MM-10833: send down computed channel props (#8953)

* MM-10833: send down computed channel props

This allows channel headers to reference channel mentions for a client
that doesn't already know about the channels in question.

We intentionally don't send down the props for the autocomplete and
search endpoints since they aren't used in that context, and would add
unnecessary overhead.

* update channel props on patch

* revert to treating channel purpose as plaintext

app/channel.go

@@ -1591,3 +1591,67 @@ func (a *App) ToggleMuteChannel(channelId string, userId string) *model.ChannelM
 	a.Srv.Store.Channel().UpdateMember(member)
 	return member
 }
+
+func (a *App) FillInChannelProps(channel *model.Channel) *model.AppError {
+	return a.FillInChannelsProps(&model.ChannelList{channel})
+}
+
+func (a *App) FillInChannelsProps(channelList *model.ChannelList) *model.AppError {
+	// Group the channels by team and call GetChannelsByNames just once per team.
+	channelsByTeam := make(map[string]model.ChannelList)
+	for _, channel := range *channelList {
+		channelsByTeam[channel.TeamId] = append(channelsByTeam[channel.TeamId], channel)
+	}
+
+	for teamId, channelList := range channelsByTeam {
+		allChannelMentions := make(map[string]bool)
+		channelMentions := make(map[*model.Channel][]string, len(channelList))
+
+		// Collect mentions across the channels so as to query just once for this team.
+		for _, channel := range channelList {
+			channelMentions[channel] = model.ChannelMentions(channel.Header)
+
+			for _, channelMention := range channelMentions[channel] {
+				allChannelMentions[channelMention] = true
+			}
+		}
+
+		allChannelMentionNames := make([]string, 0, len(allChannelMentions))
+		for channelName := range allChannelMentions {
+			allChannelMentionNames = append(allChannelMentionNames, channelName)
+		}
+
+		if len(allChannelMentionNames) > 0 {
+			mentionedChannels, err := a.GetChannelsByNames(allChannelMentionNames, teamId)
+			if err != nil {
+				return err
+			}
+
+			mentionedChannelsByName := make(map[string]*model.Channel)
+			for _, channel := range mentionedChannels {
+				mentionedChannelsByName[channel.Name] = channel
+			}
+
+			for _, channel := range channelList {
+				channelMentionsProp := make(map[string]interface{}, len(channelMentions[channel]))
+				for _, channelMention := range channelMentions[channel] {
+					if mentioned, ok := mentionedChannelsByName[channelMention]; ok {
+						if mentioned.Type == model.CHANNEL_OPEN {
+							channelMentionsProp[mentioned.Name] = map[string]interface{}{
+								"display_name": mentioned.DisplayName,
+							}
+						}
+					}
+				}
+
+				if len(channelMentionsProp) > 0 {
+					channel.AddProp("channel_mentions", channelMentionsProp)
+				} else if channel.Props != nil {
+					delete(channel.Props, "channel_mentions")
+				}
+			}
+		}
+	}
+
+	return nil
+}