MergeFromString() does not work with any buffer protocol type.

[pettni]

What version of protobuf and what language are you using?
Python protobuf 5.28.2. I believe this affects all versions after the upb migration.

What operating system (Linux, Windows, ...) and version?
Linux (Amazon Linux 2)

What runtime / compiler are you using (e.g., python version or gcc version)
Python 3.12

What did you do?

python -c 'from google.protobuf import timestamp_pb2; from array import array; timestamp_pb2.Timestamp.FromString(array("l"))'

What did you expect to see
The program should run without error.

What did you see instead?

Traceback (most recent call last):
  File "<string>", line 1, in <module>
TypeError: expected bytes, array.array found

Workaround
Wrap the argument to MergeFromString in a memoryview().

---

The documentation says that MergeFromString() should work on

Any object that allows us to call `memoryview(serialized)` to access a string of 
bytes using the buffer interface.

However, I had trouble with a custom type that implements the buffer interface. I reproduced it with an array which also implements the buffer interface in the example above.

From reading the source it appears that serialized needs to be one of {memoryview, bytearray, bytes}, but does not support arbitrary buffer interface types. I checked with version 3.19.2 (before upb) and did not have the same issue there.

Would it be a good idea to to instead use PyMemoryView_FromObject (with a NULL check) instead? That seems closer to the old Python-native logic and the documentation.

Note: this is similar to #16691 (issue #15911) that added support for bytearray's, but that is still not as general as any buffer protocol type.