enable shrink the socket pool size

we found the mgo will allocate the pool size during burst traffic but won't
close the sockets any more until restart the client or server.

And the mongo document defines two related query options

- [minPoolSize](https://docs.mongodb.com/manual/reference/connection-string/#urioption.minPoolSize)
- [maxIdleTimeMS](https://docs.mongodb.com/manual/reference/connection-string/#urioption.maxIdleTimeMS)

By implementing these two options, it could shrink the pool to minPoolSize after
the sockets introduced by burst traffic timeout.

The idea comes from https://github.com/JodeZer/mgo , he investigated
this issue and provide the initial commits.

I found there are still some issue in sockets maintenance, and had a PR against
his repo JodeZer#1 .

This commit include JodeZer's commits and my fix, and I simplified the data structure.
What's in this commit could be described as this figure:

+------------------------+
| Session | <-------+ Add options here
+------------------------+

+------------------------+
| Cluster | <-------+ Add options here
+------------------------+

+------------------------+
| Server | <-------+*Add options here
| | *add timestamp when recycle a socket +---+
| +-----------+ | +---+ *periodically check the unused sockets |
| | shrinker <------+ and reclaim the timeout sockets. +---+
| +-----------+ | |
| | |
+------------------------+ |
|
+------------------------+ |
| Socket | <-------+ Add a field for last used times+---------+
+------------------------+

Signed-off-by: Wang Xu <gnawux@gmail.com>

cluster.go

@@ -48,21 +48,23 @@ import (
 
 type mongoCluster struct {
 	sync.RWMutex
-	serverSynced sync.Cond
-	userSeeds    []string
-	dynaSeeds    []string
-	servers      mongoServers
-	masters      mongoServers
-	references   int
-	syncing      bool
-	direct       bool
-	failFast     bool
-	syncCount    uint
-	setName      string
-	cachedIndex  map[string]bool
-	sync         chan bool
-	dial         dialer
-	appName      string
+	serverSynced  sync.Cond
+	userSeeds     []string
+	dynaSeeds     []string
+	servers       mongoServers
+	masters       mongoServers
+	references    int
+	syncing       bool
+	direct        bool
+	failFast      bool
+	syncCount     uint
+	setName       string
+	cachedIndex   map[string]bool
+	sync          chan bool
+	dial          dialer
+	appName       string
+	minPoolSize   int
+	maxIdleTimeMS int
 }
 
 func newCluster(userSeeds []string, direct, failFast bool, dial dialer, setName string, appName string) *mongoCluster {
@@ -437,11 +439,13 @@ func (cluster *mongoCluster) syncServersLoop() {
 func (cluster *mongoCluster) server(addr string, tcpaddr *net.TCPAddr) *mongoServer {
 	cluster.RLock()
 	server := cluster.servers.Search(tcpaddr.String())
+	minPoolSize := cluster.minPoolSize
+	maxIdleTimeMS := cluster.maxIdleTimeMS
 	cluster.RUnlock()
 	if server != nil {
 		return server
 	}
-	return newServer(addr, tcpaddr, cluster.sync, cluster.dial)
+	return newServer(addr, tcpaddr, cluster.sync, cluster.dial, minPoolSize, maxIdleTimeMS)
 }
 
 func resolveAddr(addr string) (*net.TCPAddr, error) {

server.go

@@ -55,6 +55,8 @@ type mongoServer struct {
 	pingCount     uint32
 	closed        bool
 	abended       bool
+	minPoolSize   int
+	maxIdleTimeMS int
 }
 
 type dialer struct {
@@ -76,17 +78,22 @@ type mongoServerInfo struct {
 
 var defaultServerInfo mongoServerInfo
 
-func newServer(addr string, tcpaddr *net.TCPAddr, sync chan bool, dial dialer) *mongoServer {
+func newServer(addr string, tcpaddr *net.TCPAddr, sync chan bool, dial dialer, minPoolSize, maxIdleTimeMS int) *mongoServer {
 	server := &mongoServer{
-		Addr:         addr,
-		ResolvedAddr: tcpaddr.String(),
-		tcpaddr:      tcpaddr,
-		sync:         sync,
-		dial:         dial,
-		info:         &defaultServerInfo,
-		pingValue:    time.Hour, // Push it back before an actual ping.
+		Addr:          addr,
+		ResolvedAddr:  tcpaddr.String(),
+		tcpaddr:       tcpaddr,
+		sync:          sync,
+		dial:          dial,
+		info:          &defaultServerInfo,
+		pingValue:     time.Hour, // Push it back before an actual ping.
+		minPoolSize:   minPoolSize,
+		maxIdleTimeMS: maxIdleTimeMS,
 	}
 	go server.pinger(true)
+	if maxIdleTimeMS != 0 {
+		go server.poolShrinker()
+	}
 	return server
 }
 
@@ -221,6 +228,7 @@ func (server *mongoServer) close(waitForIdle bool) {
 func (server *mongoServer) RecycleSocket(socket *mongoSocket) {
 	server.Lock()
 	if !server.closed {
+		socket.lastTimeUsed = time.Now()
 		server.unusedSockets = append(server.unusedSockets, socket)
 	}
 	server.Unlock()
@@ -346,6 +354,53 @@ func (server *mongoServer) pinger(loop bool) {
 	}
 }
 
