Merge branch 't' into t2

* t:
  X lib/zodb: zstor_2zurl: Explicitly reject MappingStorage

lib/zodb.py

@@ -371,4 +371,11 @@ def zstor_2zurl(zstor):
 
         return u
 
+    # MappingStorage:
+    if ztype == "ZODB.MappingStorage.MappingStorage":
+        raise ValueError("%s is in-RAM storage\n"  +
+            "\tin-RAM storages are not supported:\n" +
+            "\ta zurl pointing to in-RAM storage in one process would lead to\n"
+            "\tanother in-RAM storage in WCFS process." % ztype)
+
     raise NotImplementedError("don't know how to extract zurl from %r" % zstor)