use go module for dependencies (#594)

1 files changed, 699 insertions, 0 deletions

diff --git a/mediaapi/routing/download.go b/mediaapi/routing/download.go
new file mode 100644
index 00000000..9c8f43c4
--- /dev/null
+++ b/mediaapi/routing/download.go
@@ -0,0 +1,699 @@
+// Copyright 2017 Vector Creations Ltd
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package routing
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"mime"
+	"net/http"
+	"os"
+	"path/filepath"
+	"regexp"
+	"strconv"
+	"strings"
+	"sync"
+
+	"github.com/matrix-org/dendrite/clientapi/jsonerror"
+	"github.com/matrix-org/dendrite/common/config"
+	"github.com/matrix-org/dendrite/mediaapi/fileutils"
+	"github.com/matrix-org/dendrite/mediaapi/storage"
+	"github.com/matrix-org/dendrite/mediaapi/thumbnailer"
+	"github.com/matrix-org/dendrite/mediaapi/types"
+	"github.com/matrix-org/gomatrixserverlib"
+	"github.com/matrix-org/util"
+	"github.com/pkg/errors"
+	log "github.com/sirupsen/logrus"
+)
+
+const mediaIDCharacters = "A-Za-z0-9_=-"
+
+// Note: unfortunately regex.MustCompile() cannot be assigned to a const
+var mediaIDRegex = regexp.MustCompile("[" + mediaIDCharacters + "]+")
+
+// downloadRequest metadata included in or derivable from a download or thumbnail request
+// https://matrix.org/docs/spec/client_server/r0.2.0.html#get-matrix-media-r0-download-servername-mediaid
+// http://matrix.org/docs/spec/client_server/r0.2.0.html#get-matrix-media-r0-thumbnail-servername-mediaid
+type downloadRequest struct {
+	MediaMetadata      *types.MediaMetadata
+	IsThumbnailRequest bool
+	ThumbnailSize      types.ThumbnailSize
+	Logger             *log.Entry
+}
+
+// Download implements /download amd /thumbnail
+// Files from this server (i.e. origin == cfg.ServerName) are served directly
+// Files from remote servers (i.e. origin != cfg.ServerName) are cached locally.
+// If they are present in the cache, they are served directly.
+// If they are not present in the cache, they are obtained from the remote server and
+// simultaneously served back to the client and written into the cache.
+func Download(
+	w http.ResponseWriter,
+	req *http.Request,
+	origin gomatrixserverlib.ServerName,
+	mediaID types.MediaID,
+	cfg *config.Dendrite,
+	db *storage.Database,
+	client *gomatrixserverlib.Client,
+	activeRemoteRequests *types.ActiveRemoteRequests,
+	activeThumbnailGeneration *types.ActiveThumbnailGeneration,
+	isThumbnailRequest bool,
+) {
+	dReq := &downloadRequest{
+		MediaMetadata: &types.MediaMetadata{
+			MediaID: mediaID,
+			Origin:  origin,
+		},
+		IsThumbnailRequest: isThumbnailRequest,
+		Logger: util.GetLogger(req.Context()).WithFields(log.Fields{
+			"Origin":  origin,
+			"MediaID": mediaID,
+		}),
+	}
+
+	if dReq.IsThumbnailRequest {
+		width, err := strconv.Atoi(req.FormValue("width"))
+		if err != nil {
+			width = -1
+		}
+		height, err := strconv.Atoi(req.FormValue("height"))
+		if err != nil {
+			height = -1
+		}
+		dReq.ThumbnailSize = types.ThumbnailSize{
+			Width:        width,
+			Height:       height,
+			ResizeMethod: strings.ToLower(req.FormValue("method")),
+		}
+		dReq.Logger.WithFields(log.Fields{
+			"RequestedWidth":        dReq.ThumbnailSize.Width,
+			"RequestedHeight":       dReq.ThumbnailSize.Height,
+			"RequestedResizeMethod": dReq.ThumbnailSize.ResizeMethod,
+		})
+	}
+
+	// request validation
+	if req.Method != http.MethodGet {
+		dReq.jsonErrorResponse(w, util.JSONResponse{
+			Code: http.StatusMethodNotAllowed,
+			JSON: jsonerror.Unknown("request method must be GET"),
+		})
+		return
+	}
+
+	if resErr := dReq.Validate(); resErr != nil {
