fix: handle non-existent keys properly in Store.SMembers (#111)

I saw some unexpected traces when debugging some custom instrumentation
I'm adding. Basically, the service was not looking for claims in IPNI or
legacy systems even though there were no results in the cache.

Turns out that the `SMEMBERS` command doesn't return `(nil)` when the
key doesn't exist, as others commands do. Instead, it returns an empty
set (see the docs for
[SMEMBERS](https://redis.io/docs/latest/commands/smembers/#:~:text=This%20has%20the%20same%20effect%20as%20running%20SINTER%20with%20one%20argument%20key.)
and then
[SINTER](https://redis.io/docs/latest/commands/sinter/#:~:text=Keys%20that%20do%20not%20exist%20are%20considered%20to%20be%20empty%20sets.)).
Therefore, the store was returning an empty slice instead of
`ErrKeyNotFound`.

I added an explicit existence check in `SMembers` when the command
returns an empty set, assuming that it may be interesting to
differentiate between a non-existent and an empty set.

pkg/redis/redisstore.go

@@ -91,14 +91,19 @@ func (rs *Store[Key, Value]) SetExpirable(ctx context.Context, key Key, expires
 }
 
 // Members returns all deserialized set values from redis.
+// If the key does not exist, it returns ErrKeyNotFound.
 func (rs *Store[Key, Value]) Members(ctx context.Context, key Key) ([]Value, error) {
 	data, err := rs.client.SMembers(ctx, rs.keyString(key)).Result()
 	if err != nil {
-		if err == redis.Nil {
-			return nil, types.ErrKeyNotFound
-		}
 		return nil, fmt.Errorf("getting set members: %w", err)
 	}
+
+	// as opposed to other commands, SMembers doesn't return redis.Nil when the key doesn't exist, but an empty set
+	// this implementation assumes there is no need to differentiate between a non-existing key and an empty set
+	if len(data) == 0 {
+		return nil, types.ErrKeyNotFound
+	}
+
 	var values []Value
 	for _, d := range data {
 		v, err := rs.fromRedis(d)

pkg/redis/redisstore_test.go

@@ -334,7 +334,8 @@ func (m *MockRedis) SMembers(ctx context.Context, key string) *goredis.StringSli
 	}
 	val, ok := m.data[key]
 	if !ok {
-		cmd.SetErr(goredis.Nil)
+		// SMembers returns an empty set for non-existing keys
+		cmd.SetVal([]string{})
 	} else {
 		values := slices.Collect(maps.Keys(val.data))
 		cmd.SetVal(values)