[MM-25767] Quick switch users search is always falling back to the database (#14762)

* Refactor of getListOfAllowedChannelsForTeam

Also, I've fixed some problematic scenarios:

- The quick search doesn't provide team id so it was always failing
- When the teamId was empty and view restrictions too we always
  return all the channels because if we do "strings.Contains("foo", "")
  it always returns true
- There was a case, in quick search with a guest account, where you
  get an empty result because teamId is not provided

* Error if team id is not passed when searching for the channel

If we search users passing the channel id, we must pass the team id
too so we avoid returning all the channels if we remove the empty
team id restriction we have in the getListOfAllowedChannelsForTeam

There is no known reason to search for a channel but not filtering
using the team id. Even guest accounts belong to a team

store/searchlayer/user_layer.go

@@ -4,7 +4,6 @@
 package searchlayer
 
 import (
-	"net/http"
 	"strings"
 
 	"github.com/mattermost/mattermost-server/v5/mlog"
@@ -104,7 +103,7 @@ func (s *SearchUserStore) autocompleteUsersInChannelByEngine(engine searchengine
 	uchanIds := []string{}
 	nuchanIds := []string{}
 	sanitizedTerm := sanitizeSearchTerm(term)
-	if options.ListOfAllowedChannels != nil && !strings.Contains(strings.Join(options.ListOfAllowedChannels, "."), channelId) {
+	if channelId != "" && options.ListOfAllowedChannels != nil && !strings.Contains(strings.Join(options.ListOfAllowedChannels, "."), channelId) {
 		nuchanIds, err = engine.SearchUsersInTeam(teamId, options.ListOfAllowedChannels, sanitizedTerm, options)
 	} else {
 		uchanIds, nuchanIds, err = engine.SearchUsersInChannel(teamId, channelId, options.ListOfAllowedChannels, sanitizedTerm, options)
@@ -146,45 +145,49 @@ func (s *SearchUserStore) autocompleteUsersInChannelByEngine(engine searchengine
 	return autocomplete, nil
 }
 
+// getListOfAllowedChannelsForTeam return the list of allowed channels to search user based on the
+//	next scenarios:
+//		- If there isn't view restrictions (team or channel) and no team id to filter them, then all
+//		  channels are allowed (nil return)
+//	   	- If we receive a team Id and either we don't have view restrictions or the provided team id is included in the
+//		  list of restricted teams, then we return all the team channels
+//		- If we don't receive team id or the provided team id is not in the list of allowed teams to search of and we
+//		  don't have channel restrictions then we return an empty result because we cannot get channels
+//		- If we receive channels restrictions we get:
+//			- If we don't have team id, we get those restricted channels (guest accounts and quick search)
+//			- If we have a team id then we only return those restricted channels that belongs to that team
 func (s *SearchUserStore) getListOfAllowedChannelsForTeam(teamId string, viewRestrictions *model.ViewUsersRestrictions) ([]string, *model.AppError) {
-	if len(teamId) == 0 {
-		return nil, model.NewAppError("SearchUserStore", "store.search_user_store.empty_team_id", nil, "", http.StatusInternalServerError)
-	}
-
 	var listOfAllowedChannels []string
 	if viewRestrictions == nil && teamId == "" {
+		// nil return without error means all channels are allowed
 		return nil, nil
 	}
 
-	if viewRestrictions == nil || strings.Contains(strings.Join(viewRestrictions.Teams, "."), teamId) {
+	if teamId != "" && (viewRestrictions == nil || strings.Contains(strings.Join(viewRestrictions.Teams, "."), teamId)) {
 		channels, err := s.rootStore.Channel().GetTeamChannels(teamId)
 		if err != nil {
 			return nil, err
 		}
-		channelIds := []string{}
 		for _, channel := range *channels {
-			channelIds = append(channelIds, channel.Id)
+			listOfAllowedChannels = append(listOfAllowedChannels, channel.Id)
 		}
-
-		return channelIds, nil
+		return listOfAllowedChannels, nil
 	}
 
-	if len(viewRestrictions.Channels) == 0 {
-		return []string{}, nil
-	}
-
-	channels, err := s.rootStore.Channel().GetChannelsByIds(viewRestrictions.Channels, false)
-
-	if err != nil {
-		return nil, err
-	}
-	for _, c := range channels {
-		if c.TeamId == teamId {
-			listOfAllowedChannels = append(listOfAllowedChannels, c.Id)
+	if len(viewRestrictions.Channels) > 0 {
+		channels, err := s.rootStore.Channel().GetChannelsByIds(viewRestrictions.Channels, false)
+		if err != nil {
+			return nil, err
 		}
+		for _, c := range channels {
+			if teamId == "" || (teamId != "" && c.TeamId == teamId) {
+				listOfAllowedChannels = append(listOfAllowedChannels, c.Id)
+			}
+		}
+		return listOfAllowedChannels, nil
 	}
 
-	return listOfAllowedChannels, nil
+	return []string{}, nil
 }
 
 func (s *SearchUserStore) AutocompleteUsersInChannel(teamId, channelId, term string, options *model.UserSearchOptions) (*model.UserAutocompleteInChannel, *model.AppError) {
@@ -195,7 +198,7 @@ func (s *SearchUserStore) AutocompleteUsersInChannel(teamId, channelId, term str
 				mlog.Error("Encountered error on AutocompleteUsersInChannel.", mlog.String("search_engine", engine.GetName()), mlog.Err(err))
 				continue
 			}
-			if len(listOfAllowedChannels) == 0 {
+			if listOfAllowedChannels != nil && len(listOfAllowedChannels) == 0 {
 				return &model.UserAutocompleteInChannel{}, nil
 			}
 			options.ListOfAllowedChannels = listOfAllowedChannels