Fix issue where a member is forced to leave a room when the invite is marked deleted (#2839)

Proposed fix for issue:
https://github.com/matrix-org/dendrite/issues/2838

Suppose bob received invites to spaceA and spaceB.
When Bob joins spaceA, we add an OutputEvent event to retire the invite.
This sets the invite to "deleted" in the database. This makes sense.

The bug is in stream_invites.go. Triggered when bob received a new
invite for spaceB, and does a client sync.

In the block (line 76)
`for roomID := range retiredInvites

   if _, ok := req.Response.Rooms.Invite[roomID]; ok {
	continue
   }

   if _, ok := req.Response.Rooms.Join[roomID]; ok {
	continue
  }
...
` 

Bob is not in either maps even though he had just accepted the invite
for spaceA. Consequently, the spaceA invite is treated as a retired
invite, and a membership Leave event is generated. What bob sees is that
after accepting the invite to spaceB, he lose access to spaceA.


### Pull Request Checklist

<!-- Please read
https://matrix-org.github.io/dendrite/development/contributing before
submitting your pull request -->

* [ ] I have added tests for PR _or_ I have justified why this PR
doesn't need tests.
* [x ] Pull request includes a [sign off below using a legally
identifiable
name](https://matrix-org.github.io/dendrite/development/contributing#sign-off)
_or_ I have already signed off privately

Signed-off-by: `Tak Wai Wong <tak@hntlabs.com>`

Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>

1 files changed, 7 insertions, 5 deletions

diff --git a/syncapi/streams/stream_invite.go b/syncapi/streams/stream_invite.go
index 700f25c1..e4de30e1 100644
--- a/syncapi/streams/stream_invite.go
+++ b/syncapi/streams/stream_invite.go
@@ -4,6 +4,7 @@ import (
 	"context"
 	"crypto/sha256"
 	"encoding/base64"
+	"math"
 	"strconv"
 	"time"
 
@@ -74,12 +75,14 @@ func (p *InviteStreamProvider) IncrementalSync(
 		return to
 	}
 	for roomID := range retiredInvites {
-		if _, ok := req.Response.Rooms.Invite[roomID]; ok {
-			continue
-		}
-		if _, ok := req.Response.Rooms.Join[roomID]; ok {
+		membership, _, err := snapshot.SelectMembershipForUser(ctx, roomID, req.Device.UserID, math.MaxInt64)
+		// Skip if the user is an existing member of the room.
+		// Otherwise, the NewLeaveResponse will eject the user from the room unintentionally
+		if membership == gomatrixserverlib.Join ||
+			err != nil {
 			continue
 		}
+
 		lr := types.NewLeaveResponse()
 		h := sha256.Sum256(append([]byte(roomID), []byte(strconv.FormatInt(int64(to), 10))...))
 		lr.Timeline.Events = append(lr.Timeline.Events, gomatrixserverlib.ClientEvent{
@@ -93,7 +96,6 @@ func (p *InviteStreamProvider) IncrementalSync(
 			Content:        gomatrixserverlib.RawJSON(`{"membership":"leave"}`),
 		})
 		req.Response.Rooms.Leave[roomID] = lr
-
 	}
 
 	return maxID