fix(tests): reap orphaned descendants after each test

conftest sets PR_SET_CHILD_SUBREAPER so daemonized descendants reparent
to the pytest session, but the framework only waitpid()s the firecracker
PID. Other helpers (screen, ssh, vhost-user backends, socat-forked cat)
linger as zombies, and their queued signals stay charged against the
RLIMIT_SIGPENDING pool until reaped, which can cause later signal sends
to be dropped.

Add a per-test reap loop that drains exited descendants on teardown,
ordered after the microVM is killed via a microvm_factory dependency.

Signed-off-by: Riccardo Mancini <mancio@amazon.com>

Author: Manciukic

tests/conftest.py

@@ -60,6 +60,23 @@
     raise OSError(ctypes.get_errno(), "prctl(PR_SET_CHILD_SUBREAPER) failed")
 
 
+def reap_orphaned_children():
+    """Reap descendants that have reparented to this subreaper session.
+
+    Non-blocking; safe to call repeatedly. Returns the number reaped.
+    """
+    reaped = 0
+    while True:
+        try:
+            pid, _status = os.waitpid(-1, os.WNOHANG)
+        except ChildProcessError:
+            break
+        if pid == 0:
+            break
+        reaped += 1
+    return reaped
+
+
 METRICS = get_metrics_logger()
 PHASE_REPORT_KEY = pytest.StashKey[dict[str, pytest.CollectReport]]()
 
@@ -126,6 +143,17 @@ def record_props(request, record_property):
     record_property("description", function_docstring)
 
 
+@pytest.fixture(scope="function")
+def reap_orphans():
+    """Reap orphaned descendants after each test.
+
+    Teardown runs after the microVM is killed because `microvm_factory`
+    depends on this fixture (fixtures finalize in reverse setup order).
+    """
+    yield
+    reap_orphaned_children()
+
+
 def pytest_runtest_logreport(report):
     """Send general test metrics to CloudWatch"""
 
@@ -374,8 +402,12 @@ def get(self, _netns_id):
 
 
 @pytest.fixture()
-def microvm_factory(request, record_property, results_dir, netns_factory):
-    """Fixture to create microvms simply."""
+def microvm_factory(request, record_property, results_dir, netns_factory, reap_orphans):
+    """Fixture to create microvms simply.
+
+    Depends on `reap_orphans` for teardown ordering (reaping runs after
+    the VMs are killed).
+    """
 
     binary_dir = request.config.getoption("--binary-dir") or DEFAULT_BINARY_DIR
     if isinstance(binary_dir, str):