plugin/forward: erase expired connections by timer (#1782)

* plugin/forward: erase expired connection by timer

 - in previous implementation, the expired connections resided in
   cache until new request to the same upstream/protocol came. In
   case if the upstream was unhealthy new request may come long time
   later or may not come at all. All this time expired connections
   held system resources (file descriptors, ephemeral ports). In my
   fix the expired connections and related resources are released
   by timer
 - decreased the complexity of taking connection from cache. The list
   of connections is treated as stack (LIFO queue), i.e. the connection
   is taken from the end of queue (the most fresh connection) and
   returned to the end (as it was implemented before). The remarkable
   thing is that all connections in the stack appear to be ordered by
   'used' field
 - the cleanup() method finds the first good (not expired) connection
   in stack with binary search, since all connections are ordered by
   'used' field

* fix race conditions

* minor enhancement

* add comments

plugin/forward/persistent.go

@@ -3,6 +3,7 @@ package forward
 import (
 	"crypto/tls"
 	"net"
+	"sort"
 	"time"
 
 	"github.com/miekg/dns"
@@ -37,7 +38,6 @@ func newTransport(addr string, tlsConfig *tls.Config) *transport {
 		ret:    make(chan *dns.Conn),
 		stop:   make(chan bool),
 	}
-	go func() { t.connManager() }()
 	return t
 }
 
@@ -53,28 +53,26 @@ func (t *transport) len() int {
 
 // connManagers manages the persistent connection cache for UDP and TCP.
 func (t *transport) connManager() {
-
+	ticker := time.NewTicker(t.expire)
 Wait:
 	for {
 		select {
 		case proto := <-t.dial:
-			// Yes O(n), shouldn't put millions in here. We walk all connection until we find the first
-			// one that is usuable.
-			i := 0
-			for i = 0; i < len(t.conns[proto]); i++ {
-				pc := t.conns[proto][i]
+			// take the last used conn - complexity O(1)
+			if stack := t.conns[proto]; len(stack) > 0 {
+				pc := stack[len(stack)-1]
 				if time.Since(pc.used) < t.expire {
 					// Found one, remove from pool and return this conn.
-					t.conns[proto] = t.conns[proto][i+1:]
+					t.conns[proto] = stack[:len(stack)-1]
 					t.ret <- pc.c
 					continue Wait
 				}
-				// This conn has expired. Close it.
-				pc.c.Close()
+				// clear entire cache if the last conn is expired
+				t.conns[proto] = nil
+				// now, the connections being passed to closeConns() are not reachable from
+				// transport methods anymore. So, it's safe to close them in a separate goroutine
+				go closeConns(stack)
 			}
-
-			// Not conns were found. Connect to the upstream to create one.
-			t.conns[proto] = t.conns[proto][i:]
 			SocketGauge.WithLabelValues(t.addr).Set(float64(t.len()))
 
 			t.ret <- nil
@@ -96,13 +94,53 @@ Wait:
 
 			t.conns["tcp-tls"] = append(t.conns["tcp-tls"], &persistConn{conn, time.Now()})
 
+		case <-ticker.C:
+			t.cleanup(false)
+
 		case <-t.stop:
+			t.cleanup(true)
 			close(t.ret)
 			return
 		}
 	}
 }
 
+// closeConns closes connections.
+func closeConns(conns []*persistConn) {
+	for _, pc := range conns {
+		pc.c.Close()
+	}
+}
+
+// cleanup removes connections from cache.
+func (t *transport) cleanup(all bool) {
+	staleTime := time.Now().Add(-t.expire)
+	for proto, stack := range t.conns {
+		if len(stack) == 0 {
+			continue
+		}
+		if all {
+			t.conns[proto] = nil
+			// now, the connections being passed to closeConns() are not reachable from
+			// transport methods anymore. So, it's safe to close them in a separate goroutine
+			go closeConns(stack)
+			continue
+		}
+		if stack[0].used.After(staleTime) {
+			continue
+		}
+
+		// connections in stack are sorted by "used"
+		good := sort.Search(len(stack), func(i int) bool {
+			return stack[i].used.After(staleTime)
+		})
+		t.conns[proto] = stack[good:]
+		// now, the connections being passed to closeConns() are not reachable from
+		// transport methods anymore. So, it's safe to close them in a separate goroutine
+		go closeConns(stack[:good])
+	}
+}
+
 // Dial dials the address configured in transport, potentially reusing a connection or creating a new one.
 func (t *transport) Dial(proto string) (*dns.Conn, bool, error) {
 	// If tls has been configured; use it.
@@ -128,6 +166,9 @@ func (t *transport) Dial(proto string) (*dns.Conn, bool, error) {
 // Yield return the connection to transport for reuse.
 func (t *transport) Yield(c *dns.Conn) { t.yield <- c }
 
+// Start starts the transport's connection manager.
+func (t *transport) Start() { go t.connManager() }
+
 // Stop stops the transport's connection manager.
 func (t *transport) Stop() { close(t.stop) }