Phase 6: media

Implements PLAN.md Phase 6: the media repository. Clients upload files,
reserve an mxc:// URI and upload to it later, download and ask for
thumbnails, all through the authenticated media endpoints of Matrix 1.11;
other servers download this server's media through the federation media
endpoints, and media uploaded elsewhere is fetched over federation on
first use and kept.

internal/mediaapi is the component. Content goes into a BlobStore named
by its SHA-256, so the same file uploaded twice is stored once and the
hash doubles as the ETag; the metadata that maps an mxc:// URI onto a
blob is in media_repository and media_thumbnails. The local-filesystem
store writes to a temporary file and renames it into place once hashed,
so nothing ever refers to half a file. A storage driver can bring its own
store through storage.BlobProvider, which is where PocketBase's
filesystem abstraction (local or S3) plugs in with Phase 10.

Reserved uploads belong to the user who made them, take one upload,
lapse after a day, and are capped per user; a download of one waits for
its content, woken in memory the moment it arrives, for as long as
timeout_ms allows.

Thumbnails come from a fixed set of configured sizes, so a thousand
requested sizes cannot mean a thousand resizes. They are made on first
request and kept, never enlarge, honour a JPEG's EXIF orientation, and
point at the original when it already fits. The pixel count is checked
from the header before anything is decoded, at most four resizes run at
once, and concurrent requests for the same thumbnail share one.

Remote media is fetched through the signed client: the multipart/mixed
answer of Matrix 1.11, with a fallback to the legacy download endpoint
for servers that answer M_UNRECOGNIZED, and Location redirects followed
by a dialer that refuses loopback, private and link-local addresses.
Concurrent requests share one fetch, detached from the request that
started it. With federation off nothing is fetched, and what was cached
before is still served.

Downloads carry a sandboxing CSP, nosniff, inline Content-Disposition
only for the types the spec lists as safe, and private immutable caching.
The unauthenticated legacy download endpoints are off by default behind
media.legacy_unauthenticated_downloads.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

server/internal/federationapi/media.go

@@ -0,0 +1,97 @@
+package federationapi
+
+import (
+	"io"
+	"mime/multipart"
+	"net/http"
+	"net/textproto"
+
+	"github.com/dariuszjaniak/warp/server/internal/httputil"
+	"github.com/dariuszjaniak/warp/server/internal/mediaapi"
+)
+
+// The media endpoints of Matrix 1.11: how another server downloads media
+// uploaded here. They serve local media only — a server asking for a
+// third server's media asks that server — and answer in multipart/mixed,
+// a JSON part for metadata (an empty object, for now) and then the
+// content.
+
+func (s *Server) registerMediaRoutes(authed func(string, authedHandler)) {
+	if s.media == nil {
+		return
+	}
+	const v1 = "/_matrix/federation/v1"
+	authed("GET "+v1+"/media/download/{mediaID}", s.handleMediaDownload)
+	authed("GET "+v1+"/media/thumbnail/{mediaID}", s.handleMediaThumbnail)
+}
+
+func (s *Server) handleMediaDownload(w http.ResponseWriter, r *http.Request, _ string, _ []byte) error {
+	wait, err := mediaapi.WaitParam(r)
+	if err != nil {
+		return err
+	}
+	content, err := s.media.Download(r.Context(), s.serverName, r.PathValue("mediaID"),
+		mediaapi.FetchOptions{Wait: wait, LocalOnly: true})
+	if err != nil {
+		return mediaapi.HTTPError(err)
+	}
+	defer func() { _ = content.Body.Close() }()
+	writeMultipart(w, r, content, mediaapi.ContentDisposition(content.ContentType, content.FileName))
+	return nil
+}
+
+func (s *Server) handleMediaThumbnail(w http.ResponseWriter, r *http.Request, _ string, _ []byte) error {
+	req, err := mediaapi.ThumbnailParams(r)
+	if err != nil {
+		return err
+	}
+	wait, err := mediaapi.WaitParam(r)
+	if err != nil {
+		return err
+	}
+	content, err := s.media.Thumbnail(r.Context(), s.serverName, r.PathValue("mediaID"), req,
+		mediaapi.FetchOptions{Wait: wait, LocalOnly: true})
+	if err != nil {
+		return mediaapi.HTTPError(err)
+	}
+	defer func() { _ = content.Body.Close() }()
+	writeMultipart(w, r, content,
+		mediaapi.ContentDisposition(content.ContentType, mediaapi.ThumbnailFileName(content.ContentType)))
+	return nil
+}
+
+// writeMultipart sends content the way the federation media endpoints do.
+//
+// The status goes out first, so a failure after it can only cut the
+// answer short — which the other server sees as a broken multipart body —
+// and is only logged.
+func writeMultipart(w http.ResponseWriter, r *http.Request, content mediaapi.Content, disposition string) {
+	mw := multipart.NewWriter(w)
+	w.Header().Set("Content-Type", "multipart/mixed; boundary="+mw.Boundary())
+	mediaapi.SetSecurityHeaders(w.Header())
+	w.WriteHeader(http.StatusOK)
+
+	err := func() error {
+		metadata, err := mw.CreatePart(textproto.MIMEHeader{"Content-Type": {"application/json"}})
+		if err != nil {
+			return err
+		}
+		if _, err := metadata.Write([]byte("{}")); err != nil {
+			return err
+		}
+		part, err := mw.CreatePart(textproto.MIMEHeader{
+			"Content-Type":        {content.ContentType},
+			"Content-Disposition": {disposition},
+		})
+		if err != nil {
+			return err
+		}
+		if _, err := io.Copy(part, content.Body); err != nil {
+			return err
+		}
+		return mw.Close()
+	}()
+	if err != nil {
+		httputil.LoggerFrom(r.Context()).Debug("sending media over federation failed", "error", err)
+	}
+}

