Added a new crasher that targets mutating the underlying storage of a buffer. …

Added a new crasher that targets mutating the underlying storage of a buffer.  All work done by Armin Rigo.

Lib/test/crashers/buffer_mutate.py

+#
+# The various methods of bufferobject.c (here buffer_subscript()) call
+# get_buf() before calling potentially more Python code (here via
+# PySlice_GetIndicesEx()).  But get_buf() already returned a void*
+# pointer.  This void* pointer can become invalid if the object
+# underlying the buffer is mutated (here a bytearray object).
+#
+# As usual, please keep in mind that the three "here" in the sentence
+# above are only examples.  Each can be changed easily and lead to
+# another crasher.
+#
+# This crashes for me on Linux 32-bits with CPython 2.6 and 2.7
+# with a segmentation fault.
+#
+
+
+class PseudoIndex(object):
+    def __index__(self):
+        for c in "foobar"*n:
+            a.append(c)
+        return n * 4
+
+
+for n in range(1, 100000, 100):
+    a = bytearray("test"*n)
+    buf = buffer(a)
+
+    s = buf[:PseudoIndex():1]
+    #print repr(s)
+    #assert s == "test"*n