transactionalUndo(): When we're undoing the last revision of an

object, don't rely on its prevrevid pointing to the previous metadata
record.  That's because if the last revision was an undo record, it
will have a prevrevid pointing to the revision previous to the undone
one (phew! :).  Instead, use a cursor to find the previous metadata
record to restore.

supportsTransactionalUndo(): Added.

lib/python/BDBStorage/BDBFullStorage.py

@@ -4,7 +4,7 @@ See Minimal.py for an implementation of Berkeley storage that does not support
 undo or versioning.
 """
 
-# $Revision: 1.14 $
+# $Revision: 1.15 $
 __version__ = '0.1'
 
 import struct
@@ -631,16 +631,34 @@ class Full(BerkeleyBase):
                 # the state changes couldn't be resolved.
                 revid = self._serials[oid]
                 if revid == tid:
-                    # We can always undo the last transaction
                     vid, nvrevid, lrevid, prevrevid = struct.unpack(
                         '>8s8s8s8s', self._metadata[oid+tid])
-                    if prevrevid == ZERO:
-                        # We're undoing the object's creation.  The only thing
-                        # to undo from there is the zombification of the
-                        # object, i.e. restore the current revision.
-                        newrevs.append((oid, vid+nvrevid+DNE+revid))
-                    else:
-                        newrevs.append((oid, self._metadata[oid+prevrevid]))
+                    # We can always undo the last transaction.  The prevrevid
+                    # pointer doesn't necessarily point to the previous
+                    # transaction, if the revision we're undoing was itself an
+                    # undo.  Use a cursor to find the previous revision of
+                    # this object.
+                    mdc = self._metadata.cursor()
+                    try:
+                        mdc.set(oid+revid)
+                        mrec = mdc.prev()
+                        # If we're undoing the first record, either for the
+                        # whole system or for this object, just write a
+                        # zombification record
+                        if not mrec or mrec[0][:8] <> oid:
+                            newrevs.append((oid, vid+nvrevid+DNE+revid))
+                            continue
+                        # BAW: If the revid of this object record is the same
+                        # as the revid we're being asked to undo, then I think
+                        # we have a problem (since the storage invariant is
+                        # that it doesn't retain metadata records for multiple
+                        # modifications of the object in the same txn).
+                        if mrec[0][8:] == revid:
+                            raise StorageSystemError
+                        # All is good, so just restore this metadata record
+                        newrevs.append((oid, mrec[1]))
+                    finally:
+                        mdc.close()
                 else:
                     # We need to compare the lrevid (pickle pointers) of the
                     # transaction previous to the current one, and the
@@ -934,6 +952,9 @@ class Full(BerkeleyBase):
             self._lock_release()
 
     # Other interface assertions
+    def supportsTransactionalUndo(self):
+        return 1
+
     def supportsUndo(self):
         return 1
 

lib/python/BDBStorage/Full.py

@@ -4,7 +4,7 @@ See Minimal.py for an implementation of Berkeley storage that does not support
 undo or versioning.
 """
 
-# $Revision: 1.14 $
+# $Revision: 1.15 $
 __version__ = '0.1'
 
 import struct
@@ -631,16 +631,34 @@ class Full(BerkeleyBase):
                 # the state changes couldn't be resolved.
                 revid = self._serials[oid]
                 if revid == tid:
-                    # We can always undo the last transaction
                     vid, nvrevid, lrevid, prevrevid = struct.unpack(
                         '>8s8s8s8s', self._metadata[oid+tid])
-                    if prevrevid == ZERO:
-                        # We're undoing the object's creation.  The only thing
-                        # to undo from there is the zombification of the
-                        # object, i.e. restore the current revision.
-                        newrevs.append((oid, vid+nvrevid+DNE+revid))
-                    else:
-                        newrevs.append((oid, self._metadata[oid+prevrevid]))
+                    # We can always undo the last transaction.  The prevrevid
+                    # pointer doesn't necessarily point to the previous
+                    # transaction, if the revision we're undoing was itself an
+                    # undo.  Use a cursor to find the previous revision of
+                    # this object.
+                    mdc = self._metadata.cursor()
+                    try:
+                        mdc.set(oid+revid)
+                        mrec = mdc.prev()
+                        # If we're undoing the first record, either for the
+                        # whole system or for this object, just write a
+                        # zombification record
+                        if not mrec or mrec[0][:8] <> oid:
+                            newrevs.append((oid, vid+nvrevid+DNE+revid))
+                            continue
+                        # BAW: If the revid of this object record is the same
+                        # as the revid we're being asked to undo, then I think
+                        # we have a problem (since the storage invariant is
+                        # that it doesn't retain metadata records for multiple
+                        # modifications of the object in the same txn).
+                        if mrec[0][8:] == revid:
+                            raise StorageSystemError
+                        # All is good, so just restore this metadata record
+                        newrevs.append((oid, mrec[1]))
+                    finally:
+                        mdc.close()
                 else:
                     # We need to compare the lrevid (pickle pointers) of the
                     # transaction previous to the current one, and the
@@ -934,6 +952,9 @@ class Full(BerkeleyBase):
             self._lock_release()
 
     # Other interface assertions
+    def supportsTransactionalUndo(self):
+        return 1
+
     def supportsUndo(self):
         return 1