Resize favicons before storing them

Some websites are using images of O(10kB) when not )O(100kB) for their
favicons. As miniflux only displays them with a 16x16 resolution, let's do our
best to resize them before storing them in the database. This should make
miniflux consume less bandwidth when serving pages, for the joy of mobile users
on a small data plan.

Of course, images that already are 16x16 aren't resized.

go.mod

@@ -13,6 +13,7 @@ require (
 	github.com/prometheus/client_golang v1.20.5
 	github.com/tdewolff/minify/v2 v2.21.2
 	golang.org/x/crypto v0.31.0
+	golang.org/x/image v0.23.0
 	golang.org/x/net v0.32.0
 	golang.org/x/oauth2 v0.24.0
 	golang.org/x/term v0.27.0

go.sum

@@ -70,6 +70,8 @@ golang.org/x/crypto v0.0.0-20190308221718-c2843e01d9a2/go.mod h1:djNgcEr1/C05ACk
 golang.org/x/crypto v0.0.0-20210921155107-089bfa567519/go.mod h1:GvvjBRRGRdwPK5ydBHafDWAxML/pGHZbMvKqRZ5+Abc=
 golang.org/x/crypto v0.31.0 h1:ihbySMvVjLAeSH1IbfcRTkD/iNscyz8rGzjF/E5hV6U=
 golang.org/x/crypto v0.31.0/go.mod h1:kDsLvtWBEx7MV9tJOj9bnXsPbxwJQ6csT/x4KIN4Ssk=
+golang.org/x/image v0.23.0 h1:HseQ7c2OpPKTPVzNjG5fwJsOTCiiwS4QdsYi5XU6H68=
+golang.org/x/image v0.23.0/go.mod h1:wJJBTdLfCCf3tiHa1fNxpZmUI4mmoZvwMCPP0ddoNKY=
 golang.org/x/mod v0.6.0-dev.0.20220419223038-86c51ed26bb4/go.mod h1:jJ57K6gSWd91VN4djpZkiMVwK6gcyfeH4XE8wZrZaV4=
 golang.org/x/mod v0.8.0/go.mod h1:iBbtSCu2XBx23ZKBPSOrRkjjQPZFPuis4dIYUhu/chs=
 golang.org/x/net v0.0.0-20190620200207-3b0461eec859/go.mod h1:z5CRVTTTmAJ677TzLLGU+0bjPO0LkuOLi4/5GtJWs/s=

internal/reader/icon/finder.go

@@ -4,12 +4,18 @@
 package icon // import "miniflux.app/v2/internal/reader/icon"
 
 import (
+	"bytes"
 	"encoding/base64"
 	"fmt"
+	"image"
+	"image/gif"
+	"image/jpeg"
+	"image/png"
 	"io"
 	"log/slog"
 	"net/url"
 	"regexp"
+	"slices"
 	"strings"
 
 	"miniflux.app/v2/internal/config"
@@ -19,6 +25,7 @@ import (
 	"miniflux.app/v2/internal/urllib"
 
 	"github.com/PuerkitoBio/goquery"
+	"golang.org/x/image/draw"
 	"golang.org/x/net/html/charset"
 )
 
@@ -180,9 +187,59 @@ func (f *IconFinder) DownloadIcon(iconURL string) (*model.Icon, error) {
 		Content:  responseBody,
 	}
 
