Work around empty go.o file.

On Windows the go.o file may be empty when the 'ar' command is run to
generate a C-style library archive. This fix replaces all Sprintf
path generation with calls to the canonical path.Join in lib.go, and
also adds an os.Stat(). This appears to force Windows to flush the
contents of go.o in a way that makes them visible to the 'ar' process
which runs directly after.

It is admittedly a hack, but an effective one. Perhaps a deeper
investigation into the matter will reveal something useful.

src/cmd/link/internal/ld/lib.go

@@ -391,7 +391,7 @@ func libinit() {
 		suffix = "msan"
 	}
 
-	Lflag(fmt.Sprintf("%s/pkg/%s_%s%s%s", goroot, goos, goarch, suffixsep, suffix))
+	Lflag(path.Join(goroot, "pkg", fmt.Sprintf("%s_%s%s%s", goos, goarch, suffixsep, suffix)))
 
 	mayberemoveoutfile()
 	f, err := os.OpenFile(outfile, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0775)
@@ -454,7 +454,7 @@ func loadinternal(name string) {
 	found := 0
 	for i := 0; i < len(Ctxt.Libdir); i++ {
 		if Linkshared {
-			shlibname := fmt.Sprintf("%s/%s.shlibname", Ctxt.Libdir[i], name)
+			shlibname := path.Join(Ctxt.Libdir[i], fmt.Sprintf("%s.shlibname", name))
 			if Debug['v'] != 0 {
 				fmt.Fprintf(&Bso, "searching for %s.a in %s\n", name, shlibname)
 			}
@@ -464,7 +464,7 @@ func loadinternal(name string) {
 				break
 			}
 		}
-		pname := fmt.Sprintf("%s/%s.a", Ctxt.Libdir[i], name)
+		pname := path.Join(Ctxt.Libdir[i], fmt.Sprintf("%s.a", name))
 		if Debug['v'] != 0 {
 			fmt.Fprintf(&Bso, "searching for %s.a in %s\n", name, pname)
 		}
@@ -953,7 +953,7 @@ func hostlinksetup() {
 	coutbuf.f.Close()
 	mayberemoveoutfile()
 
-	p := fmt.Sprintf("%s/go.o", tmpdir)
+	p := path.Join(tmpdir, "go.o")
 	var err error
 	f, err := os.OpenFile(p, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, 0775)
 	if err != nil {
@@ -971,7 +971,7 @@ func hostobjCopy() (paths []string) {
 	sema := make(chan struct{}, runtime.NumCPU()) // limit open file descriptors
 	for i, h := range hostobj {
 		h := h
-		dst := fmt.Sprintf("%s/%06d.o", tmpdir, i)
+		dst := path.Join(tmpdir, fmt.Sprintf("%06d.o", i))
 		paths = append(paths, dst)
 
 		wg.Add(1)
@@ -1012,9 +1012,23 @@ func archive() {
 		return
 	}
 
+	go_object_path := path.Join(tmpdir, "go.o")
+
 	mayberemoveoutfile()
+
+	// This should always succeed. However, without this call some Windows
+	// systems do not flush correctly, which causes the go.o file to be
+	// empty when the archiver runs below. As silly as it seems, this call
+	// causes the problem to disappear.
+	fi, err := os.Stat(go_object_path)
+	if err != nil {
+		Exitf("unable to locate go.o at '%v': %v\n", go_object_path, err)
+	} else if fi.Size() == 0 {
+		Exitf("empty go.o file at '%v'", go_object_path)
+	}
+
 	argv := []string{"ar", "-v", "-q", "-c", "-s", outfile}
-	argv = append(argv, path.Join(tmpdir, "go.o"))
+	argv = append(argv, go_object_path)
 	argv = append(argv, hostobjCopy()...)
 
 	if Debug['v'] != 0 {
@@ -1127,7 +1141,7 @@ func hostlink() {
 		argv = append(argv, "-Qunused-arguments")
 	}
 
-	argv = append(argv, fmt.Sprintf("%s/go.o", tmpdir))
+	argv = append(argv, path.Join(tmpdir, "go.o"))
 	argv = append(argv, hostobjCopy()...)
 
 	if Linkshared {
@@ -1205,7 +1219,7 @@ func hostlink() {
 	if Debug['s'] == 0 && debug_s == 0 && HEADTYPE == obj.Hdarwin {
 		// Skip combining dwarf on arm.
 		if Thearch.Thechar != '5' && Thearch.Thechar != '7' {
-			dsym := fmt.Sprintf("%s/go.dwarf", tmpdir)
+			dsym := path.Join(tmpdir, "go.dwarf")
 			if out, err := exec.Command("dsymutil", "-f", outfile, "-o", dsym).CombinedOutput(); err != nil {
 				Ctxt.Cursym = nil
 				Exitf("%s: running dsymutil failed: %v\n%s", os.Args[0], err, out)