Support `HEAD` on `/download`, per MSC4120 (#578)

* Support `HEAD` on `/download`, per MSC4120

https://github.com/matrix-org/matrix-spec-proposals/pull/4120

* Changelog

CHANGELOG.md

@@ -10,6 +10,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
 ### Added
 
 * New datastore option to ignore Redis cache when downloading media served by a `publicBaseUrl`. This can help ensure more requests get redirected to the CDN.
+* `HEAD /download` is now supported, as per [MSC4120](https://github.com/matrix-org/matrix-spec-proposals/pull/4120).
 
 ### Fixed
 

api/r0/download.go

@@ -53,6 +53,13 @@ func DownloadMedia(r *http.Request, rctx rcontext.RequestContext, user _apimeta.
 		return _responses.BadRequest("timeout_ms does not appear to be an integer")
 	}
 
+	recordOnly := false
+	if r.Method == http.MethodHead {
+		rctx.Log.Debug("HEAD request received - changing parameters")
+		//downloadRemote = false // we allow the download to go through to ensure proper metadata is returned
+		recordOnly = true
+	}
+
 	rctx = rctx.LogWithFields(logrus.Fields{
 		"mediaId":       mediaId,
 		"server":        server,
@@ -70,6 +77,7 @@ func DownloadMedia(r *http.Request, rctx rcontext.RequestContext, user _apimeta.
 		FetchRemoteIfNeeded: downloadRemote,
 		BlockForReadUntil:   blockFor,
 		CanRedirect:         canRedirect,
+		RecordOnly:          recordOnly,
 	})
 	if err != nil {
 		var redirect datastores.RedirectError

api/routes.go

@@ -33,8 +33,8 @@ func buildRoutes() http.Handler {
 	register([]string{"PUT"}, PrefixMedia, "upload/:server/:mediaId", mxV3, router, makeRoute(_routers.RequireAccessToken(r0.UploadMediaAsync), "upload_async", counter))
 	register([]string{"POST"}, PrefixMedia, "upload", mxSpecV3Transition, router, makeRoute(_routers.RequireAccessToken(r0.UploadMediaSync), "upload", counter))
 	downloadRoute := makeRoute(_routers.OptionalAccessToken(r0.DownloadMedia), "download", counter)
-	register([]string{"GET"}, PrefixMedia, "download/:server/:mediaId/:filename", mxSpecV3Transition, router, downloadRoute)
-	register([]string{"GET"}, PrefixMedia, "download/:server/:mediaId", mxSpecV3Transition, router, downloadRoute)
+	register([]string{"GET", "HEAD"}, PrefixMedia, "download/:server/:mediaId/:filename", mxSpecV3Transition, router, downloadRoute)
+	register([]string{"GET", "HEAD"}, PrefixMedia, "download/:server/:mediaId", mxSpecV3Transition, router, downloadRoute)
 	register([]string{"GET"}, PrefixMedia, "thumbnail/:server/:mediaId", mxSpecV3Transition, router, makeRoute(_routers.OptionalAccessToken(r0.ThumbnailMedia), "thumbnail", counter))
 	register([]string{"GET"}, PrefixMedia, "preview_url", mxSpecV3TransitionCS, router, makeRoute(_routers.RequireAccessToken(r0.PreviewUrl), "url_preview", counter))
 	register([]string{"GET"}, PrefixMedia, "identicon/*seed", mxR0, router, makeRoute(_routers.OptionalAccessToken(r0.Identicon), "identicon", counter))