+		dReq.jsonErrorResponse(w, *resErr)
+		return
+	}
+
+	metadata, err := dReq.doDownload(
+		req.Context(), w, cfg, db, client,
+		activeRemoteRequests, activeThumbnailGeneration,
+	)
+	if err != nil {
+		// TODO: Handle the fact we might have started writing the response
+		dReq.jsonErrorResponse(w, util.ErrorResponse(err))
+		return
+	}
+
+	if metadata == nil {
+		dReq.jsonErrorResponse(w, util.JSONResponse{
+			Code: http.StatusNotFound,
+			JSON: jsonerror.NotFound("File not found"),
+		})
+		return
+	}
+
+}
+
+func (r *downloadRequest) jsonErrorResponse(w http.ResponseWriter, res util.JSONResponse) {
+	// Marshal JSON response into raw bytes to send as the HTTP body
+	resBytes, err := json.Marshal(res.JSON)
+	if err != nil {
+		r.Logger.WithError(err).Error("Failed to marshal JSONResponse")
+		// this should never fail to be marshalled so drop err to the floor
+		res = util.MessageResponse(http.StatusInternalServerError, "Internal Server Error")
+		resBytes, _ = json.Marshal(res.JSON)
+	}
+
+	// Set status code and write the body
+	w.WriteHeader(res.Code)
+	r.Logger.WithField("code", res.Code).Infof("Responding (%d bytes)", len(resBytes))
+
+	// we don't really care that much if we fail to write the error response
+	w.Write(resBytes) // nolint: errcheck
+}
+
+// Validate validates the downloadRequest fields
+func (r *downloadRequest) Validate() *util.JSONResponse {
+	if !mediaIDRegex.MatchString(string(r.MediaMetadata.MediaID)) {
+		return &util.JSONResponse{
+			Code: http.StatusNotFound,
+			JSON: jsonerror.NotFound(fmt.Sprintf("mediaId must be a non-empty string using only characters in %v", mediaIDCharacters)),
+		}
+	}
+	// Note: the origin will be validated either by comparison to the configured server name of this homeserver
+	// or by a DNS SRV record lookup when creating a request for remote files
+	if r.MediaMetadata.Origin == "" {
+		return &util.JSONResponse{
+			Code: http.StatusNotFound,
+			JSON: jsonerror.NotFound("serverName must be a non-empty string"),
+		}
+	}
+
+	if r.IsThumbnailRequest {
+		if r.ThumbnailSize.Width <= 0 || r.ThumbnailSize.Height <= 0 {
+			return &util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: jsonerror.Unknown("width and height must be greater than 0"),
+			}
+		}
+		// Default method to scale if not set
+		if r.ThumbnailSize.ResizeMethod == "" {
+			r.ThumbnailSize.ResizeMethod = types.Scale
+		}
+		if r.ThumbnailSize.ResizeMethod != types.Crop && r.ThumbnailSize.ResizeMethod != types.Scale {
+			return &util.JSONResponse{
+				Code: http.StatusBadRequest,
+				JSON: jsonerror.Unknown("method must be one of crop or scale"),
+			}
+		}
+	}
+	return nil
+}
+
+func (r *downloadRequest) doDownload(
+	ctx context.Context,
+	w http.ResponseWriter,
+	cfg *config.Dendrite,
+	db *storage.Database,
+	client *gomatrixserverlib.Client,
+	activeRemoteRequests *types.ActiveRemoteRequests,
+	activeThumbnailGeneration *types.ActiveThumbnailGeneration,
+) (*types.MediaMetadata, error) {
+	// check if we have a record of the media in our database
+	mediaMetadata, err := db.GetMediaMetadata(
+		ctx, r.MediaMetadata.MediaID, r.MediaMetadata.Origin,
+	)
+	if err != nil {
+		return nil, errors.Wrap(err, "error querying the database")
+	}
+	if mediaMetadata == nil {
+		if r.MediaMetadata.Origin == cfg.Matrix.ServerName {
+			// If we do not have a record and the origin is local, the file is not found
+			return nil, nil
+		}
+		// If we do not have a record and the origin is remote, we need to fetch it and respond with that file
+		resErr := r.getRemoteFile(
+			ctx, client, cfg, db, activeRemoteRequests, activeThumbnailGeneration,
+		)
+		if resErr != nil {
+			return nil, resErr
+		}
+	} else {
+		// If we have a record, we can respond from the local file
+		r.MediaMetadata = mediaMetadata
+	}
+	return r.respondFromLocalFile(
+		ctx, w, cfg.Media.AbsBasePath, activeThumbnailGeneration,
+		cfg.Media.MaxThumbnailGenerators, db,
+		cfg.Media.DynamicThumbnails, cfg.Media.ThumbnailSizes,
+	)
+}
+
+// respondFromLocalFile reads a file from local storage and writes it to the http.ResponseWriter
+// If no file was found then returns nil, nil
+func (r *downloadRequest) respondFromLocalFile(
+	ctx context.Context,
+	w http.ResponseWriter,
+	absBasePath config.Path,
+	activeThumbnailGeneration *types.ActiveThumbnailGeneration,
+	maxThumbnailGenerators int,
+	db *storage.Database,
+	dynamicThumbnails bool,
+	thumbnailSizes []config.ThumbnailSize,
+) (*types.MediaMetadata, error) {
+	filePath, err := fileutils.GetPathFromBase64Hash(r.MediaMetadata.Base64Hash, absBasePath)
+	if err != nil {
+		return nil, errors.Wrap(err, "failed to get file path from metadata")
+	}
+	file, err := os.Open(filePath)
+	defer file.Close() // nolint: errcheck, staticcheck, megacheck
+	if err != nil {
+		return nil, errors.Wrap(err, "failed to open file")
+	}
+	stat, err := file.Stat()
+	if err != nil {
+		return nil, errors.Wrap(err, "failed to stat file")
+	}
+
+	if r.MediaMetadata.FileSizeBytes > 0 && int64(r.MediaMetadata.FileSizeBytes) != stat.Size() {
+		r.Logger.WithFields(log.Fields{
+			"fileSizeDatabase": r.MediaMetadata.FileSizeBytes,
+			"fileSizeDisk":     stat.Size(),
+		}).Warn("File size in database and on-disk differ.")
+		return nil, errors.New("file size in database and on-disk differ")
+	}
+
+	var responseFile *os.File
+	var responseMetadata *types.MediaMetadata
+	if r.IsThumbnailRequest {
+		thumbFile, thumbMetadata, resErr := r.getThumbnailFile(
+			ctx, types.Path(filePath), activeThumbnailGeneration, maxThumbnailGenerators,
+			db, dynamicThumbnails, thumbnailSizes,
+		)
+		if thumbFile != nil {
+			defer thumbFile.Close() // nolint: errcheck
+		}
+		if resErr != nil {
+			return nil, resErr
+		}
+		if thumbFile == nil {
+			r.Logger.WithFields(log.Fields{
+				"UploadName":    r.MediaMetadata.UploadName,
+				"Base64Hash":    r.MediaMetadata.Base64Hash,
+				"FileSizeBytes": r.MediaMetadata.FileSizeBytes,
+				"ContentType":   r.MediaMetadata.ContentType,
+			}).Info("No good thumbnail found. Responding with original file.")
+			responseFile = file
+			responseMetadata = r.MediaMetadata
+		} else {
+			r.Logger.Info("Responding with thumbnail")
+			responseFile = thumbFile
+			responseMetadata = thumbMetadata.MediaMetadata
+		}
+	} else {
+		r.Logger.WithFields(log.Fields{
+			"UploadName":    r.MediaMetadata.UploadName,
+			"Base64Hash":    r.MediaMetadata.Base64Hash,
+			"FileSizeBytes": r.MediaMetadata.FileSizeBytes,
+			"ContentType":   r.MediaMetadata.ContentType,
+		}).Info("Responding with file")
+		responseFile = file
+		responseMetadata = r.MediaMetadata
+	}
+
+	w.Header().Set("Content-Type", string(responseMetadata.ContentType))
+	w.Header().Set("Content-Length", strconv.FormatInt(int64(responseMetadata.FileSizeBytes), 10))
+	contentSecurityPolicy := "default-src 'none';" +
+		" script-src 'none';" +
+		" plugin-types application/pdf;" +
+		" style-src 'unsafe-inline';" +
+		" object-src 'self';"
+	w.Header().Set("Content-Security-Policy", contentSecurityPolicy)
+
+	if _, err := io.Copy(w, responseFile); err != nil {
+		return nil, errors.Wrap(err, "failed to copy from cache")
+	}
+	return responseMetadata, nil
+}
+
+// Note: Thumbnail generation may be ongoing asynchronously.
