b"".join should release GIL in case of memoryview

[googlyrahman]

Bug report

Bug description:

CPython PR #17757 allowed the GIL to be released during b"".join() operations. However, this optimization currently only triggers if every passed object is of type bytes; it falls back to acquiring the GIL if it encounters mutable buffers. While reacquiring the GIL is necessary for mutable types like bytearray, we can still safely drop the GIL for memoryview objects that expose immutable bytes.

Please refer to fsspec/gcsfs#795 (comment) for more context.

cc: @martindurant

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

[martindurant]

Additional information:

At this line, anything but bytes causes the GIL to be retained. However, any buffer has a readonly attribute, so the copying can be guaranteed to be safe even if the inputs aren't bytes.

This is critical for our application, where we wish to join large bytes segments, without intermediate copies (i.e., using memoryview) without blocking (i.e., on an offload thread, dropping the GIL).