datastore: add EnableKeyConversion for compatibility with Cloud Datas…

…tore encoded keys (#192)

Adds compatibility with encoded keys generated by the Cloud Datastore package's (cloud.google.com/go/datastore) Key.Encode function.

This package, and the Cloud Datastore package, both use b64-encoded protobufs as the key encoding format, however the protobufs are different, so care must be taken to try and decode to/from both proto formats.

Co-authored-by: Luke <lukemc@google.com>
Co-authored-by: Chris Broadfoot <cbro@golang.org>

README.md

@@ -71,3 +71,30 @@ A few APIs were cleaned up, and there are some differences:
   [blobstore package](https://google.golang.org/appengine/blobstore).
 * `appengine/socket` is not required on App Engine flexible environment / Managed VMs.
   Use the standard `net` package instead.
+
+## Key Encode/Decode compatibiltiy to help with datastore library migrations
+
+Key compatibility updates have been added to help customers transition from google.golang.org/appengine/datastore to cloud.google.com/go/datastore.
+The `EnableKeyConversion` enables automatic conversion from a key encoded with cloud.google.com/go/datastore to google.golang.org/appengine/datastore key type.
+
+### Enabling key conversion
+
+Enable key conversion by calling `EnableKeyConversion(ctx)` in the `/_ah/startup` handler for basic and manual scaling or any handler in automatic scaling.
+
+#### 1. Basic or manual scaling
+
+This startup handler will enable key conversion for all handlers in the service.
+
+```
+http.HandleFunc("/_ah/start", func(w http.ResponseWriter, r *http.Request) {
+    datastore.EnableKeyConversion(appengine.NewContext(r))
+})
+```
+
+#### 2. Automatic scaling
+
+`/_ah/start` is not supported for automatic scaling and `/_ah/warmup` is not guaranteed to run, so you must call `datastore.EnableKeyConversion(appengine.NewContext(r))`
+before you use code that needs key conversion.
+
+You may want to add this to each of your handlers, or introduce middleware where it's called.
+`EnableKeyConversion` is safe for concurrent use. Any call to it after the first is ignored.

datastore/internal/cloudkey/cloudkey.go

@@ -0,0 +1,120 @@
+// Copyright 2019 Google Inc. All rights reserved.
+// Use of this source code is governed by the Apache 2.0
+// license that can be found in the LICENSE file.
+
+// Package cloudpb is a subset of types and functions, copied from cloud.google.com/go/datastore.
+//
+// They are copied here to provide compatibility to decode keys generated by the cloud.google.com/go/datastore package.
+package cloudkey
+
+import (
+	"encoding/base64"
+	"errors"
+	"strings"
+
+	"github.com/golang/protobuf/proto"
+	cloudpb "google.golang.org/appengine/datastore/internal/cloudpb"
+)
+
+/////////////////////////////////////////////////////////////////////
+// Code below is copied from https://github.com/googleapis/google-cloud-go/blob/master/datastore/datastore.go
+/////////////////////////////////////////////////////////////////////
+
+var (
+	// ErrInvalidKey is returned when an invalid key is presented.
+	ErrInvalidKey = errors.New("datastore: invalid key")
+)
+
+/////////////////////////////////////////////////////////////////////
+// Code below is copied from https://github.com/googleapis/google-cloud-go/blob/master/datastore/key.go
+/////////////////////////////////////////////////////////////////////
+
+// Key represents the datastore key for a stored entity.
+type Key struct {
+	// Kind cannot be empty.
+	Kind string
+	// Either ID or Name must be zero for the Key to be valid.
+	// If both are zero, the Key is incomplete.
+	ID   int64
+	Name string
+	// Parent must either be a complete Key or nil.
+	Parent *Key
+
+	// Namespace provides the ability to partition your data for multiple
+	// tenants. In most cases, it is not necessary to specify a namespace.
+	// See docs on datastore multitenancy for details:
+	// https://cloud.google.com/datastore/docs/concepts/multitenancy
+	Namespace string
+}
+
+// DecodeKey decodes a key from the opaque representation returned by Encode.
+func DecodeKey(encoded string) (*Key, error) {
+	// Re-add padding.
+	if m := len(encoded) % 4; m != 0 {
+		encoded += strings.Repeat("=", 4-m)
+	}
+
+	b, err := base64.URLEncoding.DecodeString(encoded)
+	if err != nil {
+		return nil, err
+	}
+
+	pKey := new(cloudpb.Key)
+	if err := proto.Unmarshal(b, pKey); err != nil {
+		return nil, err
+	}
+	return protoToKey(pKey)
+}
+
+// valid returns whether the key is valid.
+func (k *Key) valid() bool {
+	if k == nil {
+		return false
+	}
+	for ; k != nil; k = k.Parent {
+		if k.Kind == "" {
+			return false
+		}
+		if k.Name != "" && k.ID != 0 {
+			return false
+		}
+		if k.Parent != nil {
+			if k.Parent.Incomplete() {
+				return false
+			}
+			if k.Parent.Namespace != k.Namespace {
+				return false
+			}
+		}
+	}
+	return true
+}
+
+// Incomplete reports whether the key does not refer to a stored entity.
+func (k *Key) Incomplete() bool {
+	return k.Name == "" && k.ID == 0
+}
+
+// protoToKey decodes a protocol buffer representation of a key into an
+// equivalent *Key object. If the key is invalid, protoToKey will return the
+// invalid key along with ErrInvalidKey.
+func protoToKey(p *cloudpb.Key) (*Key, error) {
+	var key *Key
+	var namespace string
+	if partition := p.PartitionId; partition != nil {
+		namespace = partition.NamespaceId
+	}
+	for _, el := range p.Path {
+		key = &Key{
+			Namespace: namespace,
+			Kind:      el.Kind,
+			ID:        el.GetId(),
+			Name:      el.GetName(),
+			Parent:    key,
+		}
+	}
+	if !key.valid() { // Also detects key == nil.
+		return key, ErrInvalidKey
+	}
+	return key, nil
+}