server/internal/federationapi/server.go

@@ -16,6 +16,7 @@ import (
 	"github.com/dariuszjaniak/warp/server/internal/federation"
 	"github.com/dariuszjaniak/warp/server/internal/httputil"
 	"github.com/dariuszjaniak/warp/server/internal/matrix/id"
+	"github.com/dariuszjaniak/warp/server/internal/mediaapi"
 	"github.com/dariuszjaniak/warp/server/internal/roomserver"
 	"github.com/dariuszjaniak/warp/server/internal/storage"
 	"github.com/dariuszjaniak/warp/server/internal/syncapi"
@@ -55,8 +56,13 @@ type ServerOptions struct {
 	Ephemeral  EphemeralSink
 	Profiles   Profiles
 	Store      storage.FederationStore
-	Logger     *slog.Logger
-	Now        func() time.Time
+
+	// Media serves this server's media to others. Nil leaves the media
+	// endpoints unregistered.
+	Media *mediaapi.API
+
+	Logger *slog.Logger
+	Now    func() time.Time
 }
 
 // Server is the inbound Server-Server API.
@@ -68,6 +74,7 @@ type Server struct {
 	ephemeral  EphemeralSink
 	profiles   Profiles
 	store      storage.FederationStore
+	media      *mediaapi.API
 	log        *slog.Logger
 	now        func() time.Time
 }
@@ -94,6 +101,7 @@ func NewServer(opts ServerOptions) (*Server, error) {
 		ephemeral:  opts.Ephemeral,
 		profiles:   opts.Profiles,
 		store:      opts.Store,
+		media:      opts.Media,
 		log:        opts.Logger,
 		now:        opts.Now,
 	}, nil
@@ -142,6 +150,8 @@ func (s *Server) RegisterRoutes(r *httputil.Router) {
 	authed("GET "+v1+"/query/profile", s.handleQueryProfile)
 	authed("GET "+v1+"/publicRooms", s.handlePublicRoomsGet)
 	authed("POST "+v1+"/publicRooms", s.handlePublicRoomsPost)
+
+	s.registerMediaRoutes(authed)
 }
 
 // authenticated checks X-Matrix and the allow/deny lists before the