Work around bug in bigcache. (#259)

Bigcache will use arbitrary amounts of memory if HardMaxCacheSize is less than the number of shards.

Work around this by setting max size to the number of shards if it's too small. This means we can't have a cache smaller than 4MB.

pkg/uhttp/gocache.go

@@ -207,6 +207,12 @@ func NewGoCache(ctx context.Context, cfg CacheConfig) (*GoCache, error) {
 	config := bigCache.DefaultConfig(time.Duration(cfg.TTL) * time.Second)
 	config.Verbose = cfg.LogDebug
 	config.Shards = 4
+	if cfg.MaxSize > 0 && cfg.MaxSize < config.Shards {
+		// BigCache's config.maximumShardSizeInBytes does integer division, which returns zero if there are more shards than megabytes.
+		// Zero means unlimited cache size on each shard, so max size is effectively ignored.
+		// Work around this bug by increasing the max size to the number of shards. (4, so 4MB)
+		cfg.MaxSize = config.Shards
+	}
 	config.HardMaxCacheSize = cfg.MaxSize // value in MB, 0 value means no size limit
 	cache, err := bigCache.New(ctx, config)
 	if err != nil {