linkpreview pkg

util/linkpreview/linkpreview.go

@@ -0,0 +1,93 @@
+package linkpreview
+
+import (
+	"context"
+	"net/http"
+	"path/filepath"
+	"strings"
+
+	"github.com/otiai10/opengraph"
+)
+
+func New() LinkPreview {
+	return &linkPreview{}
+}
+
+type LinkType string
+
+const (
+	LinkTypeHtml       LinkType = "html"
+	LinkTypeImage      LinkType = "image"
+	LinkTypeVideo      LinkType = "video"
+	LinkTypeText       LinkType = "text"
+	LinkTypeUnexpected LinkType = "unexpected"
+
+	// read no more than 400 kb
+	maxBytesToRead = 400000
+)
+
+type LinkPreview interface {
+	Fetch(ctx context.Context, url string) (Info, error)
+}
+
+type Info struct {
+	Title       string
+	Description string
+	ImageUrl    string
+	Type        LinkType
+}
+
+type linkPreview struct{}
+
+func (l *linkPreview) Fetch(ctx context.Context, url string) (Info, error) {
+	rt := &proxyRoundTripper{RoundTripper: http.DefaultTransport}
+	client := &http.Client{Transport: rt}
+	og, err := opengraph.FetchWithContext(ctx, url, client)
+	if err != nil {
+		if resp := rt.lastResponse; resp != nil && resp.StatusCode == http.StatusOK {
+			return l.makeNonHtml(url, resp)
+		}
+		return Info{}, err
+	}
+	return l.convertOGToInfo(og), nil
+}
+
+func (l *linkPreview) convertOGToInfo(og *opengraph.OpenGraph) (i Info) {
+	i = Info{
+		Title:       og.Title,
+		Description: og.Description,
+		Type:        LinkTypeHtml,
+	}
+	if len(og.Image) != 0 {
+		i.ImageUrl = og.Image[0].URL
+	}
+	return
+}
+
+func (l *linkPreview) makeNonHtml(url string, resp *http.Response) (i Info, err error) {
+	ct := resp.Header.Get("Content-Type")
+	i.Title = filepath.Base(url)
+	if strings.HasPrefix(ct, "image/") {
+		i.Type = LinkTypeImage
+		i.ImageUrl = url
+	} else if strings.HasPrefix(ct, "text/") {
+		i.Type = LinkTypeText
+	} else {
+		i.Type = LinkTypeUnexpected
+	}
+	return
+}
+
+type proxyRoundTripper struct {
+	http.RoundTripper
+	lastResponse *http.Response
+}
+
+func (p *proxyRoundTripper) RoundTrip(req *http.Request) (*http.Response, error) {
+	resp, err := p.RoundTripper.RoundTrip(req)
+	if err == nil {
+		p.lastResponse = resp
+		resp.Body = http.MaxBytesReader(nil, resp.Body, maxBytesToRead)
+	}
+	return resp, err
+}