Pkg.build should launch a separate Julia process?

[stevengj]

I'm wondering if Pkg.update should launch a separate Julia process, so that it builds in a "clean" Julia environment, similar to Base.compilecache.

Once you import Foo, subsequent import Foo calls in the same Julia process just load the Foo you already imported, even if Foo has changed on disk. This has caused me lots of problems in Pkg.update, apparently because something like the following sequence of events occurs:

1. BinDeps or Conda was already loaded in a Julia session
2. Pkg.update updates BinDeps or Conda
3. Another package is updated and tries to run Pkg.build using the new BinDeps or Conda
4. This fails, because either (a) precompilation on BinDeps fails because of error loading just-precompiled module if an imported module was loaded before precompiling #12508 or (b) it's not precompiled (Conda isn't, currently) but Pkg.build expects some feature of the latest version that is not present in the already-imported version

This would be solved by having Pkg.update launch a new Julia process.

cc: @wildart, @StefanKarpinski

[StefanKarpinski]

Seems like a pretty good idea.

[wildart]

I agree with the proposed. I guess we are talking more of Pkg.build rather then Pkg.update.

[stevengj]

That's true, it should be Pkg.build.

[stevengj]

Of course, if you spawn a new process for each Pkg.build, as opposed to once per Pkg.update, it will slow things down a bit.

[wildart]

Process spawning time is negligible in comparison to external build time.