+func (server *mongoServer) poolShrinker() {
+	ticker := time.NewTicker(1 * time.Minute)
+	for _ = range ticker.C {
+		if server.closed {
+			ticker.Stop()
+			return
+		}
+		server.Lock()
+		unused := len(server.unusedSockets)
+		if unused < server.minPoolSize {
+			server.Unlock()
+			continue
+		}
+		now := time.Now()
+		end := 0
+		reclaimMap := map[*mongoSocket]struct{}{}
+		// Because the acquirision and recycle are done at the tail of array,
+		// the head is always the oldest unused socket.
+		for _, s := range server.unusedSockets[:unused-server.minPoolSize] {
+			if s.lastTimeUsed.Add(time.Duration(server.maxIdleTimeMS) * time.Millisecond).After(now) {
+				break
+			}
+			end++
+			reclaimMap[s] = struct{}{}
+		}
+		tbr := server.unusedSockets[:end]
+		if end > 0 {
+			next := make([]*mongoSocket, unused-end)
+			copy(next, server.unusedSockets[end:])
+			server.unusedSockets = next
+			remainSockets := []*mongoSocket{}
+			for _, s := range server.liveSockets {
+				if _, ok := reclaimMap[s]; !ok {
+					remainSockets = append(remainSockets, s)
+				}
+			}
+			server.liveSockets = remainSockets
+			stats.conn(-1*end, server.info.Master)
+		}
+		server.Unlock()
+
+		for _, s := range tbr {
+			s.Close()
+		}
+	}
+}
+
 type mongoServerSlice []*mongoServer
 
 func (s mongoServerSlice) Len() int {

session.go

@@ -271,6 +271,15 @@ const (
 //        Defines the per-server socket pool limit. Defaults to 4096.
 //        See Session.SetPoolLimit for details.
 //
+//     minPoolSize=<limit>
+//
+//        Defines the per-server socket pool minium size. Defaults to 0.
+//
+//     maxIdleTimeMS=<millisecond>
+//
+//        The maximum number of milliseconds that a connection can remain idle in the pool
+//        before being removed and closed.
+//
 //     appName=<appName>
 //
 //        The identifier of this client application. This parameter is used to
@@ -322,6 +331,8 @@ func ParseURL(url string) (*DialInfo, error) {
 	appName := ""
 	readPreferenceMode := Primary
 	var readPreferenceTagSets []bson.D
+	minPoolSize := 0
+	maxIdleTimeMS := 0
 	for _, opt := range uinfo.options {
 		switch opt.key {
 		case "authSource":
@@ -368,6 +379,22 @@ func ParseURL(url string) (*DialInfo, error) {
 				doc = append(doc, bson.DocElem{Name: strings.TrimSpace(kvp[0]), Value: strings.TrimSpace(kvp[1])})
 			}
 			readPreferenceTagSets = append(readPreferenceTagSets, doc)
+		case "minPoolSize":
+			minPoolSize, err = strconv.Atoi(opt.value)
+			if err != nil {
+				return nil, errors.New("bad value for minPoolSize: " + opt.value)
+			}
+			if minPoolSize < 0 {
+				return nil, errors.New("bad value (negtive) for minPoolSize: " + opt.value)
+			}
+		case "maxIdleTimeMS":
+			maxIdleTimeMS, err = strconv.Atoi(opt.value)
+			if err != nil {
+				return nil, errors.New("bad value for maxIdleTimeMS: " + opt.value)
+			}
+			if maxIdleTimeMS < 0 {
+				return nil, errors.New("bad value (negtive) for maxIdleTimeMS: " + opt.value)
+			}
 		case "connect":
 			if opt.value == "direct" {
 				direct = true
@@ -402,6 +429,8 @@ func ParseURL(url string) (*DialInfo, error) {
 			TagSets: readPreferenceTagSets,
 		},
 		ReplicaSetName: setName,
+		MinPoolSize:    minPoolSize,
+		MaxIdleTimeMS:  maxIdleTimeMS,
 	}
 	return &info, nil
 }
@@ -475,6 +504,14 @@ type DialInfo struct {
 	// cluster and establish connections with further servers too.
 	Direct bool
 
+	// MinPoolSize defines The minimum number of connections in the connection pool.
+	// Defaults to 0.
+	MinPoolSize int
+
+	//The maximum number of milliseconds that a connection can remain idle in the pool
+	// before being removed and closed.
+	MaxIdleTimeMS int
+
 	// DialServer optionally specifies the dial function for establishing
 	// connections with the MongoDB servers.
 	DialServer func(addr *ServerAddr) (net.Conn, error)
@@ -554,6 +591,10 @@ func DialWithInfo(info *DialInfo) (*Session, error) {
 	if info.PoolLimit > 0 {
 		session.poolLimit = info.PoolLimit
 	}
+
+	cluster.minPoolSize = info.MinPoolSize
+	cluster.maxIdleTimeMS = info.MaxIdleTimeMS
+
 	cluster.Release()
 
 	// People get confused when we return a session that is not actually

socket.go

@@ -54,6 +54,7 @@ type mongoSocket struct {
 	dead           error
 	serverInfo     *mongoServerInfo
 	closeAfterIdle bool
+	lastTimeUsed   time.Time // for time based idle socket release
 	sendMeta       sync.Once
 }