lib/zodb: Teach zconn_at to work on ZODB4

In 3bd82127 (lib/zodb: Add zconn_at draft (ZODB5 only)) we added
zconn_at function to find out as of which state a ZODB connection is
viewing the database. That was ZODB5-only however.

Let's add support for ZODB4 now - by requiring ZODB4-wc2 - a version of
ZODB4 with MVCC backported from ZODB5: nexedi/ZODB!1

This makes wendelin.core to work on either ZODB5 or ZODB4-wc2, but not
plain ZODB4. However as zconn_at will be used only for WCFS-integration,
non-wcfs mode will continue to work on all ZODB5, ZODB4-wc2 and plain
ZODB4.

ZBigFile + WCFS client integration will use zconn_at to open WCFS
connection that corresponds to ZODB connection.

Preliminary history:

kirr/wendelin.core@1c3b7750    X zconn_at for ZODB4

lib/tests/test_zodb.py

@@ -17,7 +17,7 @@
 #
 # See COPYING file for full licensing terms.
 # See https://www.nexedi.com/licensing for rationale and options.
-from wendelin.lib.zodb import LivePersistent, deactivate_btree, dbclose, zconn_at, zstor_2zurl, zmajor
+from wendelin.lib.zodb import LivePersistent, deactivate_btree, dbclose, zconn_at, zstor_2zurl
 from wendelin.lib.testing import getTestDB
 from wendelin.lib import testing
 from persistent import Persistent, UPTODATE, GHOST, CHANGED
@@ -236,7 +236,6 @@ def test_deactivate_btree():
 
 
 # verify that zconn_at gives correct answer.
-@xfail(zmajor < 5, reason="zconn_at is TODO for ZODB4")
 @func
 def test_zconn_at():
     stor = testdb.getZODBStorage()

lib/zodb.py

@@ -156,18 +156,18 @@ def zconn_at(zconn): # -> tid
 
     # ZODB4
     #
-    # Connection:
-    #     .before     !None for historic connections
-    #
-    #     ._txn_time  - if !None - set to tid of _next_ transaction
-    #                   XXX set to None initially - what to do?
-    #
-    #     # XXX do something like that ZODB5 is doing:
-    #     zconn._start = zconn._storage.lastTransaction() + 1
-    #     # XXX _and_ check out queued invalidations
+    # We rely on our patch in 4-nxd branch that reworks ZODB.Connection to
+    # implement MVCC via always calling loadBefore(zconn._txn_time) to load objects.
     elif zmajor == 4:
-        raise AssertionError("zconn_at: TODO: add support for ZODB4")
+        assert 'conn:MVCC-via-loadBefore-only' in ZODB.nxd_patches, \
+            "https://lab.nexedi.com/nexedi/ZODB/merge_requests/1"
 
+        if zconn._mvcc_storage:
+            raise NotImplementedError("Connection.at for IMVCCStorage is not implemented")
+        if zconn.before is not None:
+            return before2at(zconn.before)      # historical connection
+        else:
+            return before2at(zconn._txn_time)   # "current" connection
 
     # ZODB3
     else: