add custom cache for google cloud storage

This is a bit cleaner than the gcs cache that was vendored in, is
properly licensed, and uses Google's application default credentials,
which just magically works when run from AppEngine and GCE.

internal/gcscache/gcscache.go

@@ -0,0 +1,84 @@
+// Package s3cache provides an httpcache.Cache implementation that stores
+// cached values on Google Cloud Storage.
+package gcscache
+
+import (
+	"context"
+	"crypto/md5"
+	"encoding/hex"
+	"io"
+	"io/ioutil"
+	"log"
+	"path"
+
+	"cloud.google.com/go/storage"
+)
+
+var ctx = context.Background()
+
+type cache struct {
+	bucket *storage.BucketHandle
+	prefix string
+}
+
+func (c *cache) Get(key string) ([]byte, bool) {
+	r, err := c.object(key).NewReader(ctx)
+	if err != nil {
+		if err != storage.ErrObjectNotExist {
+			log.Printf("error reading from gcs: %v", err)
+		}
+		return nil, false
+	}
+	defer r.Close()
+
+	value, err := ioutil.ReadAll(r)
+	if err != nil {
+		log.Printf("error reading from gcs: %v", err)
+		return nil, false
+	}
+
+	return value, true
+}
+
+func (c *cache) Set(key string, value []byte) {
+	w := c.object(key).NewWriter(ctx)
+	if _, err := w.Write(value); err != nil {
+		log.Printf("error writing to gcs: %v", err)
+	}
+	if err := w.Close(); err != nil {
+		log.Printf("error closing gcs object writer: %v", err)
+	}
+}
+
+func (c *cache) Delete(key string) {
+	if err := c.object(key).Delete(ctx); err != nil {
+		log.Printf("error deleting gcs object: %v", err)
+	}
+}
+
+func (c *cache) object(key string) *storage.ObjectHandle {
+	name := path.Join(c.prefix, keyToFilename(key))
+	return c.bucket.Object(name)
+}
+
+func keyToFilename(key string) string {
+	h := md5.New()
+	io.WriteString(h, key)
+	return hex.EncodeToString(h.Sum(nil))
+}
+
+// New constructs a Cache storing files in the specified GCS bucket.  If prefix
+// is not empty, objects will be prefixed with that path. Credentials should
+// be specified using one of the mechanisms supported for Application Default
+// Credentials (see https://cloud.google.com/docs/authentication/production)
+func New(bucket, prefix string) (*cache, error) {
+	client, err := storage.NewClient(ctx)
+	if err != nil {
+		return nil, err
+	}
+
+	return &cache{
+		prefix: prefix,
+		bucket: client.Bucket(bucket),
+	}, nil
+}