std.Thread.Pool: back to spawning all threads in initialization because
it's overall simpler. This scheme requires init to be passed a pointer
to the struct.
std.process.Child: implement integration with thread pool jobserver. The
environment variable is called `JOBSERVERV2`. The API works based on
assigning a thread pool to the child process.
build runner: store the thread pool in std.Build.Graph so that it can be
passed to child processes during the make phase.
Fix not allocating +1 pollfds in previous commit.
I renamed std.process.Child.CreateProcessSupportedExtension to WindowsExtension
and made it public to avoid duplicating the list of extensions.
While here, I also improved it to not misreport OOM from std.fs.realpathAlloc()
as a generic failure to find the program, but instead panic like the rest of the
build system does for OOM.
Closes#20314.
This fix is already in master branch for stdin, stdout, and stderr; this
commit solves the same problem but for the progress pipe.
Both fixes were originally included in one commit on this branch,
however it was split it into two so that master branch could receive the
fix before the progress branch is merged.
The added comment explains the issue here relatively well. The new
progress API made this bug obvious because it became visibly clear that
certain Compile steps were seemingly "hanging" until other steps
completed. As it turned out, these child processes had raced to spawn,
and hence one had inherited the other's stdio pipes, meaning the `poll`
call in `std.Build.Step.evalZigProcess` was not identifying the child
stdout as closed until an unrelated process terminated.
