Undeprecate loadBefore

Dieter Maurer notes that loadBefore cannot be deprecated yet because ZEO
essentially depends on the `end_tid` information returned by loadBefore
to update its cache:

https://github.com/zopefoundation/ZODB/pull/323#issuecomment-842021970

And to remove this dependency it would require to rework ZODB caching layer:

https://github.com/zopefoundation/ZODB/pull/323#issuecomment-845917355

So we cannot deprecate loadBefore until this rework is implemented first.

-> Remove general loadBefore deprecation, and emit loadBefore vs
loadBeforeEx warning only when actually hitting a "deletion" record,
because only that case is known to lead to data corruption.

CHANGES.rst

@@ -5,7 +5,7 @@
 5.6.1 (unreleased)
 ==================
 
-- Introduce a new ``loadBeforeEx`` interface and deprecate ``loadBefore``:
+- Introduce a new ``loadBeforeEx`` interface that complements ``loadBefore``:
   ``loadBeforeEx`` is simpler, provides better information for object delete
   records and can be more efficiently implemented by many storages.
   ``loadBeforeEx`` is used (and required) to fix a ``DemoStorage`` data corruption

src/ZODB/interfaces.py

@@ -710,9 +710,6 @@ class IStorage(Interface):
     def loadBefore(oid, tid):
         """Load the object data written before a transaction id
 
-        ( This method is deprecated and kept for backward-compatibility.
-          Please use loadBeforeEx instead. )
-
         If there isn't data before the object before the given
         transaction, then None is returned, otherwise three values are
         returned:
@@ -724,6 +721,8 @@ class IStorage(Interface):
         - The transaction id of the following revision, if any, or None.
 
         If the object id isn't in the storage, then POSKeyError is raised.
+
+        See also: IStorageLoadBeforeEx.loadBeforeEx .
         """
 
     def loadSerial(oid, serial):

src/ZODB/utils.py

@@ -402,21 +402,7 @@ def loadBeforeEx(storage, oid, before):
         except NotImplementedError:
             pass
 
-    # storage does not provide IStorageLoadBeforeEx - warn + fall back to loadBefore
-    if type(storage) not in _loadBeforeExWarned:
-        # there is potential race around _loadBeforeExWarned access, but due to the
-        # GIL this race cannot result in that set corruption, and can only lead
-        # to us emitting the warning twice instead of just once.
-        # -> do not spend CPU on lock and just ignore it.
-        warnings.warn(
-            "FIXME %s does not provide loadBeforeEx - emulating it via loadBefore, but ...\n"
-            "\t... 1) access will be potentially slower, and\n"
-            "\t... 2) not full semantic of loadBeforeEx could be provided.\n"
-            "\t...    this can lead to data corruption.\n"
-            "\t... -> please see https://github.com/zopefoundation/ZODB/issues/318 for details." %
-            type(storage), DeprecationWarning)
-        _loadBeforeExWarned.add(type(storage))
-
+    # storage does not provide IStorageLoadBeforeEx - fall back to loadBefore
     try:
         r = storage.loadBefore(oid, before)
     except ZODB.POSException.POSKeyError:
@@ -424,7 +410,22 @@ def loadBeforeEx(storage, oid, before):
 
     if r is None:
         # object was removed; however loadBefore does not tell when.
-        # return serial=0 - this is the "data corruption" case talked about above.
+        # return serial=0. This can, however, lead to data corruption with e.g.
+        # DemoStorage (https://github.com/zopefoundation/ZODB/issues/318), so
+        # emit corresponding warning.
+        if type(storage) not in _loadBeforeExWarned:
+            # there is potential race around _loadBeforeExWarned access, but due to the
+            # GIL this race cannot result in that set corruption, and can only lead
+            # to us emitting the warning twice instead of just once.
+            # -> do not spend CPU on lock and just ignore it.
+            warnings.warn(
+                "FIXME %s does not provide loadBeforeEx - emulating it via loadBefore, but ...\n"
+                "\t... 1) access is be potentially slower, and\n"
+                "\t... 2) not full semantic of loadBeforeEx could be provided.\n"
+                "\t...    this can lead to data corruption in the presence of delete records.\n"
+                "\t... -> please see https://github.com/zopefoundation/ZODB/issues/318 for details." %
+                type(storage), PendingDeprecationWarning)
+            _loadBeforeExWarned.add(type(storage))
         return (None, z64)
 
     data, serial, next_serial = r