ABC-228 Update GetPosts caching to work for non-60 limits (#8233)

* Update GetPosts caching to work for non-60 limits

* Only cache on limits of 30/60 and add test

* Add comments clarifying 30 and 60 limits

store/sqlstore/post_store.go

@@ -322,7 +322,10 @@ type etagPosts struct {
 
 func (s SqlPostStore) InvalidateLastPostTimeCache(channelId string) {
 	lastPostTimeCache.Remove(channelId)
-	lastPostsCache.Remove(channelId)
+
+	// Keys are "{channelid}{limit}" and caching only occurs on limits of 30 and 60
+	lastPostsCache.Remove(channelId + "30")
+	lastPostsCache.Remove(channelId + "60")
 }
 
 func (s SqlPostStore) GetEtag(channelId string, allowFromCache bool) store.StoreChannel {
@@ -439,8 +442,9 @@ func (s SqlPostStore) GetPosts(channelId string, offset int, limit int, allowFro
 			return
 		}
 
-		if allowFromCache && offset == 0 && limit == 60 {
-			if cacheItem, ok := lastPostsCache.Get(channelId); ok {
+		// Caching only occurs on limits of 30 and 60, the common limits requested by MM clients
+		if allowFromCache && offset == 0 && (limit == 60 || limit == 30) {
+			if cacheItem, ok := lastPostsCache.Get(fmt.Sprintf("%s%v", channelId, limit)); ok {
 				if s.metrics != nil {
 					s.metrics.IncrementMemCacheHitCounter("Last Posts Cache")
 				}
@@ -482,8 +486,9 @@ func (s SqlPostStore) GetPosts(channelId string, offset int, limit int, allowFro
 
 			list.MakeNonNil()
 
-			if offset == 0 && limit == 60 {
-				lastPostsCache.AddWithExpiresInSecs(channelId, list, LAST_POSTS_CACHE_SEC)
+			// Caching only occurs on limits of 30 and 60, the common limits requested by MM clients
+			if offset == 0 && (limit == 60 || limit == 30) {
+				lastPostsCache.AddWithExpiresInSecs(fmt.Sprintf("%s%v", channelId, limit), list, LAST_POSTS_CACHE_SEC)
 			}
 
 			result.Data = list