net/http: add MaxConnLifespan to Transport

Existing implementation appeared to have no way to set a connection
max lifetime. A max lifetime allows for refreshing connection lifecycle
concerns such as dns resolutions for those that need it.

When initialized to a non-zero value the connection will be closed after
the duration has passed. This change is backwards compatible.

Fixes #23427

src/net/http/transport.go

@@ -189,6 +189,10 @@ type Transport struct {
 	// uncompressed.
 	DisableCompression bool
 
+	// MaxConnLifespan controls how long a connection is allowed
+	// to be reused before it must be closed. Zero means no limit.
+	MaxConnLifespan time.Duration
+
 	// MaxIdleConns controls the maximum number of idle (keep-alive)
 	// connections across all hosts. Zero means no limit.
 	MaxIdleConns int
@@ -983,14 +987,22 @@ func (t *Transport) tryPutIdleConn(pconn *persistConn) error {
 		t.removeIdleConnLocked(oldest)
 	}
 
+	ttl, hasTtl := pconn.timeToLive()
+
 	// Set idle timer, but only for HTTP/1 (pconn.alt == nil).
 	// The HTTP/2 implementation manages the idle timer itself
 	// (see idleConnTimeout in h2_bundle.go).
-	if t.IdleConnTimeout > 0 && pconn.alt == nil {
+	if (hasTtl || t.IdleConnTimeout > 0) && pconn.alt == nil {
+
+		timeout := t.IdleConnTimeout
+		if hasTtl && ttl < timeout {
+			timeout = ttl
+		}
+
 		if pconn.idleTimer != nil {
-			pconn.idleTimer.Reset(t.IdleConnTimeout)
+			pconn.idleTimer.Reset(timeout)
 		} else {
-			pconn.idleTimer = time.AfterFunc(t.IdleConnTimeout, pconn.closeConnIfStillIdle)
+			pconn.idleTimer = time.AfterFunc(timeout, pconn.closeConnIfStillIdle)
 		}
 	}
 	pconn.idleAt = time.Now()
@@ -1020,9 +1032,10 @@ func (t *Transport) queueForIdleConn(w *wantConn) (delivered bool) {
 	// If IdleConnTimeout is set, calculate the oldest
 	// persistConn.idleAt time we're willing to use a cached idle
 	// conn.
+	now := time.Now()
 	var oldTime time.Time
 	if t.IdleConnTimeout > 0 {
-		oldTime = time.Now().Add(-t.IdleConnTimeout)
+		oldTime = now.Add(-t.IdleConnTimeout)
 	}
 
 	// Look for most recently-used idle connection.
@@ -1035,7 +1048,8 @@ func (t *Transport) queueForIdleConn(w *wantConn) (delivered bool) {
 			// See whether this connection has been idle too long, considering
 			// only the wall time (the Round(0)), in case this is a laptop or VM
 			// coming out of suspend with previously cached idle connections.
-			tooOld := !oldTime.IsZero() && pconn.idleAt.Round(0).Before(oldTime)
+			tooOld := (!oldTime.IsZero() && pconn.idleAt.Round(0).Before(oldTime)) || (!pconn.reuseDeadline.IsZero() && pconn.reuseