Pkg.precompile() needs to be moved to background task

[vtjnash]

Doing the "Alan test" today, we really need to move Pkg.precompile() to just launch a precompile task and immediately return fast. Right now, it adds a ton of latency to getting to something useful, at least half of which work is typically useless. I've always had to entirely disable it because of this problem (JULIA_PKG_PRECOMPILE_AUTO=0), but if it only ran in the background as an idle task (stopping/restarting whenever the user does any other pkg operation or precompile operation so that it can prioritize what the user actually needs), then user configuration would no longer be necessary (since we already have the pidfile locks now to avoid conflicts and so it no longer must be blocking to avoid errors).

[IanButterworth]

What's the Alan test? Install and use something on a new computer?

[vtjnash]

Ask someone named Alan to use something

[IanButterworth]

And Julia is considered Alan complete if Alan is happy? Seems reasonable tbh

[IanButterworth]

To implement this nicely I think we need UI ways to cancel and check up on Pkg.precompile(), and have it not be interrupted by ctrl-C when julia is doing something else.

So

pkg> add Foo
   Precompiling Foo in the background. Use `pkg> precompile --stop` to stop, `precompile --monitor` to monitor.

julia> # etc.

So precompile --monitor would drop you back into the typical precompilation UI.

--stop could gracefully wait for active jobs to finish.
--cancel could interrupt without showing interrupt stacktraces?

[staticfloat]

Just a note that some related discussion about the UX of this is in JuliaLang/julia#13356