MM-10381: Change the file downloading API to stream file (#9144)

* Change the file downloading to stream file

* Change file downloading to chunk only for preview

* Remove unnecessary else case

api4/file.go

@@ -154,14 +154,14 @@ func getFile(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	data, err := c.App.ReadFile(info.Path)
+	fileReader, err := c.App.FileReader(info.Path)
 	if err != nil {
 		c.Err = err
 		c.Err.StatusCode = http.StatusNotFound
 		return
 	}
 
-	err = writeFileResponse(info.Name, info.MimeType, data, forceDownload, w, r)
+	err = writeFileResponse(info.Name, info.MimeType, info.Size, fileReader, forceDownload, w, r)
 	if err != nil {
 		c.Err = err
 		return
@@ -195,10 +195,10 @@ func getFileThumbnail(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if data, err := c.App.ReadFile(info.ThumbnailPath); err != nil {
+	if fileReader, err := c.App.FileReader(info.ThumbnailPath); err != nil {
 		c.Err = err
 		c.Err.StatusCode = http.StatusNotFound
-	} else if err := writeFileResponse(info.Name, THUMBNAIL_IMAGE_TYPE, data, forceDownload, w, r); err != nil {
+	} else if err := writeFileResponse(info.Name, THUMBNAIL_IMAGE_TYPE, 0, fileReader, forceDownload, w, r); err != nil {
 		c.Err = err
 		return
 	}
@@ -264,10 +264,10 @@ func getFilePreview(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if data, err := c.App.ReadFile(info.PreviewPath); err != nil {
+	if fileReader, err := c.App.FileReader(info.PreviewPath); err != nil {
 		c.Err = err
 		c.Err.StatusCode = http.StatusNotFound
-	} else if err := writeFileResponse(info.Name, PREVIEW_IMAGE_TYPE, data, forceDownload, w, r); err != nil {
+	} else if err := writeFileResponse(info.Name, PREVIEW_IMAGE_TYPE, 0, fileReader, forceDownload, w, r); err != nil {
 		c.Err = err
 		return
 	}
@@ -325,20 +325,23 @@ func getPublicFile(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	if data, err := c.App.ReadFile(info.Path); err != nil {
+	if fileReader, err := c.App.FileReader(info.Path); err != nil {
 		c.Err = err
 		c.Err.StatusCode = http.StatusNotFound
-	} else if err := writeFileResponse(info.Name, info.MimeType, data, true, w, r); err != nil {
+	} else if err := writeFileResponse(info.Name, info.MimeType, info.Size, fileReader, true, w, r); err != nil {
 		c.Err = err
 		return
 	}
 }
 
-func writeFileResponse(filename string, contentType string, bytes []byte, forceDownload bool, w http.ResponseWriter, r *http.Request) *model.AppError {
+func writeFileResponse(filename string, contentType string, contentSize int64, fileReader io.Reader, forceDownload bool, w http.ResponseWriter, r *http.Request) *model.AppError {
 	w.Header().Set("Cache-Control", "max-age=2592000, private")
-	w.Header().Set("Content-Length", strconv.Itoa(len(bytes)))
 	w.Header().Set("X-Content-Type-Options", "nosniff")
 
+	if contentSize > 0 {
+		w.Header().Set("Content-Length", strconv.Itoa(int(contentSize)))
+	}
+
 	if contentType == "" {
 		contentType = "application/octet-stream"
 	} else {
@@ -380,7 +383,7 @@ func writeFileResponse(filename string, contentType string, bytes []byte, forceD
 	w.Header().Set("X-Frame-Options", "DENY")
 	w.Header().Set("Content-Security-Policy", "Frame-ancestors 'none'")
 
-	w.Write(bytes)
+	io.Copy(w, fileReader)
 
 	return nil
 }

app/file.go

@@ -70,6 +70,14 @@ func (a *App) ReadFile(path string) ([]byte, *model.AppError) {
 	return backend.ReadFile(path)
 }
 
+func (a *App) FileReader(path string) (io.Reader, *model.AppError) {
+	backend, err := a.FileBackend()
+	if err != nil {
+		return nil, err
+	}
+	return backend.Reader(path)
+}
+
 func (a *App) FileExists(path string) (bool, *model.AppError) {
 	backend, err := a.FileBackend()
 	if err != nil {