util: add go#util#env function

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

autoload/go/complete.vim

@@ -32,7 +32,9 @@ function! s:gocodeCommand(cmd, preargs, args)
   " we might hit cache problems, as gocode doesn't handle well different
   " GOPATHS: https://github.com/nsf/gocode/issues/239
   let old_gopath = $GOPATH
+  let old_goroot = $GOROOT
   let $GOPATH = go#path#Detect()
+  let $GOROOT = go#util#env("goroot")
 
   let socket_type = get(g:, 'go_gocode_socket_type', s:sock_type)
   let cmd = printf('%s -sock %s %s %s %s', 
@@ -45,6 +47,8 @@ function! s:gocodeCommand(cmd, preargs, args)
 
   let result = go#util#System(cmd)
   let $GOPATH = old_gopath
+  let $GOROOT = old_goroot
+
   if go#util#ShellError() != 0
     return "[\"0\", []]"
   else

autoload/go/util.vim

@@ -43,6 +43,31 @@ function! go#util#IsWin()
   return 0
 endfunction
 
+let s:env_cache = {}
+
+" env returns the go environment variable for the given key. Where key can be
+" GOARCH, GOOS, GOROOT, etc... It caches the result and returns the cached
+" version. 
+function! go#util#env(key)
+  let l:key = tolower(a:key)
+  if has_key(s:env_cache, l:key)
+    return s:env_cache[l:key]
+  endif
+
+  if executable('go')
+    let l:var = call('go#util#'.l:key, [])
+    if go#util#ShellError() != 0
+      call go#util#EchoError(printf("'go env %s' failed", toupper(l:key)))
+      return ''
+    endif
+  else
+    let l:var = eval("$".toupper(a:key))
+  endif
+
+  let s:env_cache[l:key] = l:var
+  return l:var
+endfunction
+
 function! go#util#goarch()
   return substitute(go#util#System('go env GOARCH'), '\n', '', 'g')
 endfunction