[MM-37933] Channel preference to auto-follow all threads in the channel (#21430)

* Add auto-follow feature
---------

Co-authored-by: Mattermod <mattermod@users.noreply.github.com>
Co-authored-by: Mattermost Build <build@mattermost.com>

server/model/channel_member.go

@@ -19,6 +19,9 @@ const (
 	IgnoreChannelMentionsOff        = "off"
 	IgnoreChannelMentionsOn         = "on"
 	IgnoreChannelMentionsNotifyProp = "ignore_channel_mentions"
+	ChannelAutoFollowThreadsOff     = "off"
+	ChannelAutoFollowThreadsOn      = "on"
+	ChannelAutoFollowThreads        = "channel_auto_follow_threads"
 )
 
 type ChannelUnread struct {
@@ -172,6 +175,12 @@ func (o *ChannelMember) IsValid() *AppError {
 		}
 	}
 
+	if channelAutoFollowThreads, ok := o.NotifyProps[ChannelAutoFollowThreads]; ok {
+		if len(channelAutoFollowThreads) > 3 || !IsChannelAutoFollowThreadsValid(channelAutoFollowThreads) {
+			return NewAppError("ChannelMember.IsValid", "model.channel_member.is_valid.channel_auto_follow_threads_value.app_error", nil, "channel_auto_follow_threads="+channelAutoFollowThreads, http.StatusBadRequest)
+		}
+	}
+
 	if len(o.Roles) > UserRolesMaxLength {
 		return NewAppError("ChannelMember.IsValid", "model.channel_member.is_valid.roles_limit.app_error",
 			map[string]any{"Limit": UserRolesMaxLength}, "", http.StatusBadRequest)
@@ -223,6 +232,10 @@ func IsIgnoreChannelMentionsValid(ignoreChannelMentions string) bool {
 	return ignoreChannelMentions == IgnoreChannelMentionsOn || ignoreChannelMentions == IgnoreChannelMentionsOff || ignoreChannelMentions == IgnoreChannelMentionsDefault
 }
 
+func IsChannelAutoFollowThreadsValid(channelAutoFollowThreads string) bool {
+	return channelAutoFollowThreads == ChannelAutoFollowThreadsOn || channelAutoFollowThreads == ChannelAutoFollowThreadsOff
+}
+
 func GetDefaultChannelNotifyProps() StringMap {
 	return StringMap{
 		DesktopNotifyProp:               ChannelNotifyDefault,
@@ -230,5 +243,6 @@ func GetDefaultChannelNotifyProps() StringMap {
 		PushNotifyProp:                  ChannelNotifyDefault,
 		EmailNotifyProp:                 ChannelNotifyDefault,
 		IgnoreChannelMentionsNotifyProp: IgnoreChannelMentionsDefault,
+		ChannelAutoFollowThreads:        ChannelAutoFollowThreadsOff,
 	}
 }

server/model/utils.go

@@ -41,8 +41,26 @@ const (
 var ErrMaxPropSizeExceeded = fmt.Errorf("max prop size of %d exceeded", maxPropSizeBytes)
 
 type StringInterface map[string]any
+type StringSet map[string]struct{}
 type StringArray []string
 
+func (ss StringSet) Has(val string) bool {
+	_, ok := ss[val]
+	return ok
+}
+
+func (ss StringSet) Add(val string) {
+	ss[val] = struct{}{}
+}
+
+func (ss StringSet) Val() []string {
+	keys := make([]string, 0, len(ss))
+	for k := range ss {
+		keys = append(keys, k)
+	}
+	return keys
+}
+
 func (sa StringArray) Remove(input string) StringArray {
 	for index := range sa {
 		if sa[index] == input {