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).

[picnixz]

so the copying can be guaranteed to be safe even if the inputs aren't bytes.

I'm not entirely sure about this. For instance, you can have a readonly memoryview but it does not necessarily mean that you can't update the original object in another thread, e.g.:

>>> b = bytearray(b'\x00\x01\02')
>>> m = memoryview(b).toreadonly()  # its buffer will be readonly
>>> print(m.tolist())
[0, 1, 2]
>>> b[0] = 1
>>> print(m.tolist())
[1, 1, 2]

If b is modified by another thread while the concatenation is happening, we might end up with an incorrect result. We really want the concatenation to be the value at the moment you call sep.join.

[picnixz]

cc @colesbury @kumaraditya303 (just for FT experts to confirm that I understood correctly the issue)

[colesbury]

I agree that readonly doesn't imply immutable, but it sounds like @googlyrahman may specifically be asking for memoryviews on bytesobjects. That seems doable as an additional special case.