build: multiple fixes to make "go install" work on windows

R=rsc
CC=golang-dev
https://golang.org/cl/5502054

src/buildscript_windows_386.sh

@@ -545,4 +545,4 @@ mkdir -p $WORK/cmd/go/_obj/
 gopack grc $WORK/cmd/go.a $WORK/cmd/go/_obj/_go_.6
 8l -o $WORK/cmd/go/_obj/a.out -L $WORK $WORK/cmd/go.a
 mkdir -p $GOBIN/
-cp $WORK/cmd/go/_obj/a.out $GOBIN/go
+cp $WORK/cmd/go/_obj/a.out $GOBIN/go.exe

src/buildscript_windows_amd64.sh

@@ -544,4 +544,4 @@ mkdir -p $WORK/cmd/go/_obj/
 gopack grc $WORK/cmd/go.a $WORK/cmd/go/_obj/_go_.6
 6l -o $WORK/cmd/go/_obj/a.out -L $WORK $WORK/cmd/go.a
 mkdir -p $GOBIN/
-cp $WORK/cmd/go/_obj/a.out $GOBIN/go
+cp $WORK/cmd/go/_obj/a.out $GOBIN/go.exe

src/cmd/go/build.go

@@ -15,6 +15,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"regexp"
+	"runtime"
 	"strings"
 	"sync"
 )
@@ -518,6 +519,33 @@ func (b *builder) install(a *action) error {
 	return b.copyFile(dst, src, perm)
 }
 
+// removeByRenaming removes file name by moving it to a tmp
+// directory and deleting the target if possible.
+func removeByRenaming(name string) error {
+	f, err := ioutil.TempFile("", "")
+	if err != nil {
+		return err
+	}
+	tmpname := f.Name()
+	f.Close()
+	err = os.Remove(tmpname)
+	if err != nil {
+		return err
+	}
+	err = os.Rename(name, tmpname)
+	if err != nil {
+		// assume name file does not exists,
+		// otherwise later code will fail.
+		return nil
+	}
+	err = os.Remove(tmpname)
+	if err != nil {
+		// TODO(brainman): file is locked and can't be deleted.
+		// We need to come up with a better way of doing it. 
+	}
+	return nil
+}
+
 // copyFile is like 'cp src dst'.
 func (b *builder) copyFile(dst, src string, perm uint32) error {
 	if b.nflag || b.xflag {
@@ -534,9 +562,21 @@ func (b *builder) copyFile(dst, src string, perm uint32) error {
 	defer sf.Close()
 	os.Remove(dst)
 	df, err := os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
+	if err != nil {
+		if runtime.GOOS != "windows" {
+			return err
+		}
+		// Windows does not allow to replace binary file
+		// while it is executing. We will cheat.
+		err = removeByRenaming(dst)
 		if err != nil {
 			return err
 		}
+		df, err = os.OpenFile(dst, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
+		if err != nil {
+			return err
+		}
+	}
 	_, err = io.Copy(df, sf)
 	df.Close()
 	if err != nil {

src/cmd/go/main.go

@@ -285,7 +285,7 @@ func allPackages() []string {
 			if err != nil {
 				return nil
 			}
-			name := path[len(src):]
+			name := filepath.ToSlash(path[len(src):])
 			if !have[name] {
 				pkgs = append(pkgs, name)
 				have[name] = true

src/cmd/go/pkg.go

@@ -113,6 +113,9 @@ func scanPackage(ctxt *build.Context, t *build.Tree, arg, importPath, dir string
 	if info.Package == "main" {
 		_, elem := filepath.Split(importPath)
 		targ = filepath.Join(t.BinDir(), elem)
+		if ctxt.GOOS == "windows" {
+			targ += ".exe"
+		}
 	} else {
 		targ = filepath.Join(t.PkgDir(), filepath.FromSlash(importPath)+".a")
 	}

src/pkg/mime/type_unix.go

@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
+// +build darwin freebsd linux openbsd plan9
+
 package mime
 
 import (

src/pkg/runtime/cgo/trigger.go

@@ -14,7 +14,7 @@ package cgo
 #cgo linux LDFLAGS: -lpthread
 #cgo netbsd LDFLAGS: -lpthread
 #cgo openbsd LDFLAGS: -lpthread
-#cgo windows LDFLAGS: -lm -lmthreads
+#cgo windows LDFLAGS: -lm -mthreads
 
 */
 import "C"