add GDrive provider support (#118)

* GDrive provider support
* More reliable basedir ownership
* Fix mimetype

vendor/cloud.google.com/go/internal/annotate.go

@@ -0,0 +1,54 @@
+// Copyright 2017 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package internal
+
+import (
+	"fmt"
+
+	"google.golang.org/api/googleapi"
+	"google.golang.org/grpc/status"
+)
+
+// Annotate prepends msg to the error message in err, attempting
+// to preserve other information in err, like an error code.
+//
+// Annotate panics if err is nil.
+//
+// Annotate knows about these error types:
+// - "google.golang.org/grpc/status".Status
+// - "google.golang.org/api/googleapi".Error
+// If the error is not one of these types, Annotate behaves
+// like
+//   fmt.Errorf("%s: %v", msg, err)
+func Annotate(err error, msg string) error {
+	if err == nil {
+		panic("Annotate called with nil")
+	}
+	if s, ok := status.FromError(err); ok {
+		p := s.Proto()
+		p.Message = msg + ": " + p.Message
+		return status.ErrorProto(p)
+	}
+	if g, ok := err.(*googleapi.Error); ok {
+		g.Message = msg + ": " + g.Message
+		return g
+	}
+	return fmt.Errorf("%s: %v", msg, err)
+}
+
+// Annotatef uses format and args to format a string, then calls Annotate.
+func Annotatef(err error, format string, args ...interface{}) error {
+	return Annotate(err, fmt.Sprintf(format, args...))
+}