cmd/go: bypass install to os.DevNull entirely, test mayberemovefile(os.DevNull)

Fixes #16811.

Change-Id: I7d018015f691838482ccf845d621209b96935ba4
Reviewed-on: https://go-review.googlesource.com/31657
Run-TryBot: Russ Cox <rsc@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Quentin Smith <quentin@golang.org>

src/cmd/go/build.go

@@ -470,6 +470,11 @@ func runBuild(cmd *Command, args []string) {
 		*buildO += exeSuffix
 	}
 
+	// Special case -o /dev/null by not writing at all.
+	if *buildO == os.DevNull {
+		*buildO = ""
+	}
+
 	// sanity check some often mis-used options
 	switch buildContext.Compiler {
 	case "gccgo":

src/cmd/go/build_test.go

+// Copyright 2016 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package main
+
+import (
+	"os"
+	"testing"
+)
+
+func TestRemoveDevNull(t *testing.T) {
+	fi, err := os.Lstat(os.DevNull)
+	if err != nil {
+		t.Skip(err)
+	}
+	if fi.Mode().IsRegular() {
+		t.Errorf("Lstat(%s).Mode().IsRegular() = true; expected false", os.DevNull)
+	}
+	mayberemovefile(os.DevNull)
+	_, err = os.Lstat(os.DevNull)
+	if err != nil {
+		t.Errorf("mayberemovefile(%s) did remove it; oops", os.DevNull)
+	}
+}

src/cmd/go/go_test.go

@@ -1334,9 +1334,6 @@ func TestInstallIntoGOPATH(t *testing.T) {
 
 // Issue 12407
 func TestBuildOutputToDevNull(t *testing.T) {
-	if runtime.GOOS == "plan9" {
-		t.Skip("skipping because /dev/null is a regular file on plan9")
-	}
 	tg := testgo(t)
 	defer tg.cleanup()
 	tg.setenv("GOPATH", filepath.Join(tg.pwd(), "testdata"))