Choose an appropriate python version for x.py at runtime

[jyn514]

Or, how I learned to stop worrying and love the shell.

• Start x.py as a shell script and re-execute the program with the right python interpreter
• Default to python3 -> python -> python2

Closes #71818.

r? @Mark-Simulacrum

[jyn514]

I found that this breaks on CentOS because it doesn't have which available. I'll use command -v instead.

[jyn514]

This breaks in Powershell (probably cmd.exe too, but I don't know how to install that on linux):

ResourceUnavailable: Program 'x.py' failed to run: Exec format errorAt line:1 char:1
+ ./x.py check
+ ~~~~~~~~~~~~.

[jyn514]

It looks like servo handles this by having a separate mach.bat bootstrap script, which seems unfortunate :/

[nagisa]

Are you sure this isn't missing a shebang for shell of some sort?

Running ./x.py on a NixOS system with this PR does not work:

$ git checkout origin/master
$ ./x.py
info: Downloading and building bootstrap before processing --help
      command. See src/bootstrap/README.md for help with common
      commands.
Updating only changed submodules
Updating submodule src/llvm-project
<snip>
$ git checkout origin/pr/80585
$ ./x.py
Failed to execute process './x.py'. Reason:
exec: unknown error (errno was 8)
The file './x.py' is marked as an executable but could not be run by the operating system.

[jyn514]

@nagisa most shells will execute files as scripts if they don't have a shebang, what shell are you using?

That said, if this doesn't work even on linux, it's probably not worth pursuing.

[nagisa]

I use fish and various non-shell uses would probably break as well.