cmd/godoc: update findGOROOT

After godoc stopped being part of the Go distribution, the location
of the godoc binary and the go binary can be different. Hence the old
GOROOT finding logic does not work anymore.

We shell out to "go env GOROOT" to return the new GOROOT.

Fixes golang/go#23445

Change-Id: I16e4c0798e3f5cda13d9f6546cf82808a10c4a0b
Reviewed-on: https://go-review.googlesource.com/c/tools/+/199279
Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Andrew Bonventre <andybons@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

cmd/godoc/goroot.go

@@ -6,14 +6,12 @@ package main
 
 import (
 	"os"
+	"os/exec"
 	"path/filepath"
 	"runtime"
+	"strings"
 )
 
-// Copies of functions from src/cmd/go/internal/cfg/cfg.go for
-// finding the GOROOT.
-// Keep them in sync until support is moved to a common place, if ever.
-
 func findGOROOT() string {
 	if env := os.Getenv("GOROOT"); env != "" {
 		return filepath.Clean(env)
@@ -24,51 +22,9 @@ func findGOROOT() string {
 		// depend on the executable's location.
 		return def
 	}
-	exe, err := os.Executable()
-	if err == nil {
-		exe, err = filepath.Abs(exe)
-		if err == nil {
-			if dir := filepath.Join(exe, "../.."); isGOROOT(dir) {
-				// If def (runtime.GOROOT()) and dir are the same
-				// directory, prefer the spelling used in def.
-				if isSameDir(def, dir) {
-					return def
-				}
-				return dir
-			}
-			exe, err = filepath.EvalSymlinks(exe)
-			if err == nil {
-				if dir := filepath.Join(exe, "../.."); isGOROOT(dir) {
-					if isSameDir(def, dir) {
-						return def
-					}
-					return dir
-				}
-			}
-		}
-	}
-	return def
-}
-
-// isGOROOT reports whether path looks like a GOROOT.
-//
-// It does this by looking for the path/pkg/tool directory,
-// which is necessary for useful operation of the cmd/go tool,
-// and is not typically present in a GOPATH.
-func isGOROOT(path string) bool {
-	stat, err := os.Stat(filepath.Join(path, "pkg", "tool"))
+	out, err := exec.Command("go", "env", "GOROOT").Output()
 	if err != nil {
-		return false
+		return def
 	}
-	return stat.IsDir()
-}
-
-// isSameDir reports whether dir1 and dir2 are the same directory.
-func isSameDir(dir1, dir2 string) bool {
-	if dir1 == dir2 {
-		return true
-	}
-	info1, err1 := os.Stat(dir1)
-	info2, err2 := os.Stat(dir2)
-	return err1 == nil && err2 == nil && os.SameFile(info1, info2)
+	return strings.TrimSpace(string(out))
 }