New Feature:

When transactions are aborted, new object ids allocated during the
  transaction are saved and used in subsequent transactions. This can
  help in situations where object ids are used as BTree keys and the
  sequential allocation of object ids leads to conflict errors.

src/CHANGES.txt

@@ -2,6 +2,17 @@
  Change History
 ================
 
+3.10.0a2 (2010-??-??)
+=====================
+
+New Features
+------------
+
+- When transactions are aborted, new object ids allocated during the
+  transaction are saved and used in subsequent transactions. This can
+  help in situations where object ids are used as BTree keys and the
+  sequential allocation of object ids leads to conflict errors.
+
 3.10.0a1 (2010-02-08)
 =====================
 

src/ZODB/Connection.py

@@ -104,7 +104,7 @@ class Connection(ExportImport, object):
             self._mvcc_storage = False
 
         self._normal_storage = self._storage = storage
-        self.new_oid = storage.new_oid
+        self.new_oid = db.new_oid
         self._savepoint_storage = None
 
         # Do we need to join a txn manager?
@@ -214,7 +214,7 @@ class Connection(ExportImport, object):
                             " added to a Connection.", obj)
         elif obj._p_jar is None:
             assert obj._p_oid is None
-            oid = obj._p_oid = self._storage.new_oid()
+            oid = obj._p_oid = self.new_oid()
             obj._p_jar = self
             if self._added_during_commit is not None:
                 self._added_during_commit.append(obj)
@@ -426,6 +426,7 @@ class Connection(ExportImport, object):
         if self._savepoint_storage is not None:
             self._abort_savepoint()
 
+        self._invalidate_creating()
         self._tpc_cleanup()
 
     def _abort(self):
@@ -438,6 +439,7 @@ class Connection(ExportImport, object):
                 del self._added[oid]
                 del obj._p_jar
                 del obj._p_oid
+                self._db.save_oid(oid)
             else:
 
                 # Note: If we invalidate a non-ghostifiable object
@@ -723,6 +725,7 @@ class Connection(ExportImport, object):
             self._creating = {}
 
         for oid in creating:
+            self._db.save_oid(oid)
             o = self._cache.get(oid)
             if o is not None:
                 del self._cache[oid]

src/ZODB/DB.py

@@ -384,7 +384,9 @@ class DB(object):
                  historical_timeout=300,
                  database_name='unnamed',
                  databases=None,
-                 xrefs=True):
+                 xrefs=True,
+                 max_saved_oids=999,
+                 ):
         """Create an object database.
 
         :Parameters:
@@ -480,6 +482,9 @@ class DB(object):
         self._setupUndoMethods()
         self.history = storage.history
 
+        self._saved_oids = []
+        self._max_saved_oids = max_saved_oids
+
     def _setupUndoMethods(self):
         storage = self.storage
         try:
@@ -942,6 +947,19 @@ class DB(object):
         return ContextManager(self)
 
 
+    def save_oid(self, oid):
+        if len(self._saved_oids) < self._max_saved_oids:
+            self._saved_oids.append(oid)
+
+    def new_oid(self):
+        if self._saved_oids:
+            try:
+                return self._saved_oids.pop()
+            except IndexError:
+                pass # Hm, threads
+        return self.storage.new_oid()
+
+
 class ContextManager:
     """PEP 343 context manager
     """

src/ZODB/tests/new_oids_get_reused_on_abort.test

+New OIDs get reused if a transaction aborts
+===========================================
+
+Historical note:
+
+  An OID is a terrible thing to waste.
+
+  Seriously: sequential allocation of OIDs could cause problems when
+  OIDs are used as (or as the basis of) BTree keys.  This happened
+  with Zope 3's intid utility where the object->id mapping uses an
+  object key based on the OID.  We got frequent conflict errors
+  because, in a site with many users, many objects are added at the
+  same time and conficts happened when conflicting changes caused
+  bucket splits.
+
+  Reusing an earlier allocated, but discarded OID will allow retries
+  of transactions to work because they'll use earlier OIDs which won't
+  tend to conflict with newly allocated ones.
+
+If a transaction is aborted, new OIDs assigned in the transaction are
+saved and made available for later transactions.
+
+    >>> import ZODB.tests.util, transaction
+    >>> db = ZODB.tests.util.DB()
+    >>> tm1 = transaction.TransactionManager()
+    >>> conn1 = db.open(tm1)
+    >>> conn1.root.x = ZODB.tests.util.P()
+    >>> tm1.commit()
+    >>> conn1.root.x.x = ZODB.tests.util.P()
+    >>> conn1.root.y = 1
+    >>> tm2 = transaction.TransactionManager()
+    >>> conn2 = db.open(tm2)
+    >>> conn2.root.y = ZODB.tests.util.P()
+    >>> tm2.commit()
+
+We get a conflict when we try to commit the change to the first connection:
+
+    >>> tm1.commit() # doctest: +ELLIPSIS
+    Traceback (most recent call last):
+    ...
+    ConflictError: ...
+
+    >>> tm1.abort()
+
+When we try, we get the same oid we would have gotten on the first transaction:
+
+    >>> conn1.root.x.x = ZODB.tests.util.P()
+    >>> tm1.commit()
+    >>> conn1.root.x.x._p_oid
+    '\x00\x00\x00\x00\x00\x00\x00\x03'
+
+We see this more clearly when we use add to assign the oid before the commit:
+
+    >>> conn1.root.z = ZODB.tests.util.P()
+    >>> conn1.add(conn1.root.z)
+    >>> conn1.root.z._p_oid
+    '\x00\x00\x00\x00\x00\x00\x00\x04'
+
+    >>> tm1.abort()
+
+    >>> conn2.root.a = ZODB.tests.util.P()
+    >>> conn2.add(conn2.root.a)
+    >>> conn2.root.a._p_oid
+    '\x00\x00\x00\x00\x00\x00\x00\x04'

src/ZODB/tests/testConnection.py

@@ -815,6 +815,7 @@ class StubDatabase:
 
     def __init__(self):
         self.storage = StubStorage()
+        self.new_oid = self.storage.new_oid
 
     classFactory = None
     database_name = 'stubdatabase'
@@ -823,6 +824,8 @@ class StubDatabase:
     def invalidate(self, transaction, dict_with_oid_keys, connection):
         pass
 
+    save_oid = lambda self, oid: None
+
 def test_suite():
     s = unittest.makeSuite(ConnectionDotAdd, 'check')
     s.addTest(doctest.DocTestSuite())

src/ZODB/tests/testZODB.py

@@ -11,20 +11,21 @@
 # FOR A PARTICULAR PURPOSE.
 #
 ##############################################################################
-import unittest
-import warnings
 
-import ZODB
-import ZODB.FileStorage
-import ZODB.MappingStorage
+from persistent import Persistent
+from persistent.mapping import PersistentMapping
 from ZODB.POSException import ReadConflictError, ConflictError
 from ZODB.POSException import TransactionFailedError
 from ZODB.tests.warnhook import WarningsHook
-import ZODB.tests.util
 
-from persistent import Persistent
-from persistent.mapping import PersistentMapping
+import doctest
 import transaction
+import unittest
+import warnings
+import ZODB
+import ZODB.FileStorage
+import ZODB.MappingStorage
+import ZODB.tests.util
 
 class P(Persistent):
     pass
@@ -631,7 +632,9 @@ class PoisonedObject:
         self._p_jar = poisonedjar
 
 def test_suite():
-    return unittest.makeSuite(ZODBTests, 'check')
+    suite = unittest.makeSuite(ZODBTests, 'check')
+    suite.addTest(doctest.DocFileSuite('new_oids_get_reused_on_abort.test'))
+    return suite
 
 if __name__ == "__main__":
     unittest.main(defaultTest="test_suite")

src/ZODB/tests/util.py

@@ -57,7 +57,7 @@ def pack(db):
 
 class P(persistent.Persistent):
 
-    def __init__(self, name):
+    def __init__(self, name=None):
         self.name = name
 
     def __repr__(self):