internal/build: add GoTool and document why it uses GOROOT

build/ci.go

@@ -250,10 +250,7 @@ func goTool(subcmd string, args ...string) *exec.Cmd {
 }
 
 func goToolArch(arch string, subcmd string, args ...string) *exec.Cmd {
-	gocmd := filepath.Join(runtime.GOROOT(), "bin", "go")
-	cmd := exec.Command(gocmd, subcmd)
-	cmd.Args = append(cmd.Args, args...)
-
+	cmd := build.GoTool(subcmd, args...)
 	if subcmd == "build" || subcmd == "install" || subcmd == "test" {
 		// Go CGO has a Windows linker error prior to 1.8 (https://github.com/golang/go/issues/8756).
 		// Work around issue by allowing multiple definitions for <1.8 builds.

internal/build/util.go

@@ -138,6 +138,19 @@ func CopyFile(dst, src string, mode os.FileMode) {
 	}
 }
 
+// GoTool returns the command that runs a go tool. This uses go from GOROOT instead of PATH
+// so that go commands executed by build use the same version of Go as the 'host' that runs
+// build code. e.g.
+//
+//     /usr/lib/go-1.8/bin/go run build/ci.go ...
+//
+// runs using go 1.8 and invokes go 1.8 tools from the same GOROOT. This is also important
+// because runtime.Version checks on the host should match the tools that are run.
+func GoTool(tool string, args ...string) *exec.Cmd {
+	args = append([]string{tool}, args...)
+	return exec.Command(filepath.Join(runtime.GOROOT(), "bin", "go"), args...)
+}
+
 // ExpandPackagesNoVendor expands a cmd/go import path pattern, skipping
 // vendored packages.
 func ExpandPackagesNoVendor(patterns []string) []string {
@@ -148,8 +161,7 @@ func ExpandPackagesNoVendor(patterns []string) []string {
 		}
 	}
 	if expand {
-		args := append([]string{"list"}, patterns...)
-		cmd := exec.Command(filepath.Join(runtime.GOROOT(), "bin", "go"), args...)
+		cmd := GoTool("list", patterns...)
 		out, err := cmd.CombinedOutput()
 		if err != nil {
 			log.Fatalf("package listing failed: %v\n%s", err, string(out))