Change prepare() signature in IDataManager.

The manager should raise an exception in its prepare() method rather
than returning a boolean to indicate failure.  Rationale: The txn
manager can't raise a reasonable exception, because it doesn't know
what the data manager couldn't prepare.

src/transaction/interfaces.py

@@ -57,7 +57,8 @@ class IDataManager(Interface):
     def prepare(transaction):
         """Begin two-phase commit of a transaction.
 
-        DataManager should return True or False.
+        The data manager must raise an exception if it is not prepared
+        to commit the transaction after executing prepare().
         """
 
     def abort(transaction):

src/transaction/manager.py

@@ -23,18 +23,14 @@ class AbstractTransactionManager(object):
         # commit calls _finishCommit() or abort()
         assert txn._status is Status.ACTIVE
         txn._status = Status.PREPARING
-        prepare_ok = True
         self.logger.debug("%s: prepare", txn)
         try:
             for r in txn._resources:
-                if prepare_ok and not r.prepare(txn):
-                    raise AbortError(r)
+                r.prepare(txn)
         except:
             txn._status = Status.FAILED
             raise
         txn._status = Status.PREPARED
-        # XXX An error below is intolerable.  What state to use?
-        # Need code to handle this case.
         self._finishCommit(txn)
 
     def _finishCommit(self, txn):

src/transaction/tests/test_txn.py

@@ -33,7 +33,6 @@ class TestDataManager:
     def prepare(self, txn):
         if self._fail == "prepare":
             raise RuntimeError
-        return self._vote
 
     def abort(self, txn):
         if self._fail == "abort":
@@ -126,15 +125,6 @@ class BaseTxnTests(unittest.TestCase):
         self.assertEqual(txn.status(), Status.FAILED)
         txn.abort()
 
-    def testCommitPrepareFalse(self):
-        txn = self.manager.begin()
-        txn.join(TestDataManager())
-        txn.join(TestDataManager(vote=False))
-        self.assertRaises(AbortError, txn.commit)
-        self.assertEqual(txn.status(), Status.FAILED)
-        self.assertRaises(IllegalStateError, txn.commit)
-        txn.abort()
-
     def testCommitFailure(self):
         txn = self.manager.begin()
         txn.join(TestDataManager())