Fix file moving to be safe on Windows.

Fix file moving to be safe on Windows. A shim was added
that calls into the Win32 API / MoveFileEx that prevents an error
occurring on windows if the destination already exists. This shim might
not be needed once os.Replace lands, possibly in Go 1.5.

See: golang/go#8914

[#88907658]

cached_downloader.go

@@ -22,8 +22,8 @@ type CachedDownloader interface {
 	Fetch(urlToFetch *url.URL, cacheKey string, transformer CacheTransformer, cancelChan <-chan struct{}) (io.ReadCloser, int64, error)
 }
 
-func NoopTransform(source, destination string) (int64, error) {
-	err := os.Rename(source, destination)
+func NoopTransform(source string, destination string) (int64, error) {
+	err := Replace(source, destination)
 	if err != nil {
 		return 0, err
 	}

file_cache.go

@@ -105,7 +105,7 @@ func (c *FileCache) Add(cacheKey string, sourcePath string, size int64, cachingI
 	uniqueName := fmt.Sprintf("%s-%d-%d", cacheKey, time.Now().UnixNano(), c.seq)
 	cachePath := filepath.Join(c.cachedPath, uniqueName)
 
-	err := os.Rename(sourcePath, cachePath)
+	err := Replace(sourcePath, cachePath)
 	if err != nil {
 		return nil, err
 	}

replace.go

@@ -0,0 +1,9 @@
+// +build !windows
+
+package cacheddownloader
+
+import "os"
+
+func Replace(src, dst string) error {
+     return os.Rename(src, dst)
+}

replace_windows.go

@@ -0,0 +1,34 @@
+package cacheddownloader
+
+import (
+       "syscall"
+       "unsafe"
+)
+
+func Replace(src, dst string) error {
+	kernel32, err := syscall.LoadLibrary("kernel32.dll")
+	if err != nil {
+		return err
+	}
+	defer syscall.FreeLibrary(kernel32)
+	moveFileExUnicode, err := syscall.GetProcAddress(kernel32, "MoveFileExW")
+	if err != nil {
+		return err
+	}
+
+	srcString := syscall.StringToUTF16Ptr(src)
+	dstString := syscall.StringToUTF16Ptr(dst)
+
+	srcPtr := uintptr(unsafe.Pointer(srcString))
+	dstPtr := uintptr(unsafe.Pointer(dstString))
+
+	MOVEFILE_REPLACE_EXISTING := 0x1
+	flag := uintptr(MOVEFILE_REPLACE_EXISTING)
+
+	_, _, callErr := syscall.Syscall(uintptr(moveFileExUnicode), 3, srcPtr, dstPtr, flag)
+	if callErr != 0 {
+		return callErr
+	}
+
+	return nil
+}