MM-60115, MM-62436, MM-62493: Compliance export downloads from local and s3; e2e tests (#29806)

* add ZipReader method to filestore and file/s3 backends

to merge

to merge

* add WriteStreamResponse as an alternative to WriteFileResponse

* add generated layers

* enable download link; download job endpoint now streams export dir zips

* fix MM-62493

* re-enable e2e tests--we have download links, folks

* Add tests for ZipReader in filestore and s3store

* remove unnecessary error return on ZipReader

* little cleanup

* improve tests; some refactoring: s.Nil(err) -> s.NoError(err)

* blank commit

* backwards compatability for pre-10.5 job downloads

* compress file response; better errors; better comments; PR comments

* update generated app layers

* improve/widen tests; improve comments; simplify localstore ZipReader

* regenerate layers

* follow GoDoc conventions

* update generated layers

* remove unnecessary comment

* in jobs/job-id/download, clean exportDir before sending to ZipReader

* better comments; add an error return on ZipReader

* improve file permissions

* adjust tests for new error returns

* linting

* i18n

server/channels/app/file.go

@@ -124,20 +124,43 @@ func fileReader(backend filestore.FileBackend, path string) (filestore.ReadClose
 	return result, nil
 }
 
+func zipReader(backend filestore.FileBackend, path string, deflate bool) (io.ReadCloser, *model.AppError) {
+	result, err := backend.ZipReader(path, deflate)
+	if err != nil {
+		return nil, model.NewAppError("ZipReader", "api.file.zip_file_reader.app_error", nil, "", http.StatusInternalServerError).Wrap(err)
+	}
+	return result, nil
+}
+
 func (s *Server) fileReader(path string) (filestore.ReadCloseSeeker, *model.AppError) {
 	return fileReader(s.FileBackend(), path)
 }
 
+func (s *Server) zipReader(path string, deflate bool) (io.ReadCloser, *model.AppError) {
+	return zipReader(s.FileBackend(), path, deflate)
+}
+
 func (s *Server) exportFileReader(path string) (filestore.ReadCloseSeeker, *model.AppError) {
 	return fileReader(s.ExportFileBackend(), path)
 }
 
-// Caller must close the first return value
+// FileReader returns a ReadCloseSeeker for path from the FileBackend.
+//
+// The caller is responsible for closing the returned ReadCloseSeeker.
 func (a *App) FileReader(path string) (filestore.ReadCloseSeeker, *model.AppError) {
 	return a.Srv().fileReader(path)
 }
 
-// Caller must close the first return value
+// ZipReader returns a ReadCloser for path. If deflate is true, the zip will use compression.
+//
+// The caller is responsible for closing the returned ReadCloser.
+func (a *App) ZipReader(path string, deflate bool) (io.ReadCloser, *model.AppError) {
+	return a.Srv().zipReader(path, deflate)
+}
+
+// ExportFileReader returns a ReadCloseSeeker for path from the ExportFileBackend.
+//
+// The caller is responsible for closing the returned ReadCloseSeeker.
 func (a *App) ExportFileReader(path string) (filestore.ReadCloseSeeker, *model.AppError) {
 	return a.Srv().exportFileReader(path)
 }
@@ -424,7 +447,7 @@ func parseOldFilenames(rctx request.CTX, filenames []string, channelID, userID s
 	return parsed
 }
 
-// Creates and stores FileInfos for a post created before the FileInfos table existed.
+// MigrateFilenamesToFileInfos creates and stores FileInfos for a post created before the FileInfos table existed.
 func (a *App) MigrateFilenamesToFileInfos(rctx request.CTX, post *model.Post) []*model.FileInfo {
 	if len(post.Filenames) == 0 {
 		rctx.Logger().Warn("Unable to migrate post to use FileInfos with an empty Filenames field", mlog.String("post_id", post.Id))