Fix possible data corruption after FileStorage is truncated to roll back a transaction

Multi-threaded IO support, which is new to ZODB 3.10, allows clients to read
data (load & loadBefore) even after tpc_vote has started to write a new
transaction to disk. This is done by using different 'file' objects.

Issues start when a transaction is rolled back after data has been appended
(using the writing file object). Truncating is not enough because the FilePool
may have been used concurrently to read the end of the last transaction:
file objects have their own read buffers which, in this case, may also contain
the beginning of the aborted transaction.

So a solution is to invalidate read buffers whenever they may contain wrong
data. This patch does it on truncation, which happens rarely enough to not
affect performance.

We discovered this bug in the following conditions:
- ZODB splitted in several FileStorage
- many conflicts in the first committed DB, but always resolved
- unresolved conflict in another DB
If the transaction is replayed with success (no more conflict in the other DB),
a subsequent load of the object that could be resolved in the first DB may, for
example, return a wrong serial (tid of the aborted transaction) if the layout
of the committed transaction matches that of the aborted one.

The bug usually manifests with POSKeyError & CorruptedDataError exceptions in
ZEO logs, for example while trying to resolve a conflict (and restarting the
transaction does not help, causing Site Errors in Zope). But theorically,
this could also cause silent corruption or unpickling errors at client side.

src/ZODB/FileStorage/FileStorage.py

@@ -20,6 +20,7 @@ import contextlib
 import errno
 import logging
 import os
+import sys
 import threading
 import time
 from struct import pack
@@ -722,6 +723,7 @@ class FileStorage(
                 # Hm, an error occurred writing out the data. Maybe the
                 # disk is full. We don't want any turd at the end.
                 self._file.truncate(self._pos)
+                self._files.flush()
                 raise
             self._nextpos = self._pos + (tl + 8)
 
@@ -776,6 +778,7 @@ class FileStorage(
     def _abort(self):
         if self._nextpos:
             self._file.truncate(self._pos)
+            self._files.flush()
             self._nextpos=0
             self._blob_tpc_abort()
 
@@ -2093,6 +2096,22 @@ class FilePool:
         while self._files:
             self._files.pop().close()
 
+
+    def flush(self):
+        """Empty read buffers.
+
+        This is required if they contain data of rolled back transactions.
+        """
+        with self.write_lock():
+            for f in self._files:
+                f.flush()
+
+    # Unfortunately, Python 3.x has no API to flush read buffers.
+    if sys.version_info.major > 2:
+        def flush(self):
+            with self.write_lock():
+                self.empty()
+
     def close(self):
         with self._cond:
             self.closed = True

src/ZODB/tests/testFileStorage.py

@@ -25,6 +25,7 @@ import zope.testing.setupstack
 from ZODB import POSException
 from ZODB import DB
 from ZODB.fsIndex import fsIndex
+from ZODB.utils import U64, p64, z64
 
 from ZODB.tests import StorageTestBase, BasicStorage, TransactionalUndoStorage
 from ZODB.tests import PackableStorage, Synchronization, ConflictResolution
@@ -217,7 +218,6 @@ class FileStorageTests(
         # global.
         import time
 
-        from ZODB.utils import U64, p64
         from ZODB.FileStorage.format import CorruptedError
         from ZODB.serialize import referencesf
 
@@ -285,6 +285,27 @@ class FileStorageTests(
             else:
                 self.assertNotEqual(next_oid, None)
 
+    def checkFlushAfterTruncate(self, fail=False):
+        r0 = self._dostore(z64)
+        storage = self._storage
+        t = transaction.Transaction()
+        storage.tpc_begin(t)
+        storage.store(z64, r0, b'foo', b'', t)
+        storage.tpc_vote(t)
+        # Read operations are done with separate 'file' objects with their
+        # own buffers: here, the buffer also includes voted data.
+        storage.load(z64)
+        # This must invalidate all read buffers.
+        storage.tpc_abort(t)
+        self._dostore(z64, r0, b'bar', 1)
+        # In the case that read buffers were not invalidated, return value
+        # is based on what was cached during the first load.
+        self.assertEqual(storage.load(z64)[0], b'foo' if fail else b'bar')
+
+    def checkFlushNeededAfterTruncate(self):
+        self._storage._files.flush = lambda: None
+        self.checkFlushAfterTruncate(True)
+
 class FileStorageHexTests(FileStorageTests):
 
     def open(self, **kwargs):