[release-10.11] Tighten thread membership cleanup on team membership changes (#37081)

* Tighten thread membership cleanup on team membership changes (#36764)

* Tighten thread membership cleanup on team membership changes

Ensure ThreadMembership rows are cleaned up when a user is removed
from or leaves a team, add a defense-in-depth filter on the thread
retrieval path so memberships referencing channels the user is no
longer a member of are excluded, and add a one-time migration to
clean up stale records on existing deployments.

https://mattermost.atlassian.net/browse/MM-69008

* Centralize per-channel membership removal and tighten tests

Extract the combined channel-member and thread-membership removal into
a shared helper used by both removeUserFromChannel and LeaveTeam, so
future code paths cannot revoke channel access without dropping the
dependent thread state. The channel-leave event is now logged after
the combined removal completes.

Also drop verbose test header comments and rename a test to a
behavior-focused name.

* Backfill channel members in thread storetest setups

The new ChannelMembers predicate on thread read queries filters out
ThreadMembership rows whose user has no ChannelMembers row for the
thread's channel. Several existing storetest setups bypassed the
normal write path and inserted threads/memberships without channel
members. Add the missing channel-member rows so the test data matches
the real-world invariant.

* Retrigger enterprise CI

Pick up enterprise merge e6953d4 (master into
MM-69008-thread-membership-team-leave) in the combined Enterprise
CI/tests lane, which pins the enterprise SHA at mattermost-side
dispatch time.

Co-authored-by: Maria A Nunez <maria.nunez@mattermost.com>

---------

Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Cursor Agent <cursoragent@cursor.com>

* Add MySQL migration for 000195_threadmemberships_cleanup_v2

Co-authored-by: Cursor <cursoragent@cursor.com>

* Apply pre-commit lint fixes

Co-authored-by: Cursor <cursoragent@cursor.com>

* Retrigger CI

Co-authored-by: Cursor <cursoragent@cursor.com>

---------

Co-authored-by: Mattermost Build <build@mattermost.com>
Co-authored-by: Cursor Agent <cursoragent@cursor.com>

server/channels/api4/user_test.go

@@ -7711,6 +7711,63 @@ func TestGetThreadsForUser(t *testing.T) {
 	})
 }
 
+func TestGetThreadsForUser_AfterTeamRemovalAndReinvite(t *testing.T) {
+	mainHelper.Parallel(t)
+	th := Setup(t).InitBasic()
+
+	th.App.UpdateConfig(func(cfg *model.Config) {
+		*cfg.ServiceSettings.ThreadAutoFollow = true
+		*cfg.ServiceSettings.CollapsedThreads = model.CollapsedThreadsDefaultOn
+	})
+	th.App.Srv().SetLicense(model.NewTestLicenseSKU(model.LicenseShortSkuProfessional))
+
+	admin := th.BasicUser
+	victim := th.BasicUser2
+
+	privateChannel := th.CreatePrivateChannel()
+	th.AddUserToChannel(victim, privateChannel)
+
+	defer func() {
+		require.NoError(t, th.App.Srv().Store().Post().PermanentDeleteByUser(th.Context, admin.Id))
+		require.NoError(t, th.App.Srv().Store().Post().PermanentDeleteByUser(th.Context, victim.Id))
+	}()
+
+	rootPost, _, err := th.Client.CreatePost(context.Background(), &model.Post{
+		ChannelId: privateChannel.Id,
+		Message:   "private team secret",
+	})
+	require.NoError(t, err)
+
+	victimClient := th.CreateClient()
+	th.LoginBasic2WithClient(victimClient)
+
+	_, _, err = victimClient.CreatePost(context.Background(), &model.Post{
+		ChannelId: privateChannel.Id,
+		RootId:    rootPost.Id,
+		Message:   "victim reply",
+	})
+	require.NoError(t, err)
+
+	uss, _, err := victimClient.GetUserThreads(context.Background(), victim.Id, th.BasicTeam.Id, model.GetUserThreadsOpts{Extended: true})
+	require.NoError(t, err)
+	require.Len(t, uss.Threads, 1, "sanity: victim should see their own thread before team removal")
+
+	_, err = th.SystemAdminClient.RemoveTeamMember(context.Background(), th.BasicTeam.Id, victim.Id)
+	require.NoError(t, err)
+
+	_, _, err = th.SystemAdminClient.AddTeamMember(context.Background(), th.BasicTeam.Id, victim.Id)
+	require.NoError(t, err)
+
+	th.LoginBasic2WithClient(victimClient)
+
+	uss, _, err = victimClient.GetUserThreads(context.Background(), victim.Id, th.BasicTeam.Id, model.GetUserThreadsOpts{Extended: true})
+	require.NoError(t, err)
+	for _, thr := range uss.Threads {
+		require.NotEqual(t, rootPost.Id, thr.PostId, "private-channel thread must not leak to re-invited user")
+	}
+	require.Len(t, uss.Threads, 0, "re-invited user must not receive any threads from private channels they no longer belong to")
+}
+
 func TestThreadSocketEvents(t *testing.T) {
 	mainHelper.Parallel(t)
 	th := Setup(t).InitBasic()