From e295c31ab722325945192645db28f5182359e0f3 Mon Sep 17 00:00:00 2001
From: Kegan Dougal <7190048+kegsay@users.noreply.github.com>
Date: Mon, 20 May 2024 13:57:24 +0100
Subject: [PATCH 1/2] Ensure null is not sent when we mean []

---
 sync3/extensions/e2ee.go | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/sync3/extensions/e2ee.go b/sync3/extensions/e2ee.go
index 01711d6a..1dbcad45 100644
--- a/sync3/extensions/e2ee.go
+++ b/sync3/extensions/e2ee.go
@@ -70,6 +70,12 @@ func (r *E2EERequest) ProcessInitial(ctx context.Context, res *Response, extCtx
 		extRes.OTKCounts = dd.OTKCounts
 		hasUpdates = true
 	}
+	if dd.DeviceListChanged == nil {
+		dd.DeviceListChanged = make([]string, 0)
+	}
+	if dd.DeviceListLeft == nil {
+		dd.DeviceListLeft = make([]string, 0)
+	}
 	if len(dd.DeviceListChanged) > 0 || len(dd.DeviceListLeft) > 0 {
 		extRes.DeviceLists = &E2EEDeviceList{
 			Changed: dd.DeviceListChanged,

From 451f6fb9c877af0b79fa18b6640438c594448b3f Mon Sep 17 00:00:00 2001
From: Kegan Dougal <7190048+kegsay@users.noreply.github.com>
Date: Mon, 20 May 2024 14:02:29 +0100
Subject: [PATCH 2/2] Add regression test

---
 tests-integration/extensions_test.go | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/tests-integration/extensions_test.go b/tests-integration/extensions_test.go
index b2f271f7..ed25a074 100644
--- a/tests-integration/extensions_test.go
+++ b/tests-integration/extensions_test.go
@@ -235,6 +235,32 @@ func TestExtensionE2EE(t *testing.T) {
 	})
 	m.MatchResponse(t, res, m.MatchDeviceLists(wantChanged, wantLeft))
 
+	// check that empty lists aren't serialised as null
+	v2.queueResponse(alice, sync2.SyncResponse{
+		DeviceLists: struct {
+			Changed []string `json:"changed,omitempty"`
+			Left    []string `json:"left,omitempty"`
+		}{
+			Changed: wantChanged,
+		},
+	})
+	v2.waitUntilEmpty(t, alice)
+	res = v3.mustDoV3RequestWithPos(t, aliceToken, res.Pos, sync3.Request{
+		Lists: map[string]sync3.RequestList{"a": {
+			Ranges: sync3.SliceRanges{
+				[2]int64{0, 10}, // doesn't matter
+			},
+		}},
+		// enable the E2EE extension
+		Extensions: extensions.Request{
+			E2EE: &extensions.E2EERequest{
+				Core: extensions.Core{Enabled: &boolTrue},
+			},
+		},
+	})
+	if res.Extensions.E2EE.DeviceLists.Left == nil {
+		t.Errorf("left array should be [] not null")
+	}
 }
 
 // Checks that to-device messages are passed from v2 to v3