Decouple redis dependency from blob descriptor cache

Ensure that clients can use the blob descriptor cache provider without needing
the redis package.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/storage/cache/redis/redis_test.go

@@ -0,0 +1,51 @@
+package redis
+
+import (
+	"flag"
+	"os"
+	"testing"
+	"time"
+
+	"github.com/docker/distribution/registry/storage/cache"
+	"github.com/garyburd/redigo/redis"
+)
+
+var redisAddr string
+
+func init() {
+	flag.StringVar(&redisAddr, "test.registry.storage.cache.redis.addr", "", "configure the address of a test instance of redis")
+}
+
+// TestRedisLayerInfoCache exercises a live redis instance using the cache
+// implementation.
+func TestRedisBlobDescriptorCacheProvider(t *testing.T) {
+	if redisAddr == "" {
+		// fallback to an environement variable
+		redisAddr = os.Getenv("TEST_REGISTRY_STORAGE_CACHE_REDIS_ADDR")
+	}
+
+	if redisAddr == "" {
+		// skip if still not set
+		t.Skip("please set -registry.storage.cache.redis to test layer info cache against redis")
+	}
+
+	pool := &redis.Pool{
+		Dial: func() (redis.Conn, error) {
+			return redis.Dial("tcp", redisAddr)
+		},
+		MaxIdle:   1,
+		MaxActive: 2,
+		TestOnBorrow: func(c redis.Conn, t time.Time) error {
+			_, err := c.Do("PING")
+			return err
+		},
+		Wait: false, // if a connection is not avialable, proceed without cache.
+	}
+
+	// Clear the database
+	if _, err := pool.Get().Do("FLUSHDB"); err != nil {
+		t.Fatalf("unexpected error flushing redis db: %v", err)
+	}
+
+	cache.CheckBlobDescriptorCache(t, NewRedisBlobDescriptorCacheProvider(pool))
+}