resume download after fail

close #2106
Signed-off-by: Vasiliy Tolstov <v.tolstov@selfip.ru>

common/download.go

@@ -89,7 +89,7 @@ func NewDownloadClient(c *DownloadConfig) *DownloadClient {
 // downloading it.
 type Downloader interface {
 	Cancel()
-	Download(io.Writer, *url.URL) error
+	Download(*os.File, *url.URL) error
 	Progress() uint
 	Total() uint
 }
@@ -99,6 +99,8 @@ func (d *DownloadClient) Cancel() {
 }
 
 func (d *DownloadClient) Get() (string, error) {
+	var f *os.File
+
 	// If we already have the file and it matches, then just return the target path.
 	if verify, _ := d.VerifyChecksum(d.config.TargetPath); verify {
 		log.Println("Initial checksum matched, no download needed.")
@@ -131,7 +133,7 @@ func (d *DownloadClient) Get() (string, error) {
 		}
 
 		// Otherwise, download using the downloader.
-		f, err := os.Create(finalPath)
+		f, err = os.OpenFile(finalPath, os.O_RDWR|os.O_CREATE, os.FileMode(0666))
 		if err != nil {
 			return "", err
 		}
@@ -195,9 +197,9 @@ func (*HTTPDownloader) Cancel() {
 	// TODO(mitchellh): Implement
 }
 
-func (d *HTTPDownloader) Download(dst io.Writer, src *url.URL) error {
+func (d *HTTPDownloader) Download(dst *os.File, src *url.URL) error {
 	log.Printf("Starting download: %s", src.String())
-	req, err := http.NewRequest("GET", src.String(), nil)
+	req, err := http.NewRequest("HEAD", src.String(), nil)
 	if err != nil {
 		return err
 	}
@@ -217,10 +219,15 @@ func (d *HTTPDownloader) Download(dst io.Writer, src *url.URL) error {
 		return err
 	}
 
+	req.Method = "GET"
 	if resp.StatusCode != 200 {
 		log.Printf(
 			"Non-200 status code: %d. Getting error body.", resp.StatusCode)
 
+		resp, err := httpClient.Do(req)
+		if err != nil {
+			return err
+		}
 		errorBody := new(bytes.Buffer)
 		io.Copy(errorBody, resp.Body)
 		return fmt.Errorf("HTTP error '%d'! Remote side responded:\n%s",
@@ -228,6 +235,21 @@ func (d *HTTPDownloader) Download(dst io.Writer, src *url.URL) error {
 	}
 
 	d.progress = 0
+
+	if resp.Header.Get("Accept-Ranges") == "bytes" {
+		if fi, err := dst.Stat(); err == nil {
+			if _, err = dst.Seek(0, os.SEEK_END); err == nil {
+				req.Header.Set("Range", fmt.Sprintf("bytes=%d-", fi.Size()))
+				d.progress = uint(fi.Size())
+			}
+		}
+	}
+
+	resp, err = httpClient.Do(req)
+	if err != nil {
+		return err
+	}
+
 	d.total = uint(resp.ContentLength)
 
 	var buffer [4096]byte