MM-60606: Respect allowFromCache flag in Channelstore.GetMany (#28290)

Setting the flag to false never worked, and it was never
caught because there wasn't an instance when this method
was called with allowFromCache=false.

https://mattermost.atlassian.net/browse/MM-60606

```release-note
NONE
```

server/channels/store/localcachelayer/channel_layer.go

@@ -211,7 +211,6 @@ func (s LocalCacheChannelStore) GetPinnedPostCount(channelId string, allowFromCa
 	}
 
 	count, err := s.ChannelStore.GetPinnedPostCount(channelId, allowFromCache)
-
 	if err != nil {
 		return 0, err
 	}
@@ -244,22 +243,24 @@ func (s LocalCacheChannelStore) GetMany(ids []string, allowFromCache bool) (mode
 	var foundChannels []*model.Channel
 	var channelsToQuery []string
 
-	if allowFromCache {
-		toPass := allocateCacheTargets[*model.Channel](len(ids))
-		errs := s.rootStore.doMultiReadCache(s.rootStore.roleCache, ids, toPass)
-		for i, err := range errs {
-			if err != nil {
-				if err != cache.ErrKeyNotFound {
-					s.rootStore.logger.Warn("Error in Channelstore.GetMany: ", mlog.Err(err))
-				}
-				channelsToQuery = append(channelsToQuery, ids[i])
+	if !allowFromCache {
+		return s.ChannelStore.GetMany(ids, allowFromCache)
+	}
+
+	toPass := allocateCacheTargets[*model.Channel](len(ids))
+	errs := s.rootStore.doMultiReadCache(s.rootStore.roleCache, ids, toPass)
+	for i, err := range errs {
+		if err != nil {
+			if err != cache.ErrKeyNotFound {
+				s.rootStore.logger.Warn("Error in Channelstore.GetMany: ", mlog.Err(err))
+			}
+			channelsToQuery = append(channelsToQuery, ids[i])
+		} else {
+			gotChannel := *(toPass[i].(**model.Channel))
+			if gotChannel != nil {
+				foundChannels = append(foundChannels, gotChannel)
 			} else {
-				gotChannel := *(toPass[i].(**model.Channel))
-				if gotChannel != nil {
-					foundChannels = append(foundChannels, gotChannel)
-				} else {
-					s.rootStore.logger.Warn("Found nil channel in GetMany. This is not expected")
-				}
+				s.rootStore.logger.Warn("Found nil channel in GetMany. This is not expected")
 			}
 		}
 	}

server/channels/store/localcachelayer/channel_layer_test.go

@@ -391,6 +391,19 @@ func TestChannelStoreGetManyCache(t *testing.T) {
 		assert.ElementsMatch(t, model.ChannelList{&fakeChannel, &fakeChannel2}, channels)
 		mockStore.Channel().(*mocks.ChannelStore).AssertNumberOfCalls(t, "GetMany", 2)
 	})
+
+	t.Run("passing allowCache=false should bypass cache", func(t *testing.T) {
+		mockStore := getMockStore(t)
+		mockCacheProvider := getMockCacheProvider()
+		cachedStore, err := NewLocalCacheLayer(mockStore, nil, nil, mockCacheProvider, logger)
+		require.NoError(t, err)
+
+		fakeChannel := model.Channel{Id: "channel1", Name: "channel1-name"}
+		channels, err := cachedStore.Channel().GetMany([]string{fakeChannel.Id}, false)
+		require.NoError(t, err)
+		assert.ElementsMatch(t, model.ChannelList{&fakeChannel}, channels)
+		mockStore.Channel().(*mocks.ChannelStore).AssertNumberOfCalls(t, "GetMany", 1)
+	})
 }
 
 func TestChannelStoreGetByNamesCache(t *testing.T) {

server/channels/store/localcachelayer/main_test.go

@@ -104,6 +104,7 @@ func getMockStore(t *testing.T) *mocks.Store {
 	mockChannelStore.On("Get", channelId, true).Return(&fakeChannel1, nil)
 	mockChannelStore.On("Get", channelId, false).Return(&fakeChannel1, nil)
 	mockChannelStore.On("GetMany", []string{channelId}, true).Return(model.ChannelList{&fakeChannel1}, nil)
+	mockChannelStore.On("GetMany", []string{channelId}, false).Return(model.ChannelList{&fakeChannel1}, nil)
 	mockChannelStore.On("GetMany", []string{fakeChannel2.Id}, true).Return(model.ChannelList{&fakeChannel2}, nil)
 	mockChannelStore.On("GetByNames", "team1", []string{fakeChannel1.Name}, true).Return([]*model.Channel{&fakeChannel1}, nil)
 	mockChannelStore.On("GetByNames", "team1", []string{fakeChannel2.Name}, true).Return([]*model.Channel{&fakeChannel2}, nil)