Collector #1734. Critical bug in BTree conflict resolution.

Stop silent data loss in some BTree cases where a transaction adds
a new key to a bucket while a concurrent transaction deletes all
keys from the same bucket.

Still needs porting to ZODB trunk and to 3.2 line.

branches/3.3/NEWS.txt

@@ -2,6 +2,21 @@ What's new in ZODB3 3.3.1a2?
 ============================
 Release date: DD-MMM-2005
 
+BTrees
+------
+
+Collector #1734: BTrees conflict resolution leads to index inconsistencies.
+
+Silent data loss could occur due to BTree conflict resolution when one
+transaction T1 added a new key to a BTree containing at least three buckets,
+and a concurrent transaction T2 deleted all keys in the bucket to which the
+new key was added.  Conflict resolution then created a bucket containing the
+newly added key, but the bucket remained isolated, disconnected from the
+BTree. In other words, the committed BTree didn't contain the new key added by
+T1.  Conflict resolution doesn't have enough information to repair this,
+so ``ConflictError`` is now raised in such cases.
+
+
 ZEO
 ---
 

branches/3.3/src/BTrees/MergeTemplate.c

@@ -70,7 +70,9 @@ merge_error(int p1, int p2, int p3, int reason)
  * However, it's not OK for s2 and s3 to, between them, end up deleting all
  * the keys.  This is a higher-level constraint, due to that the caller of
  * bucket_merge() doesn't have enough info to unlink the resulting empty
- * bucket from its BTree correctly.
+ * bucket from its BTree correctly.  It's also not OK if s2 or s3 are empty,
+ * because the transaction that emptied the bucket unlinked the bucket from
+ * the tree, and nothing we do here can get it linked back in again.
  *
  * Key insertion:  s2 or s3 can add a new key, provided the other transaction
  * doesn't insert the same key.  It's not OK even if they insert the same
@@ -91,6 +93,13 @@ bucket_merge(Bucket *s1, Bucket *s2, Bucket *s3)
   SetIteration i1 = {0,0,0}, i2 = {0,0,0}, i3 = {0,0,0};
   int cmp12, cmp13, cmp23, mapping, set;
 
+  /* If either "after" bucket is empty, punt. */
+  if (s2->len == 0 || s3->len == 0)
+    {
+      merge_error(-1, -1, -1, 12);
+      goto err;
+    }
+
   if (initSetIteration(&i1, OBJECT(s1), 1) < 0)
       goto err;
   if (initSetIteration(&i2, OBJECT(s2), 1) < 0)

branches/3.3/src/BTrees/tests/testConflict.py

@@ -177,7 +177,7 @@ class MappingBase(Base):
 
         test_merge(base, b1, b2, bm, 'merge insert from empty')
 
-    def testMergeEmptyAndFill(self):
+    def testFailMergeEmptyAndFill(self):
         base, b1, b2, bm, e1, e2, items = self._setupConflict()
 
         b1.clear()
@@ -185,7 +185,7 @@ class MappingBase(Base):
         b2.update(e2)
         bm.update(e2)
 
-        test_merge(base, b1, b2, bm, 'merge insert from empty')
+        test_merge(base, b1, b2, bm, 'merge insert from empty', should_fail=1)
 
     def testMergeEmpty(self):
         base, b1, b2, bm, e1, e2, items = self._setupConflict()
@@ -274,7 +274,7 @@ class SetTests(Base):
 
         test_merge(base, b1, b2, bm, 'merge insert from empty')
 
-    def testMergeEmptyAndFill(self):
+    def testFailMergeEmptyAndFill(self):
         base, b1, b2, bm, e1, e2, items = self._setupConflict()
 
         b1.clear()
@@ -282,7 +282,7 @@ class SetTests(Base):
         b2.update(e2)
         bm.update(e2)
 
-        test_merge(base, b1, b2, bm, 'merge insert from empty')
+        test_merge(base, b1, b2, bm, 'merge insert from empty', should_fail=1)
 
     def testMergeEmpty(self):
         base, b1, b2, bm, e1, e2, items = self._setupConflict()
@@ -742,6 +742,90 @@ class NastyConfict(Base, TestCase):
         else:
             self.fail("expected ConflictError")
 
+    def testConflictWithOneEmptyBucket(self):
+        # If one transaction empties a bucket, while another adds an item
+        # to the bucket, all the changes "look resolvable":  bucket conflict
+        # resolution returns a bucket containing (only) the item added by
+        # the latter transaction, but changes from the former transaction
+        # removing the bucket are uncontested:  the bucket is removed from
+        # the BTree despite that resolution thinks it's non-empty!  This
+        # was first reported by Dieter Maurer, to zodb-dev on 22 Mar 2005.
+        b = self.t
+        for i in range(0, 200, 4):
+            b[i] = i
+        # bucket 0 has 15 values: 0, 4 .. 56
+        # bucket 1 has 15 values: 60, 64 .. 116
+        # bucket 2 has 20 values: 120, 124 .. 196
+        state = b.__getstate__()
+        # Looks like:  ((bucket0, 60, bucket1, 120, bucket2), firstbucket)
+        # If these fail, the *preconditions* for running the test aren't
+        # satisfied -- the test itself hasn't been run yet.
+        self.assertEqual(len(state), 2)
+        self.assertEqual(len(state[0]), 5)
+        self.assertEqual(state[0][1], 60)
+        self.assertEqual(state[0][3], 120)
+
+        # Set up database connections to provoke conflict.
+        self.openDB()
+        r1 = self.db.open().root()
+        r1["t"] = self.t
+        transaction.commit()
+
+        r2 = self.db.open(synch=False).root()
+        copy = r2["t"]
+        # Make sure all of copy is loaded.
+        list(copy.values())
+
+        self.assertEqual(self.t._p_serial, copy._p_serial)
+
+        # Now one transaction empties the first bucket, and another adds a
+        # key to the first bucket.
+
+        for k in range(0, 60, 4):
+            del self.t[k]
+        transaction.commit()
+
+        copy[1] = 1
+
+        try:
+            transaction.commit()
+        except ConflictError, detail:
+            self.assert_(str(detail).startswith('database conflict error'))
+            transaction.abort()
+        else:
+            self.fail("expected ConflictError")
+
+        # Same thing, except commit the transactions in the opposite order.
+        b = OOBTree()
+        for i in range(0, 200, 4):
+            b[i] = i
+
+        r1 = self.db.open().root()
+        r1["t"] = b
+        transaction.commit()
+
+        r2 = self.db.open(synch=False).root()
+        copy = r2["t"]
+        # Make sure all of copy is loaded.
+        list(copy.values())
+
+        self.assertEqual(b._p_serial, copy._p_serial)
+
+        # Now one transaction empties the first bucket, and another adds a
+        # key to the first bucket.
+        b[1] = 1
+        transaction.commit()
+
+        for k in range(0, 60, 4):
+            del copy[k]
+        try:
+            transaction.commit()
+        except ConflictError, detail:
+            self.assert_(str(detail).startswith('database conflict error'))
+            transaction.abort()
+        else:
+            self.fail("expected ConflictError")
+
 
 def test_suite():
     suite = TestSuite()

branches/3.3/src/ZODB/POSException.py

@@ -187,6 +187,9 @@ class BTreesConflictError(ConflictError):
 
             # 11; conflicting changes in an internal BTree node
             'Conflicting changes in an internal BTree node',
+
+            # 12; i2 or i3 was empty
+            'Empty bucket in a transaction',
             ]
 
     def __init__(self, p1, p2, p3, reason):