address comments

go/tools/builders/stdliblist.go

@@ -156,14 +156,15 @@ func flatPackageForStd(cloneBase string, pkg *goListPackage) *flatPackage {
 // If we run "go list" with that GOROOT, this action will fail because those
 // go:embed directives will refuse to include the symlinks in the sandbox.
 //
-// To work around this, cloneGoRoot creates a copy of external/go_sdk into a new
-// cloneBase directory while retaining its path relative to the root directory.
+// To work around this, cloneGoRoot creates a copy of a subset of external/go_sdk
+// that is sufficient to call "go list" into a new cloneBase directory while
+// retaining its path relative to the root directory, e.g. "go list" needs to
+// call "compile", which needs "pkg/tool".
 // So "$OUTPUT_BASE/external/go_sdk" becomes
-// {cloneBase}/external/go_sdk", which will be set at GOROOT later.
-// This ensures that file paths in the generated JSON are still valid.
+// {cloneBase}/external/go_sdk", which will be set at GOROOT later. This ensures
+// that file paths in the generated JSON are still valid.
 //
-// cloneGoRoot returns the new GOROOT we should run
-// under.
+// cloneGoRoot returns the new GOROOT we should run under.
 func cloneGoRoot(execRoot, relativeGoroot, cloneBase string) (newGoRoot string, err error) {
 	goroot := filepath.Join(execRoot, relativeGoroot)
 	newGoRoot = filepath.Join(cloneBase, relativeGoroot)

go/tools/builders/stdliblist_test.go

@@ -4,7 +4,7 @@ import (
 	"bufio"
 	"encoding/json"
 	"fmt"
-	"io/ioutil"
+	"os"
 	"path/filepath"
 	"strings"
 	"testing"
@@ -14,7 +14,7 @@ import (
 
 func Test_stdliblist(t *testing.T) {
 	testDir := t.TempDir()
-	f, _ := ioutil.TempFile(testDir, "*")
+	outJSON := filepath.Join(testDir, "out.json")
 
 	// test files are at run file directory, but this test is run at
 	// {runfile directory}/bazel.TestWorkspace()
@@ -27,7 +27,7 @@ func Test_stdliblist(t *testing.T) {
 		t.Error("failed to find runfiles path")
 	}
 	test_args := []string{
-		fmt.Sprintf("-out=%s", f.Name()),
+		fmt.Sprintf("-out=%s", outJSON),
 		fmt.Sprintf("-sdk=%s", "go_sdk"),
 		fmt.Sprintf("-wd=%s", filepath.Dir(filepath.Clean(runFilesPath))),
 	}
@@ -36,6 +36,11 @@ func Test_stdliblist(t *testing.T) {
 	if err != nil {
 		t.Errorf("calling stdliblist got err: %v", err)
 	}
+	f, err := os.Open(outJSON)
+	if err != nil {
+		t.Errorf("cannot open output json: %v", err)
+	}
+	defer func() { _ = f.Close() }()
 	scanner := bufio.NewScanner(f)
 	for scanner.Scan() {
 		var result flatPackage