+	icon = resizeIcon(icon)
+
 	return icon, nil
 }
 
+func resizeIcon(icon *model.Icon) *model.Icon {
+	r := bytes.NewReader(icon.Content)
+
+	if !slices.Contains([]string{"image/jpeg", "image/png", "image/gif"}, icon.MimeType) {
+		slog.Info("icon isn't a png/gif/jpeg/ico, can't resize", slog.String("mimetype", icon.MimeType))
+		return icon
+	}
+
+	// Don't resize icons that we can't decode, or that already have the right size.
+	config, _, err := image.DecodeConfig(r)
+	if err != nil {
+		slog.Warn("unable to decode the metadata of the icon", slog.Any("error", err))
+		return icon
+	}
+	if config.Height <= 16 && config.Width <= 16 {
+		slog.Debug("icon don't need to be rescaled", slog.Int("height", config.Height), slog.Int("width", config.Width))
+		return icon
+	}
+
+	r.Seek(0, io.SeekStart)
+
+	var src image.Image
+	switch icon.MimeType {
+	case "image/jpeg":
+		src, err = jpeg.Decode(r)
+	case "image/png":
+		src, err = png.Decode(r)
+	case "image/gif":
+		src, err = gif.Decode(r)
+	}
+	if err != nil {
+		slog.Warn("unable to decode the icon", slog.Any("error", err))
+		return icon
+	}
+
+	dst := image.NewRGBA(image.Rect(0, 0, 16, 16))
+	draw.BiLinear.Scale(dst, dst.Rect, src, src.Bounds(), draw.Over, nil)
+
+	var b bytes.Buffer
+	if err = png.Encode(io.Writer(&b), dst); err != nil {
+		slog.Warn("unable to encode the new icon", slog.Any("error", err))
+	}
+
+	icon.Content = b.Bytes()
+	icon.MimeType = "image/png"
+	return icon
+}
+
 func findIconURLsFromHTMLDocument(body io.Reader, contentType string) ([]string, error) {
 	queries := []string{
 		"link[rel='icon' i]",

internal/reader/icon/finder_test.go

@@ -4,8 +4,13 @@
 package icon // import "miniflux.app/v2/internal/reader/icon"
 
 import (
+	"bytes"
+	"encoding/base64"
+	"image"
 	"strings"
 	"testing"
+
+	"miniflux.app/v2/internal/model"
 )
 
 func TestParseImageDataURL(t *testing.T) {
@@ -125,3 +130,52 @@ func TestParseDocumentWithWhitespaceIconURL(t *testing.T) {
 		t.Errorf(`Invalid icon URL, got %q`, iconURLs[0])
 	}
 }
+
+func TestResizeIconSmallGif(t *testing.T) {
+	data, err := base64.StdEncoding.DecodeString("R0lGODlhAQABAAAAACH5BAEKAAEALAAAAAABAAEAAAICTAEAOw==")
+	if err != nil {
+		t.Fatal(err)
+	}
+	icon := model.Icon{
+		Content:  data,
+		MimeType: "image/gif",
+	}
+	if !bytes.Equal(icon.Content, resizeIcon(&icon).Content) {
+		t.Fatalf("Converted gif smaller than 16x16")
+	}
+}
+
+func TestResizeIconPng(t *testing.T) {
+	data, err := base64.StdEncoding.DecodeString("iVBORw0KGgoAAAANSUhEUgAAABEAAAARCAYAAAA7bUf6AAAAHElEQVR42mP8z/C/noFCwDhqyKgho4aMGkIlQwBrHSpf28Yx+gAAAABJRU5ErkJggg==")
+	if err != nil {
+		t.Fatal(err)
+	}
+	icon := model.Icon{
+		Content:  data,
+		MimeType: "image/png",
+	}
+	resizedIcon := resizeIcon(&icon)
+
+	if bytes.Equal(data, resizedIcon.Content) {
+		t.Fatalf("Didn't convert png of 17x17")
+	}
+
+	config, _, err := image.DecodeConfig(bytes.NewReader(resizedIcon.Content))
+	if err != nil {
+		t.Fatalf("Couln't decode resulting png: %v", err)
+	}
+
+	if config.Height != 16 || config.Width != 16 {
+		t.Fatalf("Was expecting an image of 16x16, got %dx%d", config.Width, config.Height)
+	}
+}
+
+func TestResizeInvalidImage(t *testing.T) {
+	icon := model.Icon{
+		Content:  []byte("invalid data"),
+		MimeType: "image/gif",
+	}
+	if !bytes.Equal(icon.Content, resizeIcon(&icon).Content) {
+		t.Fatalf("Tried to convert an invalid image")
+	}
+}