Replaced the FileStorage _packt member with a _pack_is_in_progress bool.

The only values _packt ever had were z64 and None.  The only thing _packt
was used for that appeared to make sense was to block undo while a pack
was in progress, and a bool works better for that.  tids were compared
to _packt in a few spots, but since the only values _packt could have
were in {z64, None}, the comparisons were doomed to (regardless of tid)
return "greater than" (z64) or to be undefined (None; although in recent
Pythons None is reliably less than objects of other types, so "greater
than" was the only outcome possible from these comparisons under 2.3.3).
Removed these comparisons, collapsing surrounding expressions to
equivalents taking into account that tid < self._packt was always false.

src/ZODB/FileStorage/FileStorage.py

@@ -13,7 +13,7 @@
 ##############################################################################
 """Storage implementation using a log written to a single file.
 
-$Revision: 1.9 $
+$Revision: 1.10 $
 """
 
 import base64
@@ -106,10 +106,8 @@ class FileStorage(BaseStorage.BaseStorage,
                   ConflictResolution.ConflictResolvingStorage,
                   FileStorageFormatter):
 
-    # default pack time is 0
-    # XXX It's unclear what this is for.  Looks like the only values it
-    # XXX can ever have are z64 and None.
-    _packt = z64
+    # Set True while a pack is in progress; undo is blocked for the duration.
+    _pack_is_in_progress = False
 
     _records_before_save = 10000
 
@@ -1062,11 +1060,10 @@ class FileStorage(BaseStorage.BaseStorage,
             last = first - last + 1
         self._lock_acquire()
         try:
-            if self._packt is None:
+            if self._pack_is_in_progress:
                 raise UndoError(
                     'Undo is currently disabled for database maintenance.<p>')
-            us = UndoSearch(self._file, self._pos, self._packt,
-                            first, last, filter)
+            us = UndoSearch(self._file, self._pos, first, last, filter)
             while not us.finished():
                 # Hold lock for batches of 20 searches, so default search
                 # parameters will finish without letting another thread run.
@@ -1130,9 +1127,7 @@ class FileStorage(BaseStorage.BaseStorage,
                 return pos
             if stop_at_pack:
                 # check the status field of the transaction header
-                # XXX Looks like self._packt is either z64 or None, so unclear
-                # XXX what the _tid < self._packt is trying to say.
-                if h[16] == 'p' or _tid < self._packt:
+                if h[16] == 'p':
                     break
         raise UndoError("Invalid transaction id")
 
@@ -1319,16 +1314,11 @@ class FileStorage(BaseStorage.BaseStorage,
         if not self._index:
             return
 
-        # Set _packt to None for the duration, which blocks undo for the
-        # duration.
-        # XXX That appears to be the only use for _packt; if so, could be
-        # XXX cleaner.
         self._lock_acquire()
         try:
-            if self._packt != z64:
-                # Already packing.
+            if self._pack_is_in_progress:
                 raise FileStorageError, 'Already packing'
-            self._packt = None
+            self._pack_is_in_progress = True
         finally:
             self._lock_release()
 
@@ -1366,7 +1356,7 @@ class FileStorage(BaseStorage.BaseStorage,
             if p.locked:
                 self._commit_lock_release()
             self._lock_acquire()
-            self._packt = z64
+            self._pack_is_in_progress = False
             self._lock_release()
 
     def iterator(self, start=None, stop=None):
@@ -1980,10 +1970,9 @@ class Record(BaseStorage.DataRecord):
 
 class UndoSearch:
 
-    def __init__(self, file, pos, packt, first, last, filter=None):
+    def __init__(self, file, pos, first, last, filter=None):
         self.file = file
         self.pos = pos
-        self.packt = packt
         self.first = first
         self.last = last
         self.filter = filter
@@ -2011,7 +2000,7 @@ class UndoSearch:
         self.file.seek(self.pos)
         h = self.file.read(TRANS_HDR_LEN)
         tid, tl, status, ul, dl, el = struct.unpack(TRANS_HDR, h)
-        if tid < self.packt or status == 'p':
+        if status == 'p':
             self.stop = 1
             return None
         if status != ' ':