Implement namespace creation method

Since [1], Pyroute forks the namespace creation to avoid calling
destructive routine "libc.unshare(CLONE_NEWNET)" from the main
process. This implementation uses sockets between both processes
to return any error feedback sent from the child process.

This patch implements the same fork without any communication. If
the child process raises an exception other than "OSError(EEXIST)",
the child process returns 1 that is read by the the main process,
that raises a "RuntimeError" exception.

Related-Bug: #1917487

[1]81db2c98a1

Change-Id: I0294586335a71d0757803843f675124bfb450967

neutron/privileged/agent/linux/ip_lib.py

@@ -543,11 +543,19 @@ def create_netns(name, **kwargs):
 
     :param name: The name of the namespace to create
     """
-    try:
-        netns.create(name, libc=priv_linux.get_cdll())
-    except OSError as e:
-        if e.errno != errno.EEXIST:
-            raise
+    pid = os.fork()
+    if pid == 0:
+        try:
+            netns._create(name, libc=priv_linux.get_cdll())
+        except OSError as e:
+            if e.errno != errno.EEXIST:
+                os._exit(1)
+        except Exception:
+            os._exit(1)
+        os._exit(0)
+    else:
+        if os.waitpid(pid, 0)[1]:
+            raise RuntimeError(_('Error creating namespace %s' % name))
 
 
 @privileged.default.entrypoint