Some env setup steps are duplicated

[sjakobi]

~> stack path -v
Version 1.1.3, Git revision 49d96c9cbbd34ab33af02f3957b33fdd9575a7f8 (dirty) (3945 commits) x86_64 hpack-0.14.0
...
2016-08-09 15:27:07.043895: [debug] Checking whether stack was built with libgmp4 @(stack_BSx6FTAIDJFC2EPDshToqJ:Stack.Config src/Stack/Config.hs:334:5)
2016-08-09 15:27:07.044312: [debug] Run process: /usr/bin/ldd /home/simon/.local/bin/stack @(stack_BSx6FTAIDJFC2EPDshToqJ:System.Process.Read src/System/Process/Read.hs:271:3)
2016-08-09 15:27:07.055798: [debug] Process finished in 11 ms: /usr/bin/ldd /home/simon/.local/bin/stack @(stack_BSx6FTAIDJFC2EPDshToqJ:System.Process.Read src/System/Process/Read.hs:271:3)
2016-08-09 15:27:07.055984: [debug] Stack was not built with libgmp4 @(stack_BSx6FTAIDJFC2EPDshToqJ:Stack.Config src/Stack/Config.hs:338:14)
2016-08-09 15:27:07.056088: [info] Run from outside a project, using implicit global project config @(stack_BSx6FTAIDJFC2EPDshToqJ:Stack.Config src/Stack/Config.hs:469:13)
2016-08-09 15:27:07.061531: [info] Using resolver: nightly-2016-08-06 from implicit global project's config file: /home/simon/.stack/global-project/stack.yaml @(stack_BSx6FTAIDJFC2EPDshToqJ:Stack.Config src/Stack/Config.hs:483:32)
2016-08-09 15:27:07.061719: [debug] Trying to decode /home/simon/.stack/build-plan-cache/x86_64-linux/nightly-2016-08-06.cache @(stack_BSx6FTAIDJFC2EPDshToqJ:Data.Store.VersionTagged src/Data/Store/VersionTagged.hs:68:5)
2016-08-09 15:27:07.084105: [debug] Success decoding /home/simon/.stack/build-plan-cache/x86_64-linux/nightly-2016-08-06.cache @(stack_BSx6FTAIDJFC2EPDshToqJ:Data.Store.VersionTagged src/Data/Store/VersionTagged.hs:72:13)
2016-08-09 15:27:07.085371: [debug] Checking whether stack was built with libgmp4 @(stack_BSx6FTAIDJFC2EPDshToqJ:Stack.Config src/Stack/Config.hs:334:5)
2016-08-09 15:27:07.085513: [debug] Run process: /usr/bin/ldd /home/simon/.local/bin/stack @(stack_BSx6FTAIDJFC2EPDshToqJ:System.Process.Read src/System/Process/Read.hs:271:3)
2016-08-09 15:27:07.097725: [debug] Process finished in 12 ms: /usr/bin/ldd /home/simon/.local/bin/stack @(stack_BSx6FTAIDJFC2EPDshToqJ:System.Process.Read src/System/Process/Read.hs:271:3)
2016-08-09 15:27:07.097923: [debug] Stack was not built with libgmp4 @(stack_BSx6FTAIDJFC2EPDshToqJ:Stack.Config src/Stack/Config.hs:338:14)
2016-08-09 15:27:07.098027: [info] Run from outside a project, using implicit global project config @(stack_BSx6FTAIDJFC2EPDshToqJ:Stack.Config src/Stack/Config.hs:469:13)
...

[sjakobi]

Another symptom:

~/s/gitit> stack install
Warning (added by new or init): Specified resolver could not satisfy all dependencies. Some external packages have been added as dependencies.
You can suppress this message by removing it from stack.yaml

Warning (added by new or init): Specified resolver could not satisfy all dependencies. Some external packages have been added as dependencies.
You can suppress this message by removing it from stack.yaml
...

[mgsloan]

Yeah, that doesn't look so good!

[sjakobi]

git bisect lead me to this line from #2270:

The addition of loadCompilerVersion causes the BuildConfig to be loaded twice (via lcLoadBuildConfig).

To resolve this I'm thinking of the following:

• Figure out the cheapest way to get the wanted compiler (bcWantedCompiler only uses bcWantedMiniBuildPlan, so we shouldn't need to compute the entire BuildConfig).
• Reuse as much of the configuration found in the first step as possible in the construction of the BuildConfig to avoid duplicate work.

This sounds rather inelegant and like quite a bit of work to me, so if someone can think of an alternative resolution, please chime in!

Pinging @YPares.

[sjakobi]

Figure out the cheapest way to get the wanted compiler (bcWantedCompiler only uses bcWantedMiniBuildPlan, so we shouldn't need to compute the entire BuildConfig).

Actually I can't really find any potential for savings here: The compiler version seems to depend on virtually all the other information that goes into the BuildConfig. (Here's the relevant code)