Add gated cover byte fetcher

backend/internal/latest/cover.go

@@ -0,0 +1,180 @@
+package latest
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"io"
+	"mime"
+	"net"
+	"net/http"
+	"net/netip"
+	"net/url"
+	"strings"
+	"time"
+
+	"bookmarkmanager/backend/internal/store"
+)
+
+// CoverBytesFetcher retrieves one cover from its source URL. The caller owns
+// persistence; this seam keeps network policy independent from the store.
+type CoverBytesFetcher interface {
+	Fetch(ctx context.Context, sourceURL string) (body []byte, contentType string, err error)
+}
+
+// CoverResolver resolves a host before any connection is attempted. Tests
+// inject it to exercise hostile DNS results without touching the live network.
+type CoverResolver func(context.Context, string) ([]netip.Addr, error)
+
+// TLSCoverFetcher retrieves image bytes with the standard HTTPS client. Unlike
+// TLSFetcher, it does not need a browser fingerprint: cover hosts are public
+// CDNs and the response is accepted only after the destination gate passes.
+type TLSCoverFetcher struct {
+	client  *http.Client
+	resolve CoverResolver
+}
+
+var _ CoverBytesFetcher = (*TLSCoverFetcher)(nil)
+
+const coverRequestTimeout = 30 * time.Second
+
+var carrierGradeNAT = netip.MustParsePrefix("100.64.0.0/10")
+
+// NewCoverFetcher builds the production cover client with the real resolver.
+func NewCoverFetcher() *TLSCoverFetcher {
+	return NewCoverFetcherWithResolver(nil)
+}
+
+// NewCoverFetcherWithResolver builds a cover client using resolve, or the real
+// system resolver when resolve is nil.
+func NewCoverFetcherWithResolver(resolve CoverResolver) *TLSCoverFetcher {
+	if resolve == nil {
+		resolve = defaultCoverResolver
+	}
+	return newCoverFetcher(newCoverHTTPClient(resolve), resolve)
+}
+
+func newCoverFetcher(client *http.Client, resolve CoverResolver) *TLSCoverFetcher {
+	f := &TLSCoverFetcher{client: client, resolve: resolve}
+	client.CheckRedirect = func(req *http.Request, _ []*http.Request) error {
+		if err := f.validateURL(req.Context(), req.URL); err != nil {
+			return fmt.Errorf("redirect destination: %w", err)
+		}
+		return nil
+	}
+	return f
+}
+
+func defaultCoverResolver(ctx context.Context, host string) ([]netip.Addr, error) {
+	return net.DefaultResolver.LookupNetIP(ctx, "ip", host)
+}
+
+func newCoverHTTPClient(resolve CoverResolver) *http.Client {
+	base, ok := http.DefaultTransport.(*http.Transport)
+	if !ok {
+		base = &http.Transport{}
+	}
+	transport := base.Clone()
+	// A proxy would make the dial target the proxy rather than the cover host,
+	// defeating destination classification. Cover fetching is direct by design.
+	transport.Proxy = nil
+	dialer := &net.Dialer{}
+	transport.DialContext = func(ctx context.Context, network, address string) (net.Conn, error) {
+		host, port, err := net.SplitHostPort(address)
+		if err != nil {
+			return nil, fmt.Errorf("split cover address %q: %w", address, err)
+		}
+		addrs, err := resolveCoverHost(ctx, host, resolve)
+		if err != nil {
+			return nil, err
+		}
+		for _, addr := range addrs {
+			if !publicCoverAddress(addr) {
+				return nil, fmt.Errorf("cover host resolves to refused address %s", addr)
+			}
+			conn, err := dialer.DialContext(ctx, network, net.JoinHostPort(addr.String(), port))
+			if err == nil {
+				return conn, nil
+			}
+		}
+		return nil, fmt.Errorf("cover host %q has no reachable address", host)
+	}
+	return &http.Client{Transport: transport, Timeout: coverRequestTimeout}
+}
+
+func (f *TLSCoverFetcher) Fetch(ctx context.Context, sourceURL string) ([]byte, string, error) {
+	u, err := url.Parse(sourceURL)
+	if err != nil {
+		return nil, "", fmt.Errorf("parse cover URL: %w", err)
+	}
+	if err := f.validateURL(ctx, u); err != nil {
+		return nil, "", err
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, u.String(), nil)
+	if err != nil {
+		return nil, "", fmt.Errorf("build cover request: %w", err)
+	}
+	resp, err := f.client.Do(req)
+	if err != nil {
+		return nil, "", fmt.Errorf("fetch cover: %w", err)
+	}
+	defer resp.Body.Close()
+	if resp.StatusCode != http.StatusOK {
+		return nil, "", fmt.Errorf("fetch cover: status %d", resp.StatusCode)
+	}
+	contentType, _, err := mime.ParseMediaType(resp.Header.Get("Content-Type"))
+	if err != nil || !store.IsCoverContentType(contentType) {
+		return nil, "", fmt.Errorf("fetch cover: unsupported content type %q", resp.Header.Get("Content-Type"))
+	}
+	if resp.ContentLength > maxBodyBytes {
+		return nil, "", fmt.Errorf("fetch cover: response exceeds %d bytes", maxBodyBytes)
+	}
+	body, err := io.ReadAll(io.LimitReader(resp.Body, maxBodyBytes+1))
+	if err != nil {
+		return nil, "", fmt.Errorf("read cover: %w", err)
+	}
+	if len(body) > maxBodyBytes {
+		return nil, "", fmt.Errorf("fetch cover: response exceeds %d bytes", maxBodyBytes)
+	}
+	return body, contentType, nil
+}
+
+// This gate deliberately differs from fetchableSeriesURL: cover hosts are
+// site-independent CDNs, so a Site host allowlist would reject valid covers.
+func (f *TLSCoverFetcher) validateURL(ctx context.Context, u *url.URL) error {
+	if u == nil || u.Scheme != "https" || u.Host == "" || u.User != nil {
+		return errors.New("cover URL must use HTTPS without credentials")
+	}
+	host := u.Hostname()
+	if host == "" {
+		return errors.New("cover URL has no host")
+	}
+	addrs, err := resolveCoverHost(ctx, host, f.resolve)
+	if err != nil {
+		return fmt.Errorf("resolve cover host %q: %w", host, err)
+	}
+	if len(addrs) == 0 {
+		return fmt.Errorf("resolve cover host %q: no addresses", host)
+	}
+	for _, addr := range addrs {
+		if !publicCoverAddress(addr) {
+			return fmt.Errorf("cover host %q resolves to refused address %s", host, addr)
+		}
+	}
+	return nil
+}
+
+func resolveCoverHost(ctx context.Context, host string, resolve CoverResolver) ([]netip.Addr, error) {
+	if literal, err := netip.ParseAddr(host); err == nil {
+		return []netip.Addr{literal.Unmap()}, nil
+	}
+	return resolve(ctx, strings.TrimSuffix(host, "."))
+}
+
+func publicCoverAddress(addr netip.Addr) bool {
+	addr = addr.Unmap()
+	return addr.IsValid() && addr.IsGlobalUnicast() &&
+		!addr.IsLoopback() && !addr.IsPrivate() && !addr.IsLinkLocalUnicast() &&
+		!carrierGradeNAT.Contains(addr)
+}

