Add Downloader.DownloadWithAlternates().

sdg0919 · Apr 26, 2016 · 7cb027a · 7cb027a
1 parent aa96c51
commit 7cb027a
Show file tree

Hide file tree

Showing 2 changed files with 41 additions and 3 deletions.
diff --git a/downloader/download.go b/downloader/download.go
@@ -24,7 +24,8 @@ type Request struct {
 	TargetDir string
 
 	// Verify is used to ensure that the download result is correct. If
-	// no func is provided then no verification happens.
+	// the download is invalid then the func must return errors.NotValid.
+	// If no func is provided then no verification happens.
 	Verify func(*os.File) error
 }
 
@@ -113,8 +114,14 @@ func (dl *Download) run(req Request) {
 
 		if req.Verify != nil {
 			err = req.Verify(file)
-			if _, err2 := file.Seek(0, os.SEEK_SET); err2 != nil && err == nil {
-				err = err2
+			if errors.IsNotValid(err) {
+				logger.Errorf("download from %s invalid: %v", req.URL, err)
+			}
+			if _, err2 := file.Seek(0, os.SEEK_SET); err2 != nil {
+				logger.Errorf("failed to seek to beginning of file: %v", err)
+				if err == nil {
+					err = err2
+				}
 			} else {
 				logger.Infof("download verified")
 			}
@@ -133,6 +140,8 @@ func (dl *Download) run(req Request) {
 }
 
 func download(req Request, openBlob func(*url.URL) (io.ReadCloser, error)) (file *os.File, err error) {
+	logger.Infof("downloading from %s", req.URL)
+
 	dir := req.TargetDir
 	if dir == "" {
 		dir = os.TempDir()

diff --git a/downloader/downloader.go b/downloader/downloader.go
@@ -6,6 +6,7 @@ package downloader
 import (
 	"io"
 	"net/url"
+	"os"
 
 	"github.com/juju/errors"
 	"github.com/juju/loggo"
@@ -55,3 +56,31 @@ func (dlr Downloader) Download(req Request, abort <-chan struct{}) (filename str
 	}
 	return file.Name(), nil
 }
+
+// DownloadWithAlternates tries each of the provided requests until
+// one succeeds. If none succeed then the error from the most recent
+// attempt is returned. At least one request must be provided.
+func (dlr Downloader) DownloadWithAlternates(requests []Request, abort <-chan struct{}) (filename string, err error) {
+	if len(requests) == 0 {
+		return "", errors.New("no requests to try")
+	}
+
+	for _, req := range requests {
+		if err := os.MkdirAll(req.TargetDir, 0755); err != nil {
+			return "", errors.Trace(err)
+		}
+		filename, err = dlr.Download(req, abort)
+		if errors.IsNotValid(err) {
+			break
+		}
+		if err == nil {
+			break
+		}
+		logger.Errorf("download request to %s failed: %v", req.URL, err)
+		// Try the next one.
+	}
+	if err != nil {
+		return "", errors.Trace(err)
+	}
+	return filename, nil
+}