Implement "list_ns_pids" inside Neutron

Instead of using Pyroute2.netns.ns_pids() [1], while [2] is not
solved, a method to list a namespace PIDs is implemented in this
patch.

This implementation is based on the Pyroute2 method.

[1]80f6e7fcdd/pyroute2/netns/__init__.py (L163)
[2]https://github.com/svinota/pyroute2/issues/633

Change-Id: Ic00c02035a0dabf1a1efd1995c84692649c6ad27
Related-Bug: #1841753

neutron/privileged/agent/linux/ip_lib.py

@@ -11,6 +11,7 @@
 # under the License.
 
 import errno
+import os
 import socket
 
 from neutron_lib import constants
@@ -30,6 +31,8 @@ from neutron import privileged
 
 _IP_VERSION_FAMILY_MAP = {4: socket.AF_INET, 6: socket.AF_INET6}
 
+NETNS_RUN_DIR = '/var/run/netns'
+
 
 def _get_scope_name(scope):
     """Return the name of the scope (given as a number), or the scope number
@@ -189,8 +192,29 @@ def open_namespace(namespace):
 
 @privileged.default.entrypoint
 def list_ns_pids(namespace):
-    """List namespace process PIDs"""
-    return netns.ns_pids().get(namespace, [])
+    """List namespace process PIDs
+
+    Based on Pyroute2.netns.ns_pids(). Remove when
+    https://github.com/svinota/pyroute2/issues/633 is fixed.
+    """
+    ns_pids = []
+    try:
+        ns_path = os.path.join(NETNS_RUN_DIR, namespace)
+        ns_inode = os.stat(ns_path).st_ino
+    except OSError:
+        return ns_pids
+
+    for pid in os.listdir('/proc'):
+        if not pid.isdigit():
+            continue
+        try:
+            pid_path = os.path.join('/proc', pid, 'ns', 'net')
+            if os.stat(pid_path).st_ino == ns_inode:
+                ns_pids.append(int(pid))
+        except OSError:
+            continue
+
+    return ns_pids
 
 
 def _translate_ip_device_exception(e, device=None, namespace=None):