setupEnv: Run some processes concurrently

[sjakobi]

This should make most commands slightly faster.
On my machine, stack path takes ~20% less time than before, a no-op
stack build in a single-package project ~15%.

[sjakobi]

So, what I actually don't know, is whether this safe, especially querying ghc-pkg asynchronously.

Can someone with more experience with asynchronous programming take a look?

BTW here's some debug output from a stack version on top of my better-process-logging branch for #2327:

~ $ stack-bpl-async -v path --dist-dir --global-pkg-db
Version 1.1.3, Git revision 7934d6ce5eb9b841a6f248b427e2e8812fd69845 (dirty) (3753 commits) x86_64 hpack-0.14.0
2016-07-06 19:40:57.493231: [debug] Checking for project config at: /home/simon/stack.yaml @(Kkm9qBx54bb6fPiRXOOC4T:Stack.Config src/Stack/Config.hs:823:9)
2016-07-06 19:40:57.493629: [debug] Checking for project config at: /home/stack.yaml @(Kkm9qBx54bb6fPiRXOOC4T:Stack.Config src/Stack/Config.hs:823:9)
2016-07-06 19:40:57.493763: [debug] Checking for project config at: /stack.yaml @(Kkm9qBx54bb6fPiRXOOC4T:Stack.Config src/Stack/Config.hs:823:9)
2016-07-06 19:40:57.493909: [debug] No project config file found, using defaults. @(Kkm9qBx54bb6fPiRXOOC4T:Stack.Config src/Stack/Config.hs:845:13)
2016-07-06 19:40:57.496410: [debug] Checking whether stack was built with libgmp4 @(Kkm9qBx54bb6fPiRXOOC4T:Stack.Config src/Stack/Config.hs:327:5)
2016-07-06 19:40:57.496909: [debug] Run process: /usr/bin/ldd /home/simon/.local/bin/stack-bpl-async @(Kkm9qBx54bb6fPiRXOOC4T:System.Process.Read src/System/Process/Read.hs:292:3)
2016-07-06 19:40:57.512142: [debug] Process finished in 15 ms: /usr/bin/ldd /home/simon/.local/bin/stack-bpl-async @(Kkm9qBx54bb6fPiRXOOC4T:System.Process.Read src/System/Process/Read.hs:292:3)
2016-07-06 19:40:57.512561: [debug] Stack was not built with libgmp4 @(Kkm9qBx54bb6fPiRXOOC4T:Stack.Config src/Stack/Config.hs:331:14)
2016-07-06 19:40:57.512688: [info] Run from outside a project, using implicit global project config @(Kkm9qBx54bb6fPiRXOOC4T:Stack.Config src/Stack/Config.hs:462:13)
2016-07-06 19:40:57.514316: [info] Using resolver: nightly-2016-06-21 from implicit global project's config file: /home/simon/.stack/global-project/stack.yaml @(Kkm9qBx54bb6fPiRXOOC4T:Stack.Config src/Stack/Config.hs:476:32)
2016-07-06 19:40:57.514602: [debug] Trying to decode /home/simon/.stack/build-plan-cache/x86_64-linux/nightly-2016-06-21.cache @(Kkm9qBx54bb6fPiRXOOC4T:Data.Store.VersionTagged src/Data/Store/VersionTagged.hs:51:5)
2016-07-06 19:40:57.527888: [debug] Success decoding /home/simon/.stack/build-plan-cache/x86_64-linux/nightly-2016-06-21.cache @(Kkm9qBx54bb6fPiRXOOC4T:Data.Store.VersionTagged src/Data/Store/VersionTagged.hs:60:13)
2016-07-06 19:40:57.534286: [debug] Getting system compiler version @(Kkm9qBx54bb6fPiRXOOC4T:Stack.Setup src/Stack/Setup.hs:345:17)
2016-07-06 19:40:57.534606: [debug] Run process: /usr/bin/ghc --info @(Kkm9qBx54bb6fPiRXOOC4T:System.Process.Read src/System/Process/Read.hs:292:3)
