internal/testenv: add GoTool

GoToolPath requires a *testing.T to handle errors.
GoTool provides a variant that returns errors
for clients without a *testing.T,
such as that found in CL 27811.

Change-Id: I7ac8b7ec9d472894c37223c5f7b121ec823e7f61
Reviewed-on: https://go-review.googlesource.com/28787Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Rob Pike <r@golang.org>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/internal/testenv/testenv.go

@@ -11,6 +11,7 @@
 package testenv
 
 import (
+	"errors"
 	"flag"
 	"os"
 	"os/exec"
@@ -67,26 +68,36 @@ func MustHaveGoRun(t *testing.T) {
 }
 
 // GoToolPath reports the path to the Go tool.
+// It is a convenience wrapper around GoTool.
 // If the tool is unavailable GoToolPath calls t.Skip.
 // If the tool should be available and isn't, GoToolPath calls t.Fatal.
 func GoToolPath(t *testing.T) string {
 	MustHaveGoBuild(t)
+	path, err := GoTool()
+	if err != nil {
+		t.Fatal(err)
+	}
+	return path
+}
 
+// GoTool reports the path to the Go tool.
+func GoTool() (string, error) {
+	if !HasGoBuild() {
+		return "", errors.New("platform cannot run go tool")
+	}
 	var exeSuffix string
 	if runtime.GOOS == "windows" {
 		exeSuffix = ".exe"
 	}
-
 	path := filepath.Join(runtime.GOROOT(), "bin", "go"+exeSuffix)
 	if _, err := os.Stat(path); err == nil {
-		return path
+		return path, nil
 	}
-
 	goBin, err := exec.LookPath("go" + exeSuffix)
 	if err != nil {
-		t.Fatalf("cannot find go tool: %v", err)
+		return "", errors.New("cannot find go tool: " + err.Error())
 	}
-	return goBin
+	return goBin, nil
 }
 
 // HasExec reports whether the current system can start new processes