Is it possible to suppress the debug logs? Also, I'd like to run this as a background job, maybe via some go or python or javascript async code.

In this line, it makes more sense for it to be fmt.Errorf("Expected infohash to be %x but got %x", infoHash, res.InfoHash), where the expected hash is the original hash as provided by the requesting client.

// Serialize serializes the handshake to a buffer
func (h *Handshake) Serialize() []byte {
	buf := make([]byte, len(h.Pstr)+49)
...
}

Len(h.Ptr) is always >=1, the string being 19 bytes, so it will never be a problem but reading your article I would just have expected 50. 1 + 19 + 20 + 20. This allocates 19 + 49 bytes.

I just cloned the project and tried out downloading with the torrent file within project folder and found the it is not working. I found out the the the tracker response from the get request contains a failure message， “torrent not found".

Is there a way to save file to storage when downloading?

https://github.com/veggiedefender/torrent-client/blob/master/torrentfile/tracker_test.go#L28

Only 14 bytes are present, but the peer id is 20 bytes.