backend/internal/latest/cover_fetch_test.go

@@ -0,0 +1,206 @@
+package latest
+
+import (
+	"bytes"
+	"context"
+	"crypto/tls"
+	"io"
+	"net"
+	"net/http"
+	"net/http/httptest"
+	"net/netip"
+	"testing"
+)
+
+func TestCoverFetcherFetchesPublicHTTPSImage(t *testing.T) {
+	server := httptest.NewTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.TLS == nil {
+			t.Fatal("cover request was not made over TLS")
+		}
+		w.Header().Set("Content-Type", "image/jpeg")
+		io.WriteString(w, "cover-bytes")
+	}))
+	defer server.Close()
+
+	transport := server.Client().Transport.(*http.Transport).Clone()
+	transport.TLSClientConfig = &tls.Config{InsecureSkipVerify: true} // test server certificate
+	transport.DialContext = func(ctx context.Context, network, _ string) (net.Conn, error) {
+		return (&net.Dialer{}).DialContext(ctx, network, server.Listener.Addr().String())
+	}
+	client := &http.Client{Transport: transport}
+	fetcher := newCoverFetcher(client, func(context.Context, string) ([]netip.Addr, error) {
+		return []netip.Addr{netip.MustParseAddr("198.51.100.10")}, nil
+	})
+
+	body, contentType, err := fetcher.Fetch(context.Background(), "https://cdn.example/cover.jpg")
+	if err != nil {
+		t.Fatalf("Fetch: %v", err)
+	}
+	if string(body) != "cover-bytes" || contentType != "image/jpeg" {
+		t.Fatalf("Fetch = (%q, %q), want (cover-bytes, image/jpeg)", body, contentType)
+	}
+}
+
+func TestNewCoverFetcherRechecksResolverBeforeConnection(t *testing.T) {
+	var requests int
+	server := httptest.NewTLSServer(http.HandlerFunc(func(http.ResponseWriter, *http.Request) {
+		requests++
+	}))
+	defer server.Close()
+	_, port, err := net.SplitHostPort(server.Listener.Addr().String())
+	if err != nil {
+		t.Fatalf("server address: %v", err)
+	}
+
+	resolves := 0
+	fetcher := NewCoverFetcherWithResolver(func(context.Context, string) ([]netip.Addr, error) {
+		resolves++
+		if resolves == 1 {
+			return []netip.Addr{netip.MustParseAddr("198.51.100.10")}, nil
+		}
+		return []netip.Addr{netip.MustParseAddr("127.0.0.1")}, nil
+	})
+	_, _, err = fetcher.Fetch(context.Background(), "https://cdn.example:"+port+"/cover.jpg")
+	if err == nil {
+		t.Fatal("Fetch accepted a destination that became private")
+	}
+	if resolves != 2 {
+		t.Fatalf("resolver calls = %d, want preflight and dial checks", resolves)
+	}
+	if requests != 0 {
+		t.Fatalf("requests = %d, want 0", requests)
+	}
+}
+
+type roundTripFunc func(*http.Request) (*http.Response, error)
+
+func (f roundTripFunc) RoundTrip(r *http.Request) (*http.Response, error) { return f(r) }
+
+func coverResponse(status int, contentType, location string, body []byte) *http.Response {
+	header := make(http.Header)
+	if contentType != "" {
+		header.Set("Content-Type", contentType)
+	}
+	if location != "" {
+		header.Set("Location", location)
+	}
+	return &http.Response{
+		StatusCode:    status,
+		Status:        http.StatusText(status),
+		Header:        header,
+		Body:          io.NopCloser(bytes.NewReader(body)),
+		ContentLength: int64(len(body)),
+	}
+}
+
+func TestCoverFetcherRefusesUnsafeDestinationsBeforeRequest(t *testing.T) {
+	var calls int
+	client := &http.Client{Transport: roundTripFunc(func(*http.Request) (*http.Response, error) {
+		calls++
+		return coverResponse(http.StatusOK, "image/jpeg", "", []byte("must not reach network")), nil
+	})}
+	resolve := func(_ context.Context, host string) ([]netip.Addr, error) {
+		switch host {
+		case "loopback.example":
+			return []netip.Addr{netip.MustParseAddr("127.0.0.1")}, nil
+		case "private.example":
