util: add go#util#env function

[fatih]

Add new function to return cached version of Go environment variables.
With that set GOROOT everytime we execute gocode. This will prevent
gocode using the internal embedded GOROOT and will use the GOROOT from
the latest installed Go version.

Fixes #901

[ashb]

I had to add this hunk to get it working for :GoRename too:

diff --git a/autoload/go/tool.vim b/autoload/go/tool.vim
index 487b933..fbbe12b 100644
--- a/autoload/go/tool.vim
+++ b/autoload/go/tool.vim
@@ -121,7 +121,9 @@ endfunction

 function! go#tool#ExecuteInDir(cmd) abort
   let old_gopath = $GOPATH
+  let old_goroot = $GOROOT
   let $GOPATH = go#path#Detect()
+  let $GOROOT = go#util#env("goroot")

   let cd = exists('*haslocaldir') && haslocaldir() ? 'lcd ' : 'cd '
   let dir = getcwd()
@@ -133,6 +135,7 @@ function! go#tool#ExecuteInDir(cmd) abort
   endtry

   let $GOPATH = old_gopath
+  let $GOROOT = old_goroot
   return out
 endfunction

Are there any other paths that might need it to?

Edit: :GoDoc will need something similar it looks like.

[fatih]

I'm not sure why :GoDoc or :GoRename needs them. Do they also use compiled GOROOT information?

[ashb]

I guess so - they seem to have problems if there's a syntax error. Trying to use ^] I just got this error trying to jump to the code for json.Marshal

vim-go: guru: cannot find package "encoding/json" in any of:
        /Users/ash/.homebrew/Cellar/go/1.6.2/libexec/src/encoding/json (from $GOROOT)
        /Users/ash/code/go/src/encoding/json (from $GOPATH)

However addingthe fix-up into autoload/go/guru.vim didn't seem to help in this case. So maybe this won't work for all tools :(

(My test is I last ran :GoUpdateBinaries with go 1.6.2 and this morning I upgraded to go 1.6.3)

[fatih]

@FiloSottile can you try this PR and see if this fixes #901 ?

[fatih]

Another ping @ashb @FiloSottile

[ashb]

I think it makes sense to include the patch form #954 (comment) too - it helps for more of the tools, even if not every single one.

[fatih]

@ashb good call, I've forget it. I'll try to add them.

[fatih]

@ashb sorry for the late response. But I've included your patch as well. Does this look good now? Can you try it?

[ashb]

This is an improvment and fixes some of the tools, if not all of them. Lets merge it.

(guru doesn't work because it doesn't call go#tool#ExecuteInDir but uses go#uitl#System. Probably worth fixing guru in another PR. Either to use ExecuteInDir or to save/restore GOROOT itself.)