From 12dc171a606e20be2d10b7d7f2fad764a89b726c Mon Sep 17 00:00:00 2001
From: Agniva De Sarker <agnivade@yahoo.co.in>
Date: Wed, 17 Nov 2021 20:49:40 +0530
Subject: [PATCH] MM-39341: Sentry crash: nil dereference in
 *User.ClearNonProfileFields (#18966)

In some cases, an invalid participant id like an empty string
might creep into the database. It's not exactly clear
how can this happen, but if it does, then it breaks the logic
of assuming that the users slice will contain all users
in the participants slice.

To prevent this, we check if a match was found before
adding it in the slice.

https://community-daily.mattermost.com/boards/workspace/zyoahc9uapdn3xdptac6jb69ic/285b80a3-257d-41f6-8cf4-ed80ca9d92e5/495cdb4d-c13a-4992-8eb9-80cfee2819a4/7sixm4t7c6rsr3d8ydxbtfgy9gr

```release-note
NONE
```
---
 store/sqlstore/thread_store.go  |  4 +++-
 store/storetest/thread_store.go | 19 +++++++++++++++++++
 2 files changed, 22 insertions(+), 1 deletion(-)

diff --git a/store/sqlstore/thread_store.go b/store/sqlstore/thread_store.go
index 9592a1eee9..afb16b77c3 100644
--- a/store/sqlstore/thread_store.go
+++ b/store/sqlstore/thread_store.go
@@ -460,7 +460,9 @@ func (s *SqlThreadStore) GetThreadForUser(teamId string, threadMembership *model
 				break
 			}
 		}
-		participants = append(participants, participant)
+		if participant != nil {
+			participants = append(participants, participant)
+		}
 	}
 
 	result := &model.ThreadResponse{
diff --git a/store/storetest/thread_store.go b/store/storetest/thread_store.go
index 1feaf4e45e..017acd5dbf 100644
--- a/store/storetest/thread_store.go
+++ b/store/storetest/thread_store.go
@@ -451,6 +451,25 @@ func testThreadStorePopulation(t *testing.T, ss store.Store) {
 		require.NoError(t, err)
 		require.Equal(t, int64(0), th.UnreadReplies)
 	})
+
+	t.Run("Empty participantID should not appear in thread response", func(t *testing.T) {
+		newPosts := makeSomePosts()
+		opts := store.ThreadMembershipOpts{
+			Following:             true,
+			IncrementMentions:     false,
+			UpdateFollowing:       true,
+			UpdateViewedTimestamp: false,
+			UpdateParticipants:    true,
+		}
+		m, err := ss.Thread().MaintainMembership("", newPosts[0].Id, opts)
+		require.NoError(t, err)
+		m.UserId = newPosts[0].UserId
+		th, err := ss.Thread().GetThreadForUser("", m, true)
+		require.NoError(t, err)
+		for _, user := range th.Participants {
+			require.NotNil(t, user)
+		}
+	})
 }
 
 func testThreadSQLOperations(t *testing.T, ss store.Store, s SqlStore) {