BTreeItems_seek(): Even if i == pseudoindex on entry, bucket mutation

since the last call may have left us with an invalid index.  Raise
RuntimeError instead of segfaulting (or picking up random trash) if so.
A new (and previously segfaulting) test case is courtesy of Steve
Alexander.

src/BTrees/BTreeItemsTemplate.c

@@ -12,7 +12,7 @@
 
  ****************************************************************************/
 
-#define BTREEITEMSTEMPLATE_C "$Id: BTreeItemsTemplate.c,v 1.17 2002/06/22 17:22:54 tim_one Exp $\n"
+#define BTREEITEMSTEMPLATE_C "$Id: BTreeItemsTemplate.c,v 1.18 2003/01/31 20:13:02 tim_one Exp $\n"
 
 /* A BTreeItems struct is returned from calling .items(), .keys() or
  * .values() on a BTree-based data structure, and is also the result of
@@ -137,6 +137,7 @@ BTreeItems_seek(BTreeItems *self, int i)
 {
     int delta, pseudoindex, currentoffset;
     Bucket *b, *currentbucket;
+    int error;
 
     pseudoindex = self->pseudoindex;
     currentoffset = self->currentoffset;
@@ -210,6 +211,21 @@ BTreeItems_seek(BTreeItems *self, int i)
     }
 
     assert(pseudoindex == i);
+
+    /* Alas, the user may have mutated the bucket since the last time we
+     * were called, and if they deleted stuff, we may be pointing into
+     * trash memory now.
+     */
+    PER_USE_OR_RETURN(currentbucket, -1);
+    error = currentoffset < 0 || currentoffset >= currentbucket->len;
+    PER_ALLOW_DEACTIVATION(currentbucket);
+    PER_ACCESSED(currentbucket);
+    if (error) {
+	PyErr_SetString(PyExc_RuntimeError,
+	                "the bucket being iterated changed size");
+	return -1;
+    }
+
     Py_INCREF(currentbucket);
     Py_DECREF(self->currentbucket);
     self->currentbucket = currentbucket;

src/BTrees/tests/testBTrees.py

@@ -770,6 +770,30 @@ class BTreeTests(MappingBase):
         self.assertEqual(t.insert(1, 1) , 1)
         self.assertEqual(lsubtract(list(t.keys()), [0,1]) , [])
 
+    def testDamagedIterator(self):
+        # A cute one from Steve Alexander.  This caused the BTreeItems
+        # object to go insane, accessing memory beyond the allocated part
+        # of the bucket.  If it fails, the symptom is either a C-level
+        # assertion error (if the BTree code was compiled without NDEBUG),
+        # or most likely a segfault (if the BTree code was compiled with
+        # NDEBUG).
+
+        t = self.t.__class__()
+        self._populate(t, 10)
+        # In order for this to fail, it's important that k be a "lazy"
+        # iterator, referring to the BTree by indirect position (index)
+        # instead of a fully materialized list.  Then the position can
+        # end up pointing into trash memory, if the bucket pointed to
+        # shrinks.
+        k = t.keys()
+        for dummy in range(20):
+            try:
+                del t[k[0]]
+            except RuntimeError, detail:
+                self.assertEqual(str(detail), "the bucket being iterated "
+                                              "changed size")
+                break
+
 ## BTree tests
 
 class TestIOBTrees(BTreeTests, TestCase):