add support for redis cache backend

fixes #49

README.md

@@ -107,7 +107,11 @@ enabled using the `-cache` flag.  It supports the following values:
    variable be set.
  - azure URL (e.g. `azure://container-name/`) - will cache images on
    Azure Storage.  This requires `AZURESTORAGE_ACCOUNT_NAME` and
-   `AZURESTORAGE_ACCESS_KEY` environmental variables set.
+ - redis URL (e.g. `redis://hostname/`) - will cache images on
+   the specified redis host. The full URL syntax is defined by the [redis URI
+   registration](https://www.iana.org/assignments/uri-schemes/prov/redis).
+   Rather than specify password in the URI, use the `REDIS_PASSWORD`
+   environment variable.
 
 For example, to cache files on disk in the `/tmp/imageproxy` directory:
 

cmd/imageproxy/main.go

@@ -22,12 +22,15 @@ import (
 	"log"
 	"net/http"
 	"net/url"
+	"os"
 	"strings"
 
 	"github.com/PaulARoy/azurestoragecache"
 	"github.com/diegomarangoni/gcscache"
+	"github.com/garyburd/redigo/redis"
 	"github.com/gregjones/httpcache"
 	"github.com/gregjones/httpcache/diskcache"
+	rediscache "github.com/gregjones/httpcache/redis"
 	"github.com/peterbourgon/diskv"
 	"sourcegraph.com/sourcegraph/s3cache"
 	"willnorris.com/go/imageproxy"
@@ -132,6 +135,12 @@ func parseCache() (imageproxy.Cache, error) {
 		return gcscache.New(u.String()), nil
 	case "azure":
 		return azurestoragecache.New("", "", u.Host)
+	case "redis":
+		conn, err := redis.DialURL(u.String(), redis.DialPassword(os.Getenv("REDIS_PASSWORD")))
+		if err != nil {
+			return nil, err
+		}
+		return rediscache.NewWithClient(conn), nil
 	case "file":
 		fallthrough
 	default: