)
+    return template.render(ansible_facts=facts)
+
+
+def main():
+    root = repo_root()
+    content = read_all_yml(root)
+    env = Environment(undefined=StrictUndefined)
+
+    flannel_expr = extract_value(content, "flannel_iface")
+    if "default_ipv4.interface" not in flannel_expr:
+        fail("flannel_iface no longer defaults from ansible facts")
+
+    # A host whose primary interface is enp1s0 (the core #621 scenario).
+    facts = FakeAnsibleFacts("enp1s0", "192.168.30.11")
+    resolved = resolve(env, flannel_expr, facts)
+    if resolved != "enp1s0":
+        fail("flannel_iface resolved to %r, expected enp1s0" % resolved)
+
+    # A different host with a different interface must resolve independently.
+    facts2 = FakeAnsibleFacts("ens3", "192.168.30.12")
+    resolved2 = resolve(env, flannel_expr, facts2)
+    if resolved2 != "ens3":
+        fail("flannel_iface resolved to %r, expected ens3" % resolved2)
+
+    print("default-interface regression test passed")
+
+
+if __name__ == "__main__":
+    main()

.pre-commit-config.yaml

@@ -84,3 +84,11 @@ repos:
         language: system
         pass_filenames: false
         files: ^roles/k3s_server/tasks/metallb\.yml$|^\.github/scripts/test-metallb-remote-read\.sh$
+      - id: default-interface-test
+        name: default interface test
+        entry: python3 .github/scripts/test-default-interface.py
+        language: python
+        additional_dependencies:
+          - Jinja2>=3.1
+        pass_filenames: false
+        files: ^inventory/sample/group_vars/all\.yml$|^\.github/scripts/test-default-interface\.py$