2016-07-06 19:40:57.560165: [debug] Process finished in 25 ms: /usr/bin/ghc --info @(Kkm9qBx54bb6fPiRXOOC4T:System.Process.Read src/System/Process/Read.hs:292:3)
2016-07-06 19:40:57.563992: [debug] Asking GHC for its version @(Kkm9qBx54bb6fPiRXOOC4T:Stack.Setup.Installed src/Stack/Setup/Installed.hs:97:13)
2016-07-06 19:40:57.564233: [debug] Getting Cabal package version @(Kkm9qBx54bb6fPiRXOOC4T:Stack.GhcPkg src/Stack/GhcPkg.hs:165:5)
2016-07-06 19:40:57.564356: [debug] Getting global package database location @(Kkm9qBx54bb6fPiRXOOC4T:Stack.GhcPkg src/Stack/GhcPkg.hs:48:5)
2016-07-06 19:40:57.564437: [debug] Run process: /home/simon/.stack/programs/x86_64-linux/ghc-8.0.1/bin/ghc --numeric-version @(Kkm9qBx54bb6fPiRXOOC4T:System.Process.Read src/System/Process/Read.hs:292:3)
2016-07-06 19:40:57.565233: [debug] Run process: /home/simon/.stack/programs/x86_64-linux/ghc-8.0.1/bin/ghc-pkg --no-user-package-db field --simple-output Cabal version @(Kkm9qBx54bb6fPiRXOOC4T:System.Process.Read src/System/Process/Read.hs:292:3)
2016-07-06 19:40:57.565852: [debug] Run process: /home/simon/.stack/programs/x86_64-linux/ghc-8.0.1/bin/ghc-pkg --no-user-package-db list --global @(Kkm9qBx54bb6fPiRXOOC4T:System.Process.Read src/System/Process/Read.hs:292:3)
2016-07-06 19:40:57.602662: [debug] Process finished in 37 ms: /home/simon/.stack/programs/x86_64-linux/ghc-8.0.1/bin/ghc-pkg --no-user-package-db field --simple-output Cabal version @(Kkm9qBx54bb6fPiRXOOC4T:System.Process.Read src/System/Process/Read.hs:292:3)
2016-07-06 19:40:57.606331: [debug] Process finished in 34 ms: /home/simon/.stack/programs/x86_64-linux/ghc-8.0.1/bin/ghc-pkg --no-user-package-db list --global @(Kkm9qBx54bb6fPiRXOOC4T:System.Process.Read src/System/Process/Read.hs:292:3)
2016-07-06 19:40:57.607750: [debug] Process finished in 43 ms: /home/simon/.stack/programs/x86_64-linux/ghc-8.0.1/bin/ghc --numeric-version @(Kkm9qBx54bb6fPiRXOOC4T:System.Process.Read src/System/Process/Read.hs:292:3)
2016-07-06 19:40:57.607943: [debug] Resolving package entries @(Kkm9qBx54bb6fPiRXOOC4T:Stack.Setup src/Stack/Setup.hs:226:5)
2016-07-06 19:40:57.608657: [debug] Getting global package database location @(Kkm9qBx54bb6fPiRXOOC4T:Stack.GhcPkg src/Stack/GhcPkg.hs:48:5)
2016-07-06 19:40:57.608962: [debug] Run process: /home/simon/.stack/programs/x86_64-linux/ghc-8.0.1/bin/ghc-pkg --no-user-package-db list --global @(Kkm9qBx54bb6fPiRXOOC4T:System.Process.Read src/System/Process/Read.hs:292:3)
2016-07-06 19:40:57.631108: [debug] Process finished in 21 ms: /home/simon/.stack/programs/x86_64-linux/ghc-8.0.1/bin/ghc-pkg --no-user-package-db list --global @(Kkm9qBx54bb6fPiRXOOC4T:System.Process.Read src/System/Process/Read.hs:292:3)
global-pkg-db: /home/simon/.stack/programs/x86_64-linux/ghc-8.0.1/lib/ghc-8.0.1/package.conf.d
dist-dir: .stack-work/dist/x86_64-linux/Cabal-1.24.0.0

[sjakobi]

So, what I actually don't know, is whether this safe, especially querying ghc-pkg asynchronously.

Now, I figure there's no reason why ghc-pkg wouldn't be able to perform two reads concurrently.

[mgsloan]

Makes sense!