cmd/go: correctly quote environment variables in -x output

This fixes the -x output so that when it reports environment variables they
are correctly quoted for later execution by the shell.
Also fix -x output to use the right path to the pack tool, and note when
we are touching a file.

Fixes #21427

Change-Id: I323ef4edf9905b08bc26944b94183d8da2fa9675
Reviewed-on: https://go-review.googlesource.com/55350Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/go_test.go

@@ -4389,3 +4389,52 @@ func TestBuildmodePIE(t *testing.T) {
 		t.Errorf("got %q; want %q", out, "hello")
 	}
 }
+
+func TestExecBuildX(t *testing.T) {
+	if !canCgo {
+		t.Skip("skipping because cgo not enabled")
+	}
+
+	if runtime.GOOS == "plan9" || runtime.GOOS == "windows" {
+		t.Skipf("skipping because unix shell is not supported on %s", runtime.GOOS)
+	}
+
+	tg := testgo(t)
+	defer tg.cleanup()
+
+	tg.tempFile("main.go", `package main; import "C"; func main() { print("hello") }`)
+	src := tg.path("main.go")
+	obj := tg.path("main")
+	tg.run("build", "-x", "-o", obj, src)
+	sh := tg.path("test.sh")
+	err := ioutil.WriteFile(sh, []byte(tg.getStderr()), 0666)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	out, err := exec.Command(obj).CombinedOutput()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(out) != "hello" {
+		t.Fatalf("got %q; want %q", out, "hello")
+	}
+
+	err = os.Remove(obj)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	out, err = exec.Command("/bin/sh", sh).CombinedOutput()
+	if err != nil {
+		t.Fatalf("/bin/sh %s: %v\n%s", sh, err, out)
+	}
+
+	out, err = exec.Command(obj).CombinedOutput()
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(out) != "hello" {
+		t.Fatalf("got %q; want %q", out, "hello")
+	}
+}

src/cmd/go/internal/work/build.go

@@ -1963,10 +1963,16 @@ func (b *Builder) runOut(dir string, desc string, env []string, cmdargs ...inter
 	cmdline := str.StringList(cmdargs...)
 	if cfg.BuildN || cfg.BuildX {
 		var envcmdline string
-		for i := range env {
-			envcmdline += env[i]
+		for _, e := range env {
+			if j := strings.IndexByte(e, '='); j != -1 {
+				if strings.ContainsRune(e[j+1:], '\'') {
+					envcmdline += fmt.Sprintf("%s=%q", e[:j], e[j+1:])
+				} else {
+					envcmdline += fmt.Sprintf("%s='%s'", e[:j], e[j+1:])
+				}
 				envcmdline += " "
 			}
+		}
 		envcmdline += joinUnambiguously(cmdline)
 		b.Showcmd(dir, "%s", envcmdline)
 		if cfg.BuildN {
@@ -2416,7 +2422,7 @@ func (gcToolchain) pack(b *Builder, p *load.Package, objDir, afile string, ofile
 	}
 
 	if cfg.BuildN || cfg.BuildX {
-		cmdline := str.StringList("pack", "r", absAfile, absOfiles)
+		cmdline := str.StringList(base.Tool("pack"), "r", absAfile, absOfiles)
 		b.Showcmd(p.Dir, "%s # internal", joinUnambiguously(cmdline))
 	}
 	if cfg.BuildN {
@@ -3220,6 +3226,9 @@ func (b *Builder) gccSupportsFlag(flag string) bool {
 		return b
 	}
 	if b.flagCache == nil {
+		if cfg.BuildN || cfg.BuildX {
+			b.Showcmd(b.WorkDir, "touch trivial.c")
+		}
 		src := filepath.Join(b.WorkDir, "trivial.c")
 		if err := ioutil.WriteFile(src, []byte{}, 0666); err != nil {
 			return false