use a context manager to apply --no-auto-shard option

...so that it gets unapplied at end of the run_* methods

This is probably overkill, but does guard against unforeseen
side-effects if the daemon run_* methods were ever called more than
once with different kwargs.

Change-Id: Iad6a3bcc99a26550048346dffec9aee432828f3a

swift/container/sharder.py

@@ -23,6 +23,7 @@ import os
 import six
 from six.moves.urllib.parse import quote
 from eventlet import Timeout
+from contextlib import contextmanager
 
 from swift.common import internal_client
 from swift.common.constraints import check_drive, AUTO_CREATE_ACCOUNT_PREFIX
@@ -1712,14 +1713,20 @@ class ContainerSharder(ContainerReplicator):
 
         self._report_stats()
 
+    @contextmanager
     def _set_auto_shard_from_command_line(self, **kwargs):
+        conf_auto_shard = self.auto_shard
         auto_shard = kwargs.get('auto_shard', None)
         if auto_shard is not None:
             self.auto_shard = config_true_value(auto_shard)
+        try:
+            yield
+        finally:
+            self.auto_shard = conf_auto_shard
 
     def run_forever(self, *args, **kwargs):
         """Run the container sharder until stopped."""
-        self._set_auto_shard_from_command_line(**kwargs)
+        with self._set_auto_shard_from_command_line(**kwargs):
             self.reported = time.time()
             time.sleep(random() * self.interval)
             while True:
@@ -1742,7 +1749,7 @@ class ContainerSharder(ContainerReplicator):
         override_options = parse_override_options(once=True, **kwargs)
         devices_to_shard = override_options.devices or Everything()
         partitions_to_shard = override_options.partitions or Everything()
-        self._set_auto_shard_from_command_line(**kwargs)
+        with self._set_auto_shard_from_command_line(**kwargs):
             begin = self.reported = time.time()
             self._one_shard_cycle(devices_to_shard=devices_to_shard,
                                   partitions_to_shard=partitions_to_shard)