Do not print to stderr if cgo linking succeeds after retry

The rety logic introduced by 6ab73ec still emitted the stderr of the
failing compiler execution before the retry. Now, this output is cached
and only emitted if the retry also fails.

go/tools/builders/cgo2.go

@@ -23,6 +23,7 @@ package main
 import (
 	"bytes"
 	"fmt"
+	"io/ioutil"
 	"os"
 	"path/filepath"
 	"strings"
@@ -196,7 +197,8 @@ func cgo2(goenv *env, goSrcs, cgoSrcs, cSrcs, cxxSrcs, objcSrcs, objcxxSrcs, sSr
 	mainBin := filepath.Join(workDir, "_cgo_.o") // .o is a lie; it's an executable
 	args = append([]string{cc, "-o", mainBin, mainObj}, cObjs...)
 	args = append(args, combinedLdFlags...)
-	if err := goenv.runCommand(args); err != nil {
+	originalErrBuf := &bytes.Buffer{}
+	if err := goenv.runCommandToFile(os.Stdout, originalErrBuf, args); err != nil {
 		// If linking the binary for cgo fails, this is usually because the
 		// object files reference external symbols that can't be resolved yet.
 		// Since the binary is only produced to have its symbols read by the cgo
@@ -214,13 +216,20 @@ func cgo2(goenv *env, goSrcs, cgoSrcs, cSrcs, cxxSrcs, objcSrcs, objcxxSrcs, sSr
 		default:
 			allowUnresolvedSymbolsLdFlag = "-Wl,--unresolved-symbols=ignore-all"
 		}
-		if err2 := goenv.runCommand(append(args, allowUnresolvedSymbolsLdFlag)); err2 != nil {
-			// Return the original error if we can't link the binary with the
-			// additional linker flags as they may simply be incorrect for the
-			// particular compiler/linker pair and would obscure the true reason
-			// for the failure of the original command.
+		// Print and return the original error if we can't link the binary with
+		// the additional linker flags as they may simply be incorrect for the
+		// particular compiler/linker pair and would obscure the true reason for
+		// the failure of the original command.
+		if err2 := goenv.runCommandToFile(
+			os.Stdout,
+			ioutil.Discard,
+			append(args, allowUnresolvedSymbolsLdFlag),
+		); err2 != nil {
+			os.Stderr.Write(relativizePaths(originalErrBuf.Bytes()))
 			return "", nil, nil, err
 		}
+		// Do not print the original error - rerunning the command with the
+		// additional linker flag fixed it.
 	}
 
 	cgoImportsGo := filepath.Join(workDir, "_cgo_imports.go")

go/tools/builders/env.go

@@ -141,12 +141,12 @@ func (e *env) runCommand(args []string) error {
 	return err
 }
 
-// runCommandToFile executes a subprocess and writes the output to the given
-// writer.
-func (e *env) runCommandToFile(w io.Writer, args []string) error {
+// runCommandToFile executes a subprocess and writes stdout/stderr to the given
+// writers.
+func (e *env) runCommandToFile(out io.Writer, err io.Writer, args []string) error {
 	cmd := exec.Command(args[0], args[1:]...)
-	cmd.Stdout = w
-	cmd.Stderr = os.Stderr
+	cmd.Stdout = out
+	cmd.Stderr = err
 	return runAndLogCommand(cmd, e.verbose)
 }
 

go/tools/builders/info.go

@@ -48,10 +48,10 @@ func run(args []string) error {
 		}
 		defer f.Close()
 	}
-	if err := goenv.runCommandToFile(f, goenv.goCmd("version")); err != nil {
+	if err := goenv.runCommandToFile(f, os.Stderr, goenv.goCmd("version")); err != nil {
 		return err
 	}
-	if err := goenv.runCommandToFile(f, goenv.goCmd("env")); err != nil {
+	if err := goenv.runCommandToFile(f, os.Stderr, goenv.goCmd("env")); err != nil {
 		return err
 	}
 	return nil

go/tools/builders/stdliblist.go

@@ -223,7 +223,7 @@ func stdliblist(args []string) error {
 	defer jsonFile.Close()
 
 	jsonData := &bytes.Buffer{}
-	if err := goenv.runCommandToFile(jsonData, listArgs); err != nil {
+	if err := goenv.runCommandToFile(jsonData, os.Stderr, listArgs); err != nil {
 		return err
 	}
 	encoder := json.NewEncoder(jsonFile)