Skip exporting channels for deleted teams (#19494)

This corrects a bug where exports that contained deleted teams
could fail to import. Channels must be skipped that belong to these
deleted teams in order to produce a valid import bundle.

app/export.go

@@ -83,17 +83,18 @@ func (a *App) BulkExport(writer io.Writer, outPath string, opts BulkExportOpts)
 	}
 
 	mlog.Info("Bulk export: exporting teams")
-	if err := a.exportAllTeams(writer); err != nil {
+	teamNames, err := a.exportAllTeams(writer)
+	if err != nil {
 		return err
 	}
 
 	mlog.Info("Bulk export: exporting channels")
-	if err := a.exportAllChannels(writer); err != nil {
+	if err = a.exportAllChannels(writer, teamNames); err != nil {
 		return err
 	}
 
 	mlog.Info("Bulk export: exporting users")
-	if err := a.exportAllUsers(writer); err != nil {
+	if err = a.exportAllUsers(writer); err != nil {
 		return err
 	}
 
@@ -165,12 +166,13 @@ func (a *App) exportVersion(writer io.Writer) *model.AppError {
 	return a.exportWriteLine(writer, versionLine)
 }
 
-func (a *App) exportAllTeams(writer io.Writer) *model.AppError {
+func (a *App) exportAllTeams(writer io.Writer) (map[string]bool, *model.AppError) {
 	afterId := strings.Repeat("0", 26)
+	teamNames := make(map[string]bool)
 	for {
 		teams, err := a.Srv().Store.Team().GetAllForExportAfter(1000, afterId)
 		if err != nil {
-			return model.NewAppError("exportAllTeams", "app.team.get_all.app_error", nil, err.Error(), http.StatusInternalServerError)
+			return nil, model.NewAppError("exportAllTeams", "app.team.get_all.app_error", nil, err.Error(), http.StatusInternalServerError)
 		}
 
 		if len(teams) == 0 {
@@ -184,18 +186,19 @@ func (a *App) exportAllTeams(writer io.Writer) *model.AppError {
 			if team.DeleteAt != 0 {
 				continue
 			}
+			teamNames[team.Name] = true
 
 			teamLine := ImportLineFromTeam(team)
 			if err := a.exportWriteLine(writer, teamLine); err != nil {
-				return err
+				return nil, err
 			}
 		}
 	}
 
-	return nil
+	return teamNames, nil
 }
 
-func (a *App) exportAllChannels(writer io.Writer) *model.AppError {
+func (a *App) exportAllChannels(writer io.Writer, teamNames map[string]bool) *model.AppError {
 	afterId := strings.Repeat("0", 26)
 	for {
 		channels, err := a.Srv().Store.Channel().GetAllChannelsForExportAfter(1000, afterId)
@@ -215,6 +218,10 @@ func (a *App) exportAllChannels(writer io.Writer) *model.AppError {
 			if channel.DeleteAt != 0 {
 				continue
 			}
+			// Skip channels on deleted teams.
+			if ok := teamNames[channel.TeamName]; !ok {
+				continue
+			}
 
 			channelLine := ImportLineFromChannel(channel)
 			if err := a.exportWriteLine(writer, channelLine); err != nil {