chore: audit 24 and 17 cases, host kept out

tests/netguard/__init__.py

@@ -0,0 +1,33 @@
+"""The guard a test module turns on for itself and the Python it starts
+(sitecustomize). module_guard() gives the setUpModule and tearDownModule
+pair: the module fails at its end when anything tried to leave the loopback."""
+
+from __future__ import annotations
+
+import os
+from pathlib import Path
+from typing import Callable
+
+from .sitecustomize import disable, enable, refused
+
+__all__ = ["module_guard", "enable", "disable", "refused"]
+
+
+def module_guard(name: str) -> tuple[Callable[[], None], Callable[[], None]]:
+    log = Path(__file__).resolve().parents[2] / "tmp" / "tests" / f"netlog-{name}-{os.getpid()}"
+    state: dict = {}
+
+    def set_up() -> None:
+        log.parent.mkdir(parents=True, exist_ok=True)
+        state["before"] = enable(str(log))
+
+    def tear_down() -> None:
+        disable(state.pop("before", {}))
+        tried = refused(str(log))
+        log.unlink(missing_ok=True)
+        if tried:
+            raise AssertionError(
+                f"{len(tried)} connections off the loopback during {name} (pid host): "
+                + ", ".join(tried[:10]))
+
+    return set_up, tear_down