fix connection full bug

server.go

@@ -172,7 +172,21 @@ func (server *mongoServer) acquireSocketInternal(
 					}()
 				}
 				timeSpentWaiting := time.Duration(0)
-				for len(server.liveSockets)-len(server.unusedSockets) >= poolLimit && !timeoutHit {
+
+				/*
+				 * @vinllen modification.
+				 * Before:
+				 *     len(server.liveSockets)-len(server.unusedSockets) >= poolLimit
+				 * Explanation:
+				 *     This is a bug that may cause the connection full. "liveSockets" means
+				 *     unused connections and used connections, "unusedSockets" means unused connections.
+				 *     However, a connection won't be released even if it's an "unused" connection.
+				 *     So this comparision only restrict the used connections but not the total connections.
+				 * After:
+				 *     len(server.liveSockets) >= poolLimit && len(server.unusedSockets) == 0
+				 */
+				// for len(server.liveSockets)-len(server.unusedSockets) >= poolLimit && !timeoutHit {
+				for len(server.liveSockets) >= poolLimit && len(server.unusedSockets) == 0 && !timeoutHit {
 					// We only count time spent in Wait(), and not time evaluating the entire loop,
 					// so that in the happy non-blocking path where the condition above evaluates true
 					// first time, we record a nice round zero wait time.
@@ -191,14 +205,18 @@ func (server *mongoServer) acquireSocketInternal(
 				// Record that we fetched a connection of of a socket list and how long we spent waiting
 				stats.noticeSocketAcquisition(timeSpentWaiting)
 			} else {
-				if len(server.liveSockets)-len(server.unusedSockets) >= poolLimit {
+				// no socket available
+				// if len(server.liveSockets)-len(server.unusedSockets) >= poolLimit {
+				if len(server.liveSockets) >= poolLimit && len(server.unusedSockets) == 0 {
 					server.Unlock()
 					return nil, false, errPoolLimit
 				}
 			}
 		}
+
 		n := len(server.unusedSockets)
 		if n > 0 {
+			// use unused socket
 			socket = server.unusedSockets[n-1]
 			server.unusedSockets[n-1] = nil // Help GC.
 			server.unusedSockets = server.unusedSockets[:n-1]
@@ -209,6 +227,7 @@ func (server *mongoServer) acquireSocketInternal(
 				continue
 			}
 		} else {
+			// build new one
 			server.Unlock()
 			socket, err = server.Connect(timeout)
 			if err == nil {
@@ -464,12 +483,16 @@ func (server *mongoServer) poolShrinker() {
 			end++
 			reclaimMap[s] = struct{}{}
 		}
+
+		// elements before index "end" are expired
+
 		tbr := server.unusedSockets[:end]
 		if end > 0 {
 			next := make([]*mongoSocket, unused-end)
 			copy(next, server.unusedSockets[end:])
 			server.unusedSockets = next
 			remainSockets := []*mongoSocket{}
+			// remove unused sockets from liveSockets
 			for _, s := range server.liveSockets {
 				if _, ok := reclaimMap[s]; !ok {
 					remainSockets = append(remainSockets, s)