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.
Instead of introducing YES_COLOR, a completely new standard, into the mix
it might make more sense to instead tag along with the CLICOLOR_FORCE env var,
which dates back to at least 2000 with FreeBSD 4.1.1 and which is
supported by tools like CMake.
<https://bixense.com/clicolors/>
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.
This is a partial revert of 105db13536.
As we learned from Void Linux packaging, these options are not actually
helpful since the distribution package manager may very well want to
cross-compile the packages that it is building.
So, let's not overcomplicate things. There are already the standard
options: -Dtarget, -Dcpu, and -Ddynamic-linker.
These options are generally provided when the project generates machine
code artifacts, however, there may be a project that does no such thing,
in which case it makes sense for these options to be missing. The Zig
Build System is a general-purpose build system, after all.
