[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/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)
+}