+			return []netip.Addr{netip.MustParseAddr("10.0.0.1")}, nil
+		case "linklocal.example":
+			return []netip.Addr{netip.MustParseAddr("169.254.1.1")}, nil
+		case "unique-local.example":
+			return []netip.Addr{netip.MustParseAddr("fc00::1")}, nil
+		case "cgnat.example":
+			return []netip.Addr{netip.MustParseAddr("100.64.0.1")}, nil
+		default:
+			return []netip.Addr{netip.MustParseAddr("198.51.100.10")}, nil
+		}
+	}
+	fetcher := newCoverFetcher(client, resolve)
+
+	tests := []string{
+		"http://public.example/cover.jpg",
+		"https://127.0.0.1/cover.jpg",
+		"https://10.0.0.1/cover.jpg",
+		"https://169.254.1.1/cover.jpg",
+		"https://[fc00::1]/cover.jpg",
+		"https://100.64.0.1/cover.jpg",
+		"https://loopback.example/cover.jpg",
+		"https://private.example/cover.jpg",
+		"https://linklocal.example/cover.jpg",
+		"https://unique-local.example/cover.jpg",
+		"https://cgnat.example/cover.jpg",
+	}
+	for _, sourceURL := range tests {
+		t.Run(sourceURL, func(t *testing.T) {
+			calls = 0
+			if _, _, err := fetcher.Fetch(context.Background(), sourceURL); err == nil {
+				t.Fatal("Fetch accepted refused destination")
+			}
+			if calls != 0 {
+				t.Fatalf("network calls = %d, want 0", calls)
+			}
+		})
+	}
+}
+
+func TestCoverFetcherStopsRedirectIntoPrivateAddress(t *testing.T) {
+	var calls int
+	client := &http.Client{Transport: roundTripFunc(func(req *http.Request) (*http.Response, error) {
+		calls++
+		if req.URL.Hostname() != "cdn.example" {
+			t.Fatalf("redirect reached %s", req.URL)
+		}
+		return coverResponse(http.StatusFound, "", "https://internal.example/cover.jpg", nil), nil
+	})}
+	fetcher := newCoverFetcher(client, func(_ context.Context, host string) ([]netip.Addr, error) {
+		if host == "internal.example" {
+			return []netip.Addr{netip.MustParseAddr("192.168.1.1")}, nil
+		}
+		return []netip.Addr{netip.MustParseAddr("198.51.100.10")}, nil
+	})
+
+	if _, _, err := fetcher.Fetch(context.Background(), "https://cdn.example/cover.jpg"); err == nil {
+		t.Fatal("Fetch followed redirect into private address")
+	}
+	if calls != 1 {
+		t.Fatalf("network calls = %d, want only public first hop", calls)
+	}
+}
+
+func TestCoverFetcherRejectsOversizedBody(t *testing.T) {
+	var calls int
+	client := &http.Client{Transport: roundTripFunc(func(*http.Request) (*http.Response, error) {
+		calls++
+		response := coverResponse(http.StatusOK, "image/webp", "", bytes.Repeat([]byte("x"), maxBodyBytes+1))
+		response.ContentLength = -1
+		return response, nil
+	})}
+	fetcher := newCoverFetcher(client, func(context.Context, string) ([]netip.Addr, error) {
+		return []netip.Addr{netip.MustParseAddr("198.51.100.10")}, nil
+	})
+
+	if _, _, err := fetcher.Fetch(context.Background(), "https://cdn.example/large.webp"); err == nil {
+		t.Fatal("Fetch accepted oversized body")
+	}
+	if calls != 1 {
+		t.Fatalf("network calls = %d, want 1", calls)
+	}
+}
+
+func TestCoverFetcherRejectsNonImage(t *testing.T) {
+	var calls int
+	client := &http.Client{Transport: roundTripFunc(func(*http.Request) (*http.Response, error) {
+		calls++
+		return coverResponse(http.StatusOK, "text/html", "", []byte("challenge")), nil
+	})}
+	fetcher := newCoverFetcher(client, func(context.Context, string) ([]netip.Addr, error) {
+		return []netip.Addr{netip.MustParseAddr("198.51.100.10")}, nil
+	})
+
+	if _, _, err := fetcher.Fetch(context.Background(), "https://cdn.example/challenge"); err == nil {
+		t.Fatal("Fetch accepted non-image response")
+	}
+	if calls != 1 {
+		t.Fatalf("network calls = %d, want 1", calls)
+	}
+}

