bpo-46799: Improve multiprocessing shared_memory ShareableList

[tcl326]

This PR improves ShareableList's memory usage and performance by consolidating the metadata of each items into a single area in the shared memory block. This allow us to fetch the metadata needed to read and set item using a single struct.unpack_from call. Additionally, this MR also removes self._allocated_offsets the offsets can be obtained directly from the shared memory and there is no need to keep a copy in ShareableList. Further more, self._allocated_offsets can has a significant memory footprint when the number of items in the list is large. I am seeing a significant savings in memory footprint and increase in performance in a benchmark where a ShareableList with a list of 10,000,000 zeros is forked by 5 child processes, and each child precess iterate through the list on their own.

Additionally, this PR also include the fix to #26328, and an additional fix where we properly allocate 8 bytes of memory for a string or byte array of size 8. Previously, we would've allocated 16 bytes for a string of size 8 or byte array of size 8.

Here is the resulting memory usage plotted using https://github.com/pythonprofilers/memory_profiler

• Memory consumption of the current ShareableList in multiprocessing
  

• Memory consumption of the ShareableList proposed in this MR
  

https://bugs.python.org/issue46799

[the-knights-who-say-ni]

Hello, and thanks for your contribution!

I'm a bot set up to make sure that the project can legally accept this contribution by verifying everyone involved has signed the PSF contributor agreement (CLA).

CLA Missing

Our records indicate the following people have not signed the CLA:

@tcl326

For legal reasons we need all the people listed to sign the CLA before we can look at your contribution. Please follow the steps outlined in the CPython devguide to rectify this issue.

If you have recently signed the CLA, please wait at least one business day
before our records are updated.

You can check yourself to see if the CLA has been received.

Thanks again for the contribution, we look forward to reviewing it!

[ghost]

All commit authors signed the Contributor License Agreement.

[arhadthedev]

@tcl326 Could you sign the new CLA by clicking not signed button in the cpython-cla-bot's message, please?

[gst]

small typo sinlge