blob: Add Upload and Download methods that may be more efficient for …

…some drivers

blob/azureblob/azureblob.go

@@ -815,9 +815,17 @@ type writer struct {
 	client     *blockblob.Client
 	uploadOpts *azblob.UploadStreamOptions
 
-	w     *io.PipeWriter
-	donec chan struct{}
-	err   error
+	// Ends of an io.Pipe, created when the first byte is written.
+	pw *io.PipeWriter
+	pr *io.PipeReader
+
+	// Alternatively, upload is set to true when Upload was
+	// used to upload data.
+	upload bool
+
+	donec chan struct{} // closed when done writing
+	// The following fields will be written before donec closes:
+	err error
 }
 
 // escapeKey does all required escaping for UTF-8 strings to work with Azure.
@@ -916,50 +924,64 @@ func (b *bucket) NewTypedWriter(ctx context.Context, key string, contentType str
 	}, nil
 }
 
-// Write appends p to w. User must call Close to close the w after done writing.
+// Write appends p to w.pw. User must call Close to close the w after done writing.
 func (w *writer) Write(p []byte) (int, error) {
+	// Avoid opening the pipe for a zero-length write;
+	// the concrete can do these for empty blobs.
 	if len(p) == 0 {
 		return 0, nil
 	}
-	if w.w == nil {
-		pr, pw := io.Pipe()
-		w.w = pw
-		if err := w.open(pr); err != nil {
-			return 0, err
-		}
+	if w.pw == nil {
+		// We'll write into pw and use pr as an io.Reader for the
+		// Upload call to Azure.
+		w.pr, w.pw = io.Pipe()
+		w.open(w.pr, true)
 	}
-	return w.w.Write(p)
+	return w.pw.Write(p)
+}
+
+// Upload reads from r. Per the driver, it is guaranteed to be the only
+// write call for this writer.
+func (w *writer) Upload(r io.Reader) error {
+	w.upload = true
+	w.open(r, false)
+	return nil
 }
 
-func (w *writer) open(pr *io.PipeReader) error {
+// r may be nil if we're Closing and no data was written.
+// If closePipeOnError is true, w.pr will be closed if there's an
+// error uploading to Azure.
+func (w *writer) open(r io.Reader, closePipeOnError bool) {
 	go func() {
 		defer close(w.donec)
 
-		var body io.Reader
-		if pr == nil {
-			body = http.NoBody
-		} else {
-			body = pr
+		if r == nil {
+			r = http.NoBody
 		}
-		_, w.err = w.client.UploadStream(w.ctx, body, w.uploadOpts)
+		_, w.err = w.client.UploadStream(w.ctx, r, w.uploadOpts)
 		if w.err != nil {
-			if pr != nil {
-				pr.CloseWithError(w.err)
+			if closePipeOnError {
+				w.pr.CloseWithError(w.err)
+				w.pr = nil
 			}
-			return
 		}
 	}()
-	return nil
 }
 
 // Close completes the writer and closes it. Any error occurring during write will
 // be returned. If a writer is closed before any Write is called, Close will
 // create an empty file at the given key.
 func (w *writer) Close() error {
-	if w.w == nil {
-		w.open(nil)
-	} else if err := w.w.Close(); err != nil {
-		return err
+	if !w.upload {
+		if w.pr != nil {
+			defer w.pr.Close()
+		}
+		if w.pw == nil {
+			// We never got any bytes written. We'll write an http.NoBody.
+			w.open(nil, false)
+		} else if err := w.pw.Close(); err != nil {
+			return err
+		}
 	}
 	<-w.donec
 	return w.err