Change TransactionBuffer to use a real iterator.

This eliminates a bunch of checks that weren't exercised by the test
suite.  Not sure if they were superstition or not, but I'm willing to
risk it.

src/ZEO/ClientStorage.py

@@ -985,22 +985,7 @@ class ClientStorage(object):
         if self._cache is None:
             return
 
-        try:
-            self._tbuf.begin_iterate()
-        except ValueError, msg:
-            raise ClientStorageError, (
-                "Unexpected error reading temporary file in "
-                "client storage: %s" % msg)
-        while 1:
-            try:
-                t = self._tbuf.next()
-            except ValueError, msg:
-                raise ClientStorageError, (
-                    "Unexpected error reading temporary file in "
-                    "client storage: %s" % msg)
-            if t is None:
-                break
-            oid, version, data = t
+        for oid, version, data in self._tbuf:
             self._cache.invalidate(oid, version, tid)
             # If data is None, we just invalidate.
             if data is not None:

src/ZEO/TransactionBuffer.py

@@ -116,41 +116,33 @@ class TransactionBuffer:
         finally:
             self.lock.release()
 
-    # unchecked constraints:
-    # 1. can't call store() after begin_iterate()
-    # 2. must call clear() after iteration finishes
-
-    def begin_iterate(self):
-        """Move the file pointer in advance of iteration"""
+    def __iter__(self):
         self.lock.acquire()
         try:
             if self.closed:
                 return
             self.file.flush()
             self.file.seek(0)
-            self.unpickler = cPickle.Unpickler(self.file)
+            return TBIterator(self.file, self.count)
         finally:
             self.lock.release()
 
-    def next(self):
-        self.lock.acquire()
-        try:
-            return self._next()
-        finally:
-            self.lock.release()
+class TBIterator(object):
+
+    def __init__(self, f, count):
+        self.file = f
+        self.count = count
+        self.unpickler = cPickle.Unpickler(f)
 
-    def _next(self):
+    def __iter__(self):
+        return self
+
+    def next(self):
         """Return next tuple of data or None if EOF"""
-        if self.closed:
-            return None
         if self.count == 0:
-            del self.unpickler
-            return None
+            self.file.seek(0)
+            self.size = 0
+            raise StopIteration
         oid_ver_data = self.unpickler.load()
         self.count -= 1
         return oid_ver_data
-
-    def get_size(self):
-        """Return size of data stored in buffer (just a hint)."""
-
-        return self.size

src/ZEO/tests/testTransactionBuffer.py

@@ -35,12 +35,8 @@ class TransBufTests(unittest.TestCase):
         tbuf = TransactionBuffer()
         tbuf.store(*new_store_data())
         tbuf.invalidate(*new_invalidate_data())
-        tbuf.begin_iterate()
-        while 1:
-            o = tbuf.next()
-            if o is None:
-                break
-        tbuf.clear()
+        for o in tbuf:
+            pass
             
     def doUpdates(self, tbuf):
         data = []
@@ -52,9 +48,7 @@ class TransBufTests(unittest.TestCase):
             tbuf.invalidate(*d)
             data.append(d)
 
-        tbuf.begin_iterate()
-        for i in range(len(data)):
-            x = tbuf.next()
+        for i, x in enumerate(tbuf):
             if x[2] is None:
                 # the tbuf add a dummy None to invalidates
                 x = x[:2]