obj: report missing container when checking versioning mode

When handling an object creation operation, and when the container supposed
to host the object is missing, an unhandled exception was raised. This
exception is now converted to a proper response error.

Also rename things and write documentation.

oioswift/proxy/controllers/obj.py

@@ -190,31 +190,40 @@ def GETorHEAD(self, req):
 
         return resp
 
-    def versioning_management(self, req):
+    def enforce_versioning(self, req):
+        """
+        Enforce the versioning mode of a container just before executing
+        an object operation. This is useful when the current object is not
+        stored in the "main" container but in a shard, where the versioning
+        mode may not have been set yet.
+        """
         if not SUPPORT_VERSIONING:
             return None
 
-        container_root = req.headers.get('X-Object-Sysmeta-Oio-Bucket-Name')
-        if container_root is None:
+        root_container = req.headers.get('X-Object-Sysmeta-Oio-Bucket-Name')
+        if root_container is None:
             return None
 
+        # We can't use _get_info_from_caches as it would use local worker cache
+        # first and an update of versioning mode may not be detected.
         memcache = getattr(self.app, 'memcache', None) or \
             req.environ.get('swift.cache')
         if memcache is None:
             return None
 
-        key = "/".join(("versioning", self.account_name, container_root))
+        key = "/".join(("versioning", self.account_name, root_container))
         val = memcache.get(key)
         if val is not None:
             if val != '':
                 req.headers[FORCEVERSIONING_HEADER] = val
             return
 
-        # we can't use get_info_cache or set_info as it use local worker cache
-        # then memcache: an update of versioning may be not detected
         oio_headers = {'X-oio-req-id': self.trans_id}
-        meta = self.app.storage.container_get_properties(
-            self.account_name, container_root, headers=oio_headers)
+        try:
+            meta = self.app.storage.container_get_properties(
+                self.account_name, root_container, headers=oio_headers)
+        except exceptions.NoSuchContainer:
+            raise HTTPNotFound(request=req)
 
         val = meta['system'].get('sys.m2.policy.version', '')
         memcache.set(key, val)
@@ -412,7 +421,7 @@ def PUT(self, req):
             if aresp:
                 return aresp
 
-        self.versioning_management(req)
+        self.enforce_versioning(req)
 
         old_slo_manifest = None
         old_slo_manifest_etag = None
@@ -705,7 +714,7 @@ def DELETE(self, req):
 
         self._update_x_timestamp(req)
 
-        self.versioning_management(req)
+        self.enforce_versioning(req)
 
         return self._delete_object(req)