[BuildStream] BuildStream on Windows

[Angelos Evripiotis <angelos evripiotis gmail com>]

Hello list!

I have started to prototype "BuildStream on native Windows, without sandbox", I

think this will be a major use-case for me.

I've managed to make a series of hacks that get a "Hello World" application

to build. See [0] for the code and [1] for setup instructions.

Over the coming months, I'd like to start making changes to BuildStream that

will end up with Windows being a supported platform.

Here is the high-level roadmap that I was thinking of, and have already hacked

together:

- Add support for `multiprocessing.set_start_method('spawn')`[2] on platforms

  that don't support 'fork'.

    - Make elements and sources picklable.

    - Start being mindful of what references what, so that we're not pickling

      more than we need to. This may mean being more explicit in general about

      the split between the main and worker processes.

- Change to use some more portable versions of things, re-assess some

  assumptions, being mindful of performance across platforms.

- "Do something about signals on win32".

- Introduce win32 platform, using SandboxDummy.

- Introduce SandboxNone, which relies on copying files and good faith. I'd also

  like to try this on MacOS, to provide an alternative to the package manager

  Homebrew.

- Provide a facility for plugin developers to record and replay job

  invocations, for debugging. This works by saving the pickled job data and

  launching it again separately.

After that I'll be looking at hacking and implementing these things:

- Evaluate a job queue and persistent worker processes model. This will avoid

  the high cost of spawning new Python processes for each job. This cost isn't

  the same when forking. This approach may also help with our memory overhead.

- Evaluate ProjFS[3] on Windows, to provide some filesystem sandboxing.

- Evaluate Windows Sandbox [4] for sandboxing.

- Evaluate Windows Containers [5] for sandboxing.

Let me know what you think!

Cheers,

Angelos

Here is the MR for the prototype:

[0]: https://gitlab.com/BuildStream/buildstream/merge_requests/1298

There are repro instruction on the branch here:

[1]: https://gitlab.com/BuildStream/buildstream/blob/aevri/win32/WINDOWS.md

multiprocessing.set_start_method:

[2]: https://docs.python.org/3/library/multiprocessing.html#multiprocessing.set_start_method

Windows Projected File System (ProjFS):

[3]: https://docs.microsoft.com/en-us/windows/desktop/projfs/projected-file-system

Windows Sandbox is a new lightweight desktop environment tailored for safely running applications in isolation:

[4]: https://techcommunity.microsoft.com/t5/Windows-Kernel-Internals/Windows-Sandbox/ba-p/301849

Containers on Windows:

[5]: https://docs.microsoft.com/en-us/virtualization/windowscontainers/about/