[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/channels/app/channel.go

@@ -1354,6 +1354,10 @@ func (a *App) UpdateChannelMemberNotifyProps(c request.CTX, data map[string]stri
 		filteredProps[model.IgnoreChannelMentionsNotifyProp] = ignoreChannelMentions
 	}
 
+	if channelAutoFollowThreads, exists := data[model.ChannelAutoFollowThreads]; exists {
+		filteredProps[model.ChannelAutoFollowThreads] = channelAutoFollowThreads
+	}
+
 	member, err := a.Srv().Store().Channel().UpdateMemberNotifyProps(channelID, userID, filteredProps)
 	if err != nil {
 		var appErr *model.AppError

server/channels/app/notification.go

@@ -101,13 +101,15 @@ func (a *App) SendNotifications(c request.CTX, post *model.Post, team *model.Tea
 	}
 	channelMemberNotifyPropsMap := result.Data.(map[string]model.StringMap)
 
-	followers := make(model.StringArray, 0)
+	followers := make(model.StringSet, 0)
 	if tchan != nil {
 		result = <-tchan
 		if result.NErr != nil {
 			return nil, result.NErr
 		}
-		followers = result.Data.([]string)
+		for _, v := range result.Data.([]string) {
+			followers.Add(v)
+		}
 	}
 
 	groups := make(map[string]*model.Group)
@@ -235,6 +237,14 @@ func (a *App) SendNotifications(c request.CTX, post *model.Post, team *model.Tea
 			threadParticipants[id] = true
 		}
 
+		if channel.Type != model.ChannelTypeDirect {
+			for id, propsMap := range channelMemberNotifyPropsMap {
+				if ok := followers.Has(id); !ok && propsMap[model.ChannelAutoFollowThreads] == model.ChannelAutoFollowThreadsOn {
+					threadParticipants[id] = true
+				}
+			}
+		}
+
 		// sema is a counting semaphore to throttle the number of concurrent DB requests.
 		// A concurrency of 8 should be sufficient.
 		// We don't want to set a higher limit which can bring down the DB.
@@ -286,8 +296,8 @@ func (a *App) SendNotifications(c request.CTX, post *model.Post, team *model.Tea
 
 				followersMutex.Lock()
 				// add new followers to existing followers
-				if threadMembership.Following && !followers.Contains(userID) {
-					followers = append(followers, userID)
+				if ok := followers.Has(userID); !ok && threadMembership.Following {
+					followers.Add(userID)
 					newParticipants[userID] = true
 				}
 				followersMutex.Unlock()
@@ -330,7 +340,7 @@ func (a *App) SendNotifications(c request.CTX, post *model.Post, team *model.Tea
 
 	notificationsForCRT := &CRTNotifiers{}
 	if isCRTAllowed && post.RootId != "" {
-		for _, uid := range followers {
+		for uid := range followers {
 			profile := profileMap[uid]
 			if profile == nil || !a.IsCRTEnabledForUser(c, uid) {
 				continue
@@ -578,7 +588,7 @@ func (a *App) SendNotifications(c request.CTX, post *model.Post, team *model.Tea
 
 	// If this is a reply in a thread, notify participants
 	if isCRTAllowed && post.RootId != "" {
-		for _, uid := range followers {
+		for uid := range followers {
 			// A user following a thread but had left the channel won't get a notification
 			// https://mattermost.atlassian.net/browse/MM-36769
 			if profileMap[uid] == nil {

server/channels/app/notification_test.go

@@ -10,6 +10,7 @@ import (
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 
+	"github.com/mattermost/mattermost-server/server/v8/channels/store"
 	"github.com/mattermost/mattermost-server/server/v8/channels/utils"
 	"github.com/mattermost/mattermost-server/server/v8/model"
 	"github.com/mattermost/mattermost-server/server/v8/platform/shared/i18n"
@@ -2819,3 +2820,66 @@ func TestReplyPostNotificationsWithCRT(t *testing.T) {
 		assert.Nil(t, membership)
 	})
 }
+
+func TestChannelAutoFollowThreads(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+
+	u1 := th.BasicUser
+	u2 := th.BasicUser2
+	u3 := th.CreateUser()
+	th.LinkUserToTeam(u3, th.BasicTeam)
+	c1 := th.BasicChannel
+	th.AddUserToChannel(u2, c1)
+	th.AddUserToChannel(u3, c1)
+
+	// Set auto-follow for user 2
+	member, appErr := th.App.UpdateChannelMemberNotifyProps(th.Context, map[string]string{model.ChannelAutoFollowThreads: model.ChannelAutoFollowThreadsOn}, c1.Id, u2.Id)
+	require.Nil(t, appErr)
+	require.Equal(t, model.ChannelAutoFollowThreadsOn, member.NotifyProps[model.ChannelAutoFollowThreads])
+
+	rootPost := &model.Post{
+		ChannelId: c1.Id,
+		Message:   "root post by user3",
+		UserId:    u3.Id,
+	}
+	rpost, appErr := th.App.CreatePost(th.Context, rootPost, c1, false, true)
+	require.Nil(t, appErr)
+
+	replyPost1 := &model.Post{
+		ChannelId: c1.Id,
+		Message:   "reply post by user1",
+		UserId:    u1.Id,
+		RootId:    rpost.Id,
+	}
+	_, appErr = th.App.CreatePost(th.Context, replyPost1, c1, false, true)
+	require.Nil(t, appErr)
+
+	// user-2 starts auto-following thread
+	threadMembership, appErr := th.App.GetThreadMembershipForUser(u2.Id, rpost.Id)
+	require.Nil(t, appErr)
+	require.NotNil(t, threadMembership)
+	assert.True(t, threadMembership.Following)
+
+	// Set "following" to false
+	_, err := th.App.Srv().Store().Thread().MaintainMembership(u2.Id, rpost.Id, store.ThreadMembershipOpts{
+		Following:       false,
+		UpdateFollowing: true,
+	})
+	require.NoError(t, err)
+
+	replyPost2 := &model.Post{
+		ChannelId: c1.Id,
+		Message:   "reply post 2 by user1",
+		UserId:    u1.Id,
+		RootId:    rpost.Id,
+	}
+	_, appErr = th.App.CreatePost(th.Context, replyPost2, c1, false, true)
+	require.Nil(t, appErr)
+
+	// Do NOT start auto-following thread, once "un-followed"
+	threadMembership, appErr = th.App.GetThreadMembershipForUser(u2.Id, rpost.Id)
+	require.Nil(t, appErr)
+	require.NotNil(t, threadMembership)
+	assert.False(t, threadMembership.Following)
+}