bpo-35972: _xxsubinterpreters: Fix potential integer truncation in channel_send()

[izbyshev]

https://bugs.python.org/issue35972

[vstinner]

Since it's a private API and _xxsubinterpreters is a private module, I don't think that a NEWS entry is needed.

@ericsnowcurrently: Would you mind to review this fix?

[vstinner]

LGTM.

[ericsnowcurrently]

(marking so it doesn't get merged yet)

[bedevere-bot]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[ericsnowcurrently]

As noted on the tracker issue, LGTM. I have added a comment to _long_shared() about the consequence for 32-bit architectures. I'll merge this in a little while.

[vstinner]

    /* Note that this means the size of shareable ints is bounded by
     * sys.maxsize.  Hence on 32-bit architectures that is half the
     * size of maximum shareable ints on 64-bit.
     */

If you really need 64-bit on 32-bit system, you can use int64_t. But it means that you should change _PyCrossInterpreterData.data type.

[pablogsal]

Buildbots have detected a leak of memory blocks in this PR. I have oppened:

https://bugs.python.org/issue35984

I do not have time to look into this until this night.