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.
The host accepts N simultaneous connections and writes 1 byte to them
each. Clients connect and read 1 byte in order to obtain a thread token.
std.Thread.Pool now lazily spawns threads only when the work queue is
non-empty. I think that was a bad idea and will revert it shortly.
There is now a std.zig.initThreadPool wrapper that deals with:
* Resolving a zig cache directory into a UNIX domain socket address.
* Creating the "tmp" directory in .zig-cache but only if the listen
failed due to ENOENT.
* Deciding to connect to an existing jobserver, or become the host for
child processes.
This function accepts a WaitGroup parameter and manages the reference
counting therein. It also is infallible.
The existing `spawn` function is still handy when the job wants to
further schedule more tasks.
Free the allocated threads in the initialization of a thread pool only with pool.join instead of additionally calling allocator.free causing free to be called twice.
Resolves#18643
