box: use upload preflight check to avoid listings in file uploads

Before this change, rclone checked to see if an object existed before
doing an upload by listing the destination directory. This was very
inefficient, especially with large directories.

After this change rclone uses the pre upload check API call which
checks to see if it is OK to upload an object, and also returns the ID
of an existing object which saves rclone having to do a directory
listing.

backend/box/api/types.go

@@ -36,13 +36,13 @@ func (t *Time) UnmarshalJSON(data []byte) error {
 
 // Error is returned from box when things go wrong
 type Error struct {
-	Type        string `json:"type"`
-	Status      int    `json:"status"`
-	Code        string `json:"code"`
-	ContextInfo json.RawMessage
-	HelpURL     string `json:"help_url"`
-	Message     string `json:"message"`
-	RequestID   string `json:"request_id"`
+	Type        string          `json:"type"`
+	Status      int             `json:"status"`
+	Code        string          `json:"code"`
+	ContextInfo json.RawMessage `json:"context_info"`
+	HelpURL     string          `json:"help_url"`
+	Message     string          `json:"message"`
+	RequestID   string          `json:"request_id"`
 }
 
 // Error returns a string for the error and satisfies the error interface
@@ -132,6 +132,38 @@ type UploadFile struct {
 	ContentModifiedAt Time   `json:"content_modified_at"`
 }
 
+// PreUploadCheck is the request for upload preflight check
+type PreUploadCheck struct {
+	Name   string `json:"name"`
+	Parent Parent `json:"parent"`
+	Size   *int64 `json:"size,omitempty"`
+}
+
+// PreUploadCheckResponse is the response from upload preflight check
+// if successful
+type PreUploadCheckResponse struct {
+	UploadToken string `json:"upload_token"`
+	UploadURL   string `json:"upload_url"`
+}
+
+// PreUploadCheckConflict is returned in the ContextInfo error field
+// from PreUploadCheck when the error code is "item_name_in_use"
+type PreUploadCheckConflict struct {
+	Conflicts struct {
+		Type        string `json:"type"`
+		ID          string `json:"id"`
+		FileVersion struct {
+			Type string `json:"type"`
+			ID   string `json:"id"`
+			Sha1 string `json:"sha1"`
+		} `json:"file_version"`
+		SequenceID string `json:"sequence_id"`
+		Etag       string `json:"etag"`
+		Sha1       string `json:"sha1"`
+		Name       string `json:"name"`
+	} `json:"conflicts"`
+}
+
 // UpdateFileModTime is used in Update File Info
 type UpdateFileModTime struct {
 	ContentModifiedAt Time `json:"content_modified_at"`

backend/box/box.go

@@ -686,22 +686,80 @@ func (f *Fs) createObject(ctx context.Context, remote string, modTime time.Time,
 	return o, leaf, directoryID, nil
 }
 
+// preUploadCheck checks to see if a file can be uploaded
+//
+// It returns "", nil if the file is good to go
+// It returns "ID", nil if the file must be updated
+func (f *Fs) preUploadCheck(ctx context.Context, leaf, directoryID string, size int64) (ID string, err error) {
+	check := api.PreUploadCheck{
+		Name: f.opt.Enc.FromStandardName(leaf),
+		Parent: api.Parent{
+			ID: directoryID,
+		},
+	}
+	if size >= 0 {
+		check.Size = &size
+	}
+	opts := rest.Opts{
+		Method: "OPTIONS",
+		Path:   "/files/content/",
+	}
+	var result api.PreUploadCheckResponse
+	var resp *http.Response
+	err = f.pacer.Call(func() (bool, error) {
+		resp, err = f.srv.CallJSON(ctx, &opts, &check, &result)
+		return shouldRetry(ctx, resp, err)
+	})
+	if err != nil {
+		if apiErr, ok := err.(*api.Error); ok && apiErr.Code == "item_name_in_use" {
+			var conflict api.PreUploadCheckConflict
+			err = json.Unmarshal(apiErr.ContextInfo, &conflict)
+			if err != nil {
+				return "", errors.Wrap(err, "pre-upload check: JSON decode failed")
+			}
+			if conflict.Conflicts.Type != api.ItemTypeFile {
+				return "", errors.Wrap(err, "pre-upload check: can't overwrite non file with file")
+			}
+			return conflict.Conflicts.ID, nil
+		}
+		return "", errors.Wrap(err, "pre-upload check")
+	}
+	return "", nil
+}
+
 // Put the object
 //
 // Copy the reader in to the new object which is returned
 //
 // The new object may have been created if an error is returned
 func (f *Fs) Put(ctx context.Context, in io.Reader, src fs.ObjectInfo, options ...fs.OpenOption) (fs.Object, error) {
-	existingObj, err := f.newObjectWithInfo(ctx, src.Remote(), nil)
-	switch err {
-	case nil:
-		return existingObj, existingObj.Update(ctx, in, src, options...)
-	case fs.ErrorObjectNotFound:
-		// Not found so create it
-		return f.PutUnchecked(ctx, in, src)
-	default:
+	// If directory doesn't exist, file doesn't exist so can upload
+	remote := src.Remote()
+	leaf, directoryID, err := f.dirCache.FindPath(ctx, remote, false)
+	if err != nil {
+		if err == fs.ErrorDirNotFound {
+			return f.PutUnchecked(ctx, in, src, options...)
+		}
 		return nil, err
 	}
+
+	// Preflight check the upload, which returns the ID if the
+	// object already exists
+	ID, err := f.preUploadCheck(ctx, leaf, directoryID, src.Size())
+	if err != nil {
+		return nil, err
+	}
+	if ID == "" {
+		return f.PutUnchecked(ctx, in, src, options...)
+	}
+
+	// If object exists then create a skeleton one with just id
+	o := &Object{
+		fs:     f,
+		remote: remote,
+		id:     ID,
+	}
+	return o, o.Update(ctx, in, src, options...)
 }
 
 // PutStream uploads to the remote path with the modTime given of indeterminate size