Add `--single-process` option to regrtest to always run tests sequentially (ignore `-jN` option)

[vstinner]

The Python release process runs tests sequentially. It catchs some issues which are ignored silently when running tests in parallel.

I propose adding --sequentially option to regrtest to always run tests sequentially. It can be used to ignore any -jN option and also to re-run failed tests sequentially.

Linked PRs

• gh-119727: Add --single-process option to regrtest #119728
• gh-119727: Run tests in parallel by default #120456
• [3.13] gh-119727: Add --single-process option to regrtest (#119728) #123010
• [3.12] gh-119727: Add --single-process option to regrtest (GH-119728) #130359

[vstinner]

My motivation for this change is also a desire to change the regrtest default to -j0: run tests in parallel using the number of CPU.

[serhiy-storchaka]

This is an opposite of --multiprocess. Why not call it --singleprocess?

You can run tests with -j1, it will be sequentially, but in different processes.

[vstinner]

IMO "sequentially" is the right name. --multiprocess means in fact "running tests in parallel". Using multipe processes is just an implementation details. It could be threads or something else. The opposite of parallel is sequential. IMO it's too late to rename the --multiprocess option.

Note: there is already a --single option. --singleprocess sounds too close.

[serhiy-storchaka]

I will not insist.

The --multiprocess option has two effects: it runs the tests in parallel and runs every test is a separate process. It could be possible to run the tests in parallel using the process pull, but this is not how it is implemented now, and I do not think there is a benefit to implement such option. Currently there are three options:

• Run the tests sequentially in the same (main) process. By default.
• Run the tests sequentially in separate processes. -j1.
• Run the tests in parallel in separate processes. -jN.