test: netns: Avoid infinite hangs on exceptions in .call()s

If the callable threw an exception, then the process running inside
the netns would exit before sending back a value on the tx pipe, which
meant that `rx.recv()` would hang in the original process.

Make sure that we catch any exceptions, and send that instead of the
result over the pipe in those scenarios. Then in the original process,
re-raise the exception.

test/infamy/netns.py

@@ -139,6 +139,10 @@ class IsolatedMacVlans:
             f"{a[0]:02x}:{a[1]:02x}:{a[2]:02x}:" + \
             f"{a[3]:02x}:{a[4]:02x}:{a[5]:02x}"
 
+    class CallException:
+        def __init__(self, inner):
+            self.inner = inner
+
     def _ns_call(self, fn, tx):
         pid = self.sleeper.pid
 
@@ -150,7 +154,12 @@ class IsolatedMacVlans:
         setns(nns, CLONE_NEWNET)
         os.close(nns)
 
-        tx.send(fn())
+        try:
+            ret = fn()
+        except Exception as e:
+            ret = IsolatedMacVlans.CallException(e)
+
+        tx.send(ret)
         tx.close()
 
     def call(self, fn):
@@ -161,6 +170,9 @@ class IsolatedMacVlans:
         ret = rx.recv()
         rx.close()
         proc.join()
+
+        if type(ret) == IsolatedMacVlans.CallException:
+            raise ret.inner
         return ret
 
     def _mangle_subprocess_args(self, args, kwargs):