Check exported buffers after the default callback returns

The default callback can export the internal buffer with getbuffer().
The packer then continues and can reallocate that buffer.
The export points at freed memory after the reallocation.

Call _check_exports() after the callback returns. The packer now
raises BufferError. The pure Python packer already raises BufferError
in this case.

Author: AbhinavMir

msgpack/_packer.pyx

@@ -264,6 +264,9 @@ cdef class Packer:
             ret = self._pack_inner(o, 1, nest_limit)
             if ret == -2:
                 o = self._default(o)
+                # The callback may have exported the internal buffer.
+                # Packing on would reallocate it and invalidate the export.
+                self._check_exports()
             else:
                 return ret
         return self._pack_inner(o, 0, nest_limit)

test/test_buffer.py

@@ -47,3 +47,25 @@ def test_packer_getbuffer():
         buffer.release()
         packer.pack(42)
         assert bytes(packer) == b"\x92*\xa5hello*"
+
+
+def test_packer_getbuffer_in_default():
+    # The default callback can export the internal buffer.
+    # The packer must refuse to pack on, because packing on reallocates
+    # the buffer and leaves the export pointing at freed memory.
+    exported = []
+
+    class Unsupported:
+        pass
+
+    def default(obj):
+        exported.append(packer.getbuffer())
+        return b"A" * (2 * 1024 * 1024)
+
+    packer = Packer(default=default, autoreset=False)
+    with raises(BufferError):
+        packer.pack([Unsupported()])
+
+    assert len(exported) == 1
+    assert bytes(exported[0]) == b"\x91"
+    exported[0].release()