+// If no thumbnail was found then returns nil, nil, nil
+func (r *downloadRequest) getThumbnailFile(
+	ctx context.Context,
+	filePath types.Path,
+	activeThumbnailGeneration *types.ActiveThumbnailGeneration,
+	maxThumbnailGenerators int,
+	db *storage.Database,
+	dynamicThumbnails bool,
+	thumbnailSizes []config.ThumbnailSize,
+) (*os.File, *types.ThumbnailMetadata, error) {
+	var thumbnail *types.ThumbnailMetadata
+	var err error
+
+	if dynamicThumbnails {
+		thumbnail, err = r.generateThumbnail(
+			ctx, filePath, r.ThumbnailSize, activeThumbnailGeneration,
+			maxThumbnailGenerators, db,
+		)
+		if err != nil {
+			return nil, nil, err
+		}
+	}
+	// If dynamicThumbnails is true but there are too many thumbnails being actively generated, we can fall back
+	// to trying to use a pre-generated thumbnail
+	if thumbnail == nil {
+		var thumbnails []*types.ThumbnailMetadata
+		thumbnails, err = db.GetThumbnails(
+			ctx, r.MediaMetadata.MediaID, r.MediaMetadata.Origin,
+		)
+		if err != nil {
+			return nil, nil, errors.Wrap(err, "error looking up thumbnails")
+		}
+
+		// If we get a thumbnailSize, a pre-generated thumbnail would be best but it is not yet generated.
+		// If we get a thumbnail, we're done.
+		var thumbnailSize *types.ThumbnailSize
+		thumbnail, thumbnailSize = thumbnailer.SelectThumbnail(r.ThumbnailSize, thumbnails, thumbnailSizes)
+		// If dynamicThumbnails is true and we are not over-loaded then we would have generated what was requested above.
+		// So we don't try to generate a pre-generated thumbnail here.
+		if thumbnailSize != nil && !dynamicThumbnails {
+			r.Logger.WithFields(log.Fields{
+				"Width":        thumbnailSize.Width,
+				"Height":       thumbnailSize.Height,
+				"ResizeMethod": thumbnailSize.ResizeMethod,
+			}).Info("Pre-generating thumbnail for immediate response.")
+			thumbnail, err = r.generateThumbnail(
+				ctx, filePath, *thumbnailSize, activeThumbnailGeneration,
+				maxThumbnailGenerators, db,
+			)
+			if err != nil {
+				return nil, nil, err
+			}
+		}
+	}
+	if thumbnail == nil {
+		return nil, nil, nil
+	}
+	r.Logger = r.Logger.WithFields(log.Fields{
+		"Width":         thumbnail.ThumbnailSize.Width,
+		"Height":        thumbnail.ThumbnailSize.Height,
+		"ResizeMethod":  thumbnail.ThumbnailSize.ResizeMethod,
+		"FileSizeBytes": thumbnail.MediaMetadata.FileSizeBytes,
+		"ContentType":   thumbnail.MediaMetadata.ContentType,
+	})
+	thumbPath := string(thumbnailer.GetThumbnailPath(types.Path(filePath), thumbnail.ThumbnailSize))
+	thumbFile, err := os.Open(string(thumbPath))
+	if err != nil {
+		thumbFile.Close() // nolint: errcheck
+		return nil, nil, errors.Wrap(err, "failed to open file")
+	}
+	thumbStat, err := thumbFile.Stat()
+	if err != nil {
+		thumbFile.Close() // nolint: errcheck
+		return nil, nil, errors.Wrap(err, "failed to stat file")
+	}
+	if types.FileSizeBytes(thumbStat.Size()) != thumbnail.MediaMetadata.FileSizeBytes {
+		thumbFile.Close() // nolint: errcheck
+		return nil, nil, errors.New("thumbnail file sizes on disk and in database differ")
+	}
+	return thumbFile, thumbnail, nil
+}
+
+func (r *downloadRequest) generateThumbnail(
+	ctx context.Context,
+	filePath types.Path,
+	thumbnailSize types.ThumbnailSize,
+	activeThumbnailGeneration *types.ActiveThumbnailGeneration,
+	maxThumbnailGenerators int,
+	db *storage.Database,
+) (*types.ThumbnailMetadata, error) {
+	r.Logger.WithFields(log.Fields{
+		"Width":        thumbnailSize.Width,
+		"Height":       thumbnailSize.Height,
+		"ResizeMethod": thumbnailSize.ResizeMethod,
+	})
+	busy, err := thumbnailer.GenerateThumbnail(
+		ctx, filePath, thumbnailSize, r.MediaMetadata,
+		activeThumbnailGeneration, maxThumbnailGenerators, db, r.Logger,
+	)
+	if err != nil {
+		return nil, errors.Wrap(err, "error creating thumbnail")
+	}
+	if busy {
+		return nil, nil
+	}
+	var thumbnail *types.ThumbnailMetadata
+	thumbnail, err = db.GetThumbnail(
+		ctx, r.MediaMetadata.MediaID, r.MediaMetadata.Origin,
+		thumbnailSize.Width, thumbnailSize.Height, thumbnailSize.ResizeMethod,
+	)
+	if err != nil {
+		return nil, errors.Wrap(err, "error looking up thumbnail")
+	}
+	return thumbnail, nil
+}
+
+// getRemoteFile fetches the remote file and caches it locally
+// A hash map of active remote requests to a struct containing a sync.Cond is used to only download remote files once,
+// regardless of how many download requests are received.
+// Note: The named errorResponse return variable is used in a deferred broadcast of the metadata and error response to waiting goroutines.
+func (r *downloadRequest) getRemoteFile(
+	ctx context.Context,
+	client *gomatrixserverlib.Client,
+	cfg *config.Dendrite,
+	db *storage.Database,
+	activeRemoteRequests *types.ActiveRemoteRequests,
+	activeThumbnailGeneration *types.ActiveThumbnailGeneration,
+) (errorResponse error) {
+	// Note: getMediaMetadataFromActiveRequest uses mutexes and conditions from activeRemoteRequests
+	mediaMetadata, resErr := r.getMediaMetadataFromActiveRequest(activeRemoteRequests)
+	if resErr != nil {
+		return resErr
+	} else if mediaMetadata != nil {
+		// If we got metadata from an active request, we can respond from the local file
+		r.MediaMetadata = mediaMetadata
+	} else {
+		// Note: This is an active request that MUST broadcastMediaMetadata to wake up waiting goroutines!
+		// Note: broadcastMediaMetadata uses mutexes and conditions from activeRemoteRequests
+		defer func() {
+			// Note: errorResponse is the named return variable so we wrap this in a closure to re-evaluate the arguments at defer-time
+			if err := recover(); err != nil {
+				r.broadcastMediaMetadata(activeRemoteRequests, errors.New("paniced"))
+				panic(err)
+			}
+			r.broadcastMediaMetadata(activeRemoteRequests, errorResponse)
+		}()
+
+		// check if we have a record of the media in our database
+		mediaMetadata, err := db.GetMediaMetadata(
+			ctx, r.MediaMetadata.MediaID, r.MediaMetadata.Origin,
+		)
+		if err != nil {
+			return errors.Wrap(err, "error querying the database.")
+		}
+
+		if mediaMetadata == nil {
+			// If we do not have a record, we need to fetch the remote file first and then respond from the local file
+			err := r.fetchRemoteFileAndStoreMetadata(
+				ctx, client,
+				cfg.Media.AbsBasePath, *cfg.Media.MaxFileSizeBytes, db,
+				cfg.Media.ThumbnailSizes, activeThumbnailGeneration,
+				cfg.Media.MaxThumbnailGenerators,
+			)
+			if err != nil {
+				return errors.Wrap(err, "error querying the database.")
+			}
+		} else {
+			// If we have a record, we can respond from the local file
+			r.MediaMetadata = mediaMetadata
+		}
+	}
+	return nil
+}
+
+func (r *downloadRequest) getMediaMetadataFromActiveRequest(activeRemoteRequests *types.ActiveRemoteRequests) (*types.MediaMetadata, error) {
+	// Check if there is an active remote request for the file
+	mxcURL := "mxc://" + string(r.MediaMetadata.Origin) + "/" + string(r.MediaMetadata.MediaID)
+
+	activeRemoteRequests.Lock()
+	defer activeRemoteRequests.Unlock()
+
+	if activeRemoteRequestResult, ok := activeRemoteRequests.MXCToResult[mxcURL]; ok {
+		r.Logger.Info("Waiting for another goroutine to fetch the remote file.")
+
+		// NOTE: Wait unlocks and locks again internally. There is still a deferred Unlock() that will unlock this.
