fallback: Fix packing multidim memoryview (#527)

Fix #526

Author: methane

msgpack/fallback.py

@@ -814,7 +814,7 @@ def _pack(
                 self._pack_raw_header(n)
                 return self._buffer.write(obj)
             if check(obj, memoryview):
-                n = len(obj) * obj.itemsize
+                n = obj.nbytes
                 if n >= 2**32:
                     raise ValueError("Memoryview is too large")
                 self._pack_bin_header(n)

test/test_memoryview.py

@@ -96,3 +96,11 @@ def test_bin32_from_byte():
 
 def test_bin32_from_float():
     _runtest("f", 2**16, b"\xc6", b"\x00\x01\x00\x00", True)
+
+
+def test_multidim_memoryview():
+    # See https://github.com/msgpack/msgpack-python/issues/526
+    view = memoryview(b"\00" * 6)
+    data = view.cast(view.format, (3, 2))
+    packed = packb(data)
+    assert packed == b'\xc4\x06\x00\x00\x00\x00\x00\x00'