minstrel/internal/indexer/magnet.go

package indexer

import (
	"bytes"
	"errors"
	"net/http"
)

// MagnetLink is a download link that turned out to be a magnet. Returned as
// an error by a .torrent fetch so the grab hands the magnet to the download
// client instead. Public trackers do this routinely: Prowlarr's download link
// answers with a redirect to `magnet:?xt=…`, which Go's client would otherwise
// fail on as an unsupported scheme.
type MagnetLink struct{ URI string }

func (m *MagnetLink) Error() string { return "the download link is a magnet link" }

// AsMagnet says whether err is a magnet link, and which.
func AsMagnet(err error) (string, bool) {
	var m *MagnetLink
	if errors.As(err, &m) && m.URI != "" {
		return m.URI, true
	}
	return "", false
}

// StopAtMagnet returns a copy of c that stops at a redirect to a magnet URI
// and reports it as a *MagnetLink. Every other redirect behaves as c would.
func StopAtMagnet(c *http.Client) *http.Client {
	if c == nil {
		c = http.DefaultClient
	}
	out := *c
	next := c.CheckRedirect
	out.CheckRedirect = func(req *http.Request, via []*http.Request) error {
		if req.URL != nil && req.URL.Scheme == "magnet" {
			return &MagnetLink{URI: req.URL.String()}
		}
		if next != nil {
			return next(req, via)
		}
		if len(via) >= 10 {
			return errors.New("stopped after 10 redirects")
		}
		return nil
	}
	return &out
}

// MagnetBody is a response whose whole body is a magnet URI, which some
// indexers send with a 200 instead of a redirect. Nil otherwise.
func MagnetBody(body []byte) error {
	trimmed := bytes.TrimSpace(body)
	if bytes.HasPrefix(trimmed, []byte("magnet:")) && !bytes.ContainsAny(trimmed, " \n<") {
		return &MagnetLink{URI: string(trimmed)}
	}
	return nil
}