+		activeRemoteRequestResult.Cond.Wait()
+		if activeRemoteRequestResult.Error != nil {
+			return nil, activeRemoteRequestResult.Error
+		}
+
+		if activeRemoteRequestResult.MediaMetadata == nil {
+			return nil, nil
+		}
+
+		return activeRemoteRequestResult.MediaMetadata, nil
+	}
+
+	// No active remote request so create one
+	activeRemoteRequests.MXCToResult[mxcURL] = &types.RemoteRequestResult{
+		Cond: &sync.Cond{L: activeRemoteRequests},
+	}
+
+	return nil, nil
+}
+
+// broadcastMediaMetadata broadcasts the media metadata and error response to waiting goroutines
+// Only the owner of the activeRemoteRequestResult for this origin and media ID should call this function.
+func (r *downloadRequest) broadcastMediaMetadata(activeRemoteRequests *types.ActiveRemoteRequests, err error) {
+	activeRemoteRequests.Lock()
+	defer activeRemoteRequests.Unlock()
+	mxcURL := "mxc://" + string(r.MediaMetadata.Origin) + "/" + string(r.MediaMetadata.MediaID)
+	if activeRemoteRequestResult, ok := activeRemoteRequests.MXCToResult[mxcURL]; ok {
+		r.Logger.Info("Signalling other goroutines waiting for this goroutine to fetch the file.")
+		activeRemoteRequestResult.MediaMetadata = r.MediaMetadata
+		activeRemoteRequestResult.Error = err
+		activeRemoteRequestResult.Cond.Broadcast()
+	}
+	delete(activeRemoteRequests.MXCToResult, mxcURL)
+}
+
+// fetchRemoteFileAndStoreMetadata fetches the file from the remote server and stores its metadata in the database
+func (r *downloadRequest) fetchRemoteFileAndStoreMetadata(
+	ctx context.Context,
+	client *gomatrixserverlib.Client,
+	absBasePath config.Path,
+	maxFileSizeBytes config.FileSizeBytes,
+	db *storage.Database,
+	thumbnailSizes []config.ThumbnailSize,
+	activeThumbnailGeneration *types.ActiveThumbnailGeneration,
+	maxThumbnailGenerators int,
+) error {
+	finalPath, duplicate, err := r.fetchRemoteFile(
+		ctx, client, absBasePath, maxFileSizeBytes,
+	)
+	if err != nil {
+		return err
+	}
+
+	r.Logger.WithFields(log.Fields{
+		"Base64Hash":    r.MediaMetadata.Base64Hash,
+		"UploadName":    r.MediaMetadata.UploadName,
+		"FileSizeBytes": r.MediaMetadata.FileSizeBytes,
+		"ContentType":   r.MediaMetadata.ContentType,
+	}).Info("Storing file metadata to media repository database")
+
+	// FIXME: timeout db request
+	if err := db.StoreMediaMetadata(ctx, r.MediaMetadata); err != nil {
+		// If the file is a duplicate (has the same hash as an existing file) then
+		// there is valid metadata in the database for that file. As such we only
+		// remove the file if it is not a duplicate.
+		if !duplicate {
+			finalDir := filepath.Dir(string(finalPath))
+			fileutils.RemoveDir(types.Path(finalDir), r.Logger)
+		}
+		// NOTE: It should really not be possible to fail the uniqueness test here so
+		// there is no need to handle that separately
+		return errors.New("failed to store file metadata in DB")
+	}
+
+	go func() {
+		busy, err := thumbnailer.GenerateThumbnails(
+			context.Background(), finalPath, thumbnailSizes, r.MediaMetadata,
+			activeThumbnailGeneration, maxThumbnailGenerators, db, r.Logger,
+		)
+		if err != nil {
+			r.Logger.WithError(err).Warn("Error generating thumbnails")
+		}
+		if busy {
+			r.Logger.Warn("Maximum number of active thumbnail generators reached. Skipping pre-generation.")
