Consider redis.ErrNil as distribution.ErrBlobUnknown for Stat HGET

* Update redis.go

If the dgst key does not exist in the cache when calling HGET, `redis.String` will return an `ErrNil` which we need to translate into `distribution.ErrBlobUnknown` so that the error being returned can be properly handled. This will ensure that `SetDescriptor` is properly called from `cachedBlobStatter::Stat` for `repositoryScopedRedisBlobDescriptorService` which will update the redis cache and be considered as a Miss rather than an Error.

cc @manishtomar

* Update suite.go

Add unit test to ensure missing blobs for scoped repo properly return ErrBlobUnknown when HGET returns redis.ErrNil.

(cherry picked from commit dca6b9526a1d30dd218a9f321c4f84ecc4b5e62e)
Signed-off-by: Derek McGowan <derek@mcgstyle.net>

registry/storage/cache/cachecheck/suite.go

@@ -54,6 +54,10 @@ func checkBlobDescriptorCacheEmptyRepository(ctx context.Context, t *testing.T,
 		t.Fatalf("expected error checking for cache item with empty digest: %v", err)
 	}
 
+	if _, err := cache.Stat(ctx, "sha384:cba111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111"); err != distribution.ErrBlobUnknown {
+		t.Fatalf("expected unknown blob error with uncached repo: %v", err)
+	}
+
 	if _, err := cache.Stat(ctx, "sha384:abc111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111111"); err != distribution.ErrBlobUnknown {
 		t.Fatalf("expected unknown blob error with empty repo: %v", err)
 	}

registry/storage/cache/redis/redis.go

@@ -186,6 +186,10 @@ func (rsrbds *repositoryScopedRedisBlobDescriptorService) Stat(ctx context.Conte
 	// We allow a per repository mediatype, let's look it up here.
 	mediatype, err := redis.String(conn.Do("HGET", rsrbds.blobDescriptorHashKey(dgst), "mediatype"))
 	if err != nil {
+		if err == redis.ErrNil {
+			return distribution.Descriptor{}, distribution.ErrBlobUnknown
+		}
+
 		return distribution.Descriptor{}, err
 	}