backend/internal/latest/poller.go

@@ -16,8 +16,8 @@ type Fetcher interface {
 	Get(ctx context.Context, url string) (body string, status int, err error)
 }
 
-// CoverFetcher retrieves one kagane cover through the browser-backed path.
-type CoverFetcher interface {
+// BrowserCoverFetcher retrieves one kagane cover through the browser-backed path.
+type BrowserCoverFetcher interface {
 	Image(ctx context.Context, imageID string) (body []byte, contentType string, err error)
 }
 
@@ -42,9 +42,11 @@ type Poller struct {
 	// cannot clear. Nil disables those sites entirely rather than falling back
 	// to Fetch, which would only ever retrieve a challenge page.
 	BrowserFetch Fetcher
-	// CoverFetch is optional: it prefetches kagane covers without changing the
-	// chapter poll's result when the browser or persistence fails.
-	CoverFetch      CoverFetcher
+	// CoverFetch is optional; failures are logged and never affect the chapter poll.
+	CoverFetch BrowserCoverFetcher
+	// CoverBytesFetch is optional; it handles non-kagane sources through the same
+	// failure-isolated prefetch path.
+	CoverBytesFetch CoverBytesFetcher
 	Now             func() time.Time // injected so tests can freeze it
 	Cooldown        time.Duration
 	BrowserCooldown time.Duration
@@ -56,7 +58,11 @@ type Poller struct {
 var browserBackedSites = []string{"kagane", "novelfull"}
 
 func (p *Poller) prefetchCover(ctx context.Context, sr store.Series) {
-	if sr.Site != "kagane" || p.CoverFetch == nil {
+	if sr.Cover == "" {
+		return
+	}
+	if sr.Site == "kagane" {
+		if p.CoverFetch == nil {
 			return
 		}
 		imageID, ok := store.KaganeImageID(sr.Cover)
@@ -79,6 +85,27 @@ func (p *Poller) prefetchCover(ctx context.Context, sr store.Series) {
 		if err := p.Store.PutKaganeCover(imageID, body, contentType); err != nil {
 			log.Printf("latest poll %q: persist cover: %v", sr.Key(), err)
 		}
+		return
+	}
+	if p.CoverBytesFetch == nil {
+		return
+	}
+	_, _, found, err := p.Store.GetCover(sr.Cover)
+	if err != nil {
+		log.Printf("latest poll %q: read cover: %v", sr.Key(), err)
+		return
+	}
+	if found {
+		return
+	}
+	body, contentType, err := p.CoverBytesFetch.Fetch(ctx, sr.Cover)
+	if err != nil {
+		log.Printf("latest poll %q: fetch cover: %v", sr.Key(), err)
+		return
+	}
+	if err := p.Store.PutCover(sr.Cover, body, contentType); err != nil {
+		log.Printf("latest poll %q: persist cover: %v", sr.Key(), err)
+	}
 }
 
 // fetcherFor returns the fetcher a site needs, or nil when the site cannot be

backend/internal/latest/poller_test.go

@@ -128,6 +128,30 @@ func (f *fakeCoverFetcher) callCount() int {
 	return len(f.calls)
 }
 
+type fakeBytesCoverFetcher struct {
+	mu          sync.Mutex
+	calls       []string
+	body        []byte
+	contentType string
+	err         error
+}
+
+func (f *fakeBytesCoverFetcher) Fetch(_ context.Context, sourceURL string) ([]byte, string, error) {
+	f.mu.Lock()
+	f.calls = append(f.calls, sourceURL)
+	f.mu.Unlock()
+	if f.err != nil {
+		return nil, "", f.err
+	}
+	return f.body, f.contentType, nil
+}
+
+func (f *fakeBytesCoverFetcher) callCount() int {
+	f.mu.Lock()
+	defer f.mu.Unlock()
+	return len(f.calls)
+}
+
 // newTestPoller wires a poller with a frozen clock and no stagger, so tests run
 // instantly and deterministically.
 func newTestPoller(t *testing.T, s *store.Store, f Fetcher, at time.Time) *Poller {
@@ -144,6 +168,63 @@ func newTestPoller(t *testing.T, s *store.Store, f Fetcher, at time.Time) *Polle
 	}
 }
 
+func TestRunOncePrefetchesPublicCover(t *testing.T) {
+	s, _ := newTestStore(t)
+	const (
+		key       = "asura:chronicles-of-the-demon-faction-f886a8af"
+		seriesURL = "https://asurascans.com/comics/chronicles-of-the-demon-faction-f886a8af"
+		coverURL  = "https://cdn.example/covers/chronicles.jpg"
+	)
+	if _, err := s.Upsert(s.OwnerID(), store.Bookmark{
+		Key: key, Site: "asura", SeriesID: "chronicles-of-the-demon-faction-f886a8af",
+		SeriesURL: seriesURL, Cover: coverURL, UpdatedAt: 1000,
+	}); err != nil {
+		t.Fatalf("seed: %v", err)
+	}
+	covers := &fakeBytesCoverFetcher{body: []byte("cover-bytes"), contentType: "image/jpeg"}
+	p := &Poller{
+		Store: s, Fetch: &fakeFetcher{body: asuraSeriesFixture, status: 200}, CoverBytesFetch: covers,
+		Now: func() time.Time { return time.UnixMilli(5_000_000) }, Cooldown: time.Hour, Batch: 10,
+	}
+	p.runOnce(context.Background())
+
+	if got := covers.callCount(); got != 1 {
+		t.Fatalf("cover fetch calls = %d, want 1", got)
+	}
+	body, contentType, found, err := s.GetCover(coverURL)
+	if err != nil || !found {
+		t.Fatalf("GetCover: %v found=%v", err, found)
+	}
+	if string(body) != "cover-bytes" || contentType != "image/jpeg" {
+		t.Fatalf("stored cover = (%q, %q), want (cover-bytes, image/jpeg)", body, contentType)
+	}
+}
+
+func TestRunOnceDoesNotStoreNonImagePublicCover(t *testing.T) {
+	s, _ := newTestStore(t)
+	const (
+		key       = "asura:non-image-cover"
+		seriesURL = "https://asurascans.com/comics/non-image-cover"
+		coverURL  = "https://cdn.example/covers/challenge"
+	)
+	if _, err := s.Upsert(s.OwnerID(), store.Bookmark{
+		Key: key, Site: "asura", SeriesID: "non-image-cover", SeriesURL: seriesURL, Cover: coverURL,
+		UpdatedAt: 1000,
+	}); err != nil {
+		t.Fatalf("seed: %v", err)
+	}
+	p := &Poller{
+		Store: s, Fetch: &fakeFetcher{body: asuraSeriesFixture, status: 200},
+		CoverBytesFetch: &fakeBytesCoverFetcher{body: []byte("challenge"), contentType: "text/html"},
+		Now:             func() time.Time { return time.UnixMilli(5_000_000) }, Cooldown: time.Hour, Batch: 10,
+	}
+	p.runOnce(context.Background())
+
+	if _, _, found, err := s.GetCover(coverURL); err != nil || found {
+		t.Fatalf("non-image cover = found %v, err %v; want missing", found, err)
+	}
+}
+
 func TestRunOnceRecordsLatestChapter(t *testing.T) {
 	s, _ := newTestStore(t)
 	const url = "https://asurascans.com/comics/chronicles-of-the-demon-faction-f886a8af"
@@ -748,24 +829,30 @@ func TestRunOnceWithoutCoverFetcherStillPollsKagane(t *testing.T) {
 	}
 }
 
-func TestRunOnceDoesNotPrefetchNonKaganeCover(t *testing.T) {
+func TestRunOnceRoutesNonKaganeCoverToPublicFetcher(t *testing.T) {
 	s, _ := newTestStore(t)
 	const key = "asura:solo"
+	const coverURL = "https://asurascans.com/covers/solo.jpg"
 	if _, err := s.Upsert(s.OwnerID(), store.Bookmark{
 		Key: key, Site: "asura", SeriesID: "solo", SeriesURL: "https://asurascans.com/comics/solo",
-		Cover: "https://asurascans.com/covers/solo.jpg", UpdatedAt: 1000,
+		Cover: coverURL, UpdatedAt: 1000,
 	}); err != nil {
 		t.Fatalf("seed: %v", err)
 	}
-	covers := &fakeCoverFetcher{body: []byte("must not be fetched"), contentType: "image/webp"}
+	browserCovers := &fakeCoverFetcher{body: []byte("must not be fetched"), contentType: "image/webp"}
+	publicCovers := &fakeBytesCoverFetcher{body: []byte("public cover"), contentType: "image/webp"}
 	p := &Poller{
-		Store: s, Fetch: &fakeFetcher{body: asuraSeriesFixture, status: 200}, CoverFetch: covers,
+		Store: s, Fetch: &fakeFetcher{body: asuraSeriesFixture, status: 200}, CoverFetch: browserCovers,
+		CoverBytesFetch: publicCovers,
 		Now:             func() time.Time { return time.UnixMilli(5_000_000) }, Cooldown: time.Hour, BrowserCooldown: time.Hour, Batch: 10,
 	}
 	p.runOnce(context.Background())
 
-	if got := covers.callCount(); got != 0 {
-		t.Fatalf("cover fetch calls for asura = %d, want 0", got)
+	if got := publicCovers.callCount(); got != 1 {
+		t.Fatalf("public cover fetch calls = %d, want 1", got)
+	}
+	if got := browserCovers.callCount(); got != 0 {
+		t.Fatalf("browser cover fetch calls for asura = %d, want 0", got)
 	}
 }
 

backend/internal/store/store.go

@@ -156,9 +156,8 @@ func KaganeImageID(cover string) (string, bool) {
 	return m[1], true
 }
 
-// IsKaganeCoverContentType reports whether a fetched response is safe to store
-// and serve as a cover.
-func IsKaganeCoverContentType(contentType string) bool {
+// IsCoverContentType reports whether a fetched response is safe to store and serve.
+func IsCoverContentType(contentType string) bool {
 	switch contentType {
 	case "image/webp", "image/jpeg", "image/png", "image/avif", "image/gif":
 		return true
@@ -609,7 +608,7 @@ func (s *Store) getCover(sourceURL string) ([]byte, string, bool, error) {
 }
 
 func (s *Store) putCover(sourceURL string, body []byte, contentType string) error {
-	if !IsKaganeCoverContentType(contentType) {
+	if !IsCoverContentType(contentType) {
 		return fmt.Errorf("put cover %q: unsupported content type %q", sourceURL, contentType)
 	}
 	address := coverSourceAddress(sourceURL)
@@ -647,6 +646,18 @@ func (s *Store) putCover(sourceURL string, body []byte, contentType string) erro
 	return nil
 }
 
+// GetCover returns the immutable object addressed by its source URL. Missing
+// files are reported with ok=false so callers can retry acquisition later.
+func (s *Store) GetCover(sourceURL string) ([]byte, string, bool, error) {
+	return s.getCover(sourceURL)
+}
+
+// PutCover persists bytes under the source URL's content address. A later
+// write for the same URL cannot replace the immutable object.
+func (s *Store) PutCover(sourceURL string, body []byte, contentType string) error {
+	return s.putCover(sourceURL, body, contentType)
+}
+
 // GetKaganeCover returns one persisted cover. Missing covers are reported with
 // ok=false rather than as an error so the web handler can fetch them once.
 func (s *Store) GetKaganeCover(imageID string) ([]byte, string, bool, error) {

backend/internal/store/store_test.go

@@ -1352,3 +1352,25 @@ func TestKaganeCoverIsContentAddressedOnFilesystem(t *testing.T) {
 		t.Fatalf("covers still has body column")
 	}
 }
+
+func TestCoverStoreAcceptsAnySourceURL(t *testing.T) {
+	s := newTestStore(t)
+	const sourceURL = "https://cdn.example/covers/series.jpg"
+	want := []byte("cover-bytes")
+	if err := s.PutCover(sourceURL, want, "image/jpeg"); err != nil {
+		t.Fatalf("PutCover: %v", err)
+	}
+	got, contentType, ok, err := s.GetCover(sourceURL)
+	if err != nil {
+		t.Fatalf("GetCover: %v", err)
+	}
+	if !ok || !bytes.Equal(got, want) || contentType != "image/jpeg" {
+		t.Fatalf("GetCover = (%q, %q, %v), want (%q, image/jpeg, true)", got, contentType, ok, want)
+	}
+	if err := s.PutCover("https://cdn.example/not-image", []byte("html"), "text/html"); err == nil {
+		t.Fatal("PutCover accepted a non-image")
+	}
+	if _, _, ok, err := s.GetCover("https://cdn.example/not-image"); err != nil || ok {
+		t.Fatalf("rejected cover = found %v, err %v; want missing", ok, err)
+	}
+}

backend/internal/web/cover.go

@@ -65,7 +65,7 @@ func (h *Handler) kaganeCover(w http.ResponseWriter, r *http.Request) {
 		http.NotFound(w, r)
 		return
 	}
-	if !store.IsKaganeCoverContentType(contentType) {
+	if !store.IsCoverContentType(contentType) {
 		log.Printf("kagane cover %s: unexpected content type %q", id, contentType)
 		http.NotFound(w, r)
 		return

backend/main.go

@@ -343,8 +343,8 @@ func main() {
 
 // newLatestPoller wires the configured cooldowns and fetchers into the poller.
 func newLatestPoller(s *store.Store, cfg LatestPoll, fetch, browser latest.Fetcher) *latest.Poller {
-	var covers latest.CoverFetcher
-	if f, ok := browser.(latest.CoverFetcher); ok {
+	var covers latest.BrowserCoverFetcher
+	if f, ok := browser.(latest.BrowserCoverFetcher); ok {
 		covers = f
 	}
 	return &latest.Poller{
@@ -352,6 +352,7 @@ func newLatestPoller(s *store.Store, cfg LatestPoll, fetch, browser latest.Fetch
 		Fetch:           fetch,
 		BrowserFetch:    browser,
 		CoverFetch:      covers,
+		CoverBytesFetch: latest.NewCoverFetcher(),
 		Now:             time.Now,
 		Cooldown:        cfg.Cooldown,
 		BrowserCooldown: cfg.BrowserCooldown,