+		}
+	}()
+
+	r.Logger.WithFields(log.Fields{
+		"UploadName":    r.MediaMetadata.UploadName,
+		"Base64Hash":    r.MediaMetadata.Base64Hash,
+		"FileSizeBytes": r.MediaMetadata.FileSizeBytes,
+		"ContentType":   r.MediaMetadata.ContentType,
+	}).Infof("Remote file cached")
+
+	return nil
+}
+
+func (r *downloadRequest) fetchRemoteFile(
+	ctx context.Context,
+	client *gomatrixserverlib.Client,
+	absBasePath config.Path,
+	maxFileSizeBytes config.FileSizeBytes,
+) (types.Path, bool, error) {
+	r.Logger.Info("Fetching remote file")
+
+	// create request for remote file
+	resp, err := r.createRemoteRequest(ctx, client)
+	if err != nil {
+		return "", false, err
+	}
+	if resp == nil {
+		// Remote file not found
+		return "", false, nil
+	}
+	defer resp.Body.Close() // nolint: errcheck
+
+	// get metadata from request and set metadata on response
+	contentLength, err := strconv.ParseInt(resp.Header.Get("Content-Length"), 10, 64)
+	if err != nil {
+		r.Logger.WithError(err).Warn("Failed to parse content length")
+		return "", false, errors.Wrap(err, "invalid response from remote server")
+	}
+	if contentLength > int64(maxFileSizeBytes) {
+		// TODO: Bubble up this as a 413
+		return "", false, fmt.Errorf("remote file is too large (%v > %v bytes)", contentLength, maxFileSizeBytes)
+	}
+	r.MediaMetadata.FileSizeBytes = types.FileSizeBytes(contentLength)
+	r.MediaMetadata.ContentType = types.ContentType(resp.Header.Get("Content-Type"))
+	_, params, err := mime.ParseMediaType(resp.Header.Get("Content-Disposition"))
+	if err == nil && params["filename"] != "" {
+		r.MediaMetadata.UploadName = types.Filename(params["filename"])
+	}
+
+	r.Logger.Info("Transferring remote file")
+
+	// The file data is hashed but is NOT used as the MediaID, unlike in Upload. The hash is useful as a
+	// method of deduplicating files to save storage, as well as a way to conduct
+	// integrity checks on the file data in the repository.
+	// Data is truncated to maxFileSizeBytes. Content-Length was reported as 0 < Content-Length <= maxFileSizeBytes so this is OK.
+	hash, bytesWritten, tmpDir, err := fileutils.WriteTempFile(resp.Body, maxFileSizeBytes, absBasePath)
+	if err != nil {
+		r.Logger.WithError(err).WithFields(log.Fields{
+			"MaxFileSizeBytes": maxFileSizeBytes,
+		}).Warn("Error while downloading file from remote server")
+		fileutils.RemoveDir(tmpDir, r.Logger)
+		return "", false, errors.New("file could not be downloaded from remote server")
+	}
+
+	r.Logger.Info("Remote file transferred")
+
+	// It's possible the bytesWritten to the temporary file is different to the reported Content-Length from the remote
+	// request's response. bytesWritten is therefore used as it is what would be sent to clients when reading from the local
+	// file.
+	r.MediaMetadata.FileSizeBytes = types.FileSizeBytes(bytesWritten)
+	r.MediaMetadata.Base64Hash = hash
+
+	// The database is the source of truth so we need to have moved the file first
+	finalPath, duplicate, err := fileutils.MoveFileWithHashCheck(tmpDir, r.MediaMetadata, absBasePath, r.Logger)
+	if err != nil {
+		return "", false, errors.Wrap(err, "failed to move file")
+	}
+	if duplicate {
+		r.Logger.WithField("dst", finalPath).Info("File was stored previously - discarding duplicate")
+		// Continue on to store the metadata in the database
+	}
+
+	return types.Path(finalPath), duplicate, nil
+}
+
+func (r *downloadRequest) createRemoteRequest(
+	ctx context.Context, matrixClient *gomatrixserverlib.Client,
+) (*http.Response, error) {
+	resp, err := matrixClient.CreateMediaDownloadRequest(ctx, r.MediaMetadata.Origin, string(r.MediaMetadata.MediaID))
+	if err != nil {
+		return nil, fmt.Errorf("file with media ID %q could not be downloaded from %q", r.MediaMetadata.MediaID, r.MediaMetadata.Origin)
+	}
+
+	if resp.StatusCode != http.StatusOK {
+		if resp.StatusCode == http.StatusNotFound {
+			return nil, nil
+		}
+		r.Logger.WithFields(log.Fields{
+			"StatusCode": resp.StatusCode,
+		}).Warn("Received error response")
+		return nil, fmt.Errorf("file with media ID %q could not be downloaded from %q", r.MediaMetadata.MediaID, r.MediaMetadata.Origin)
+	}
+
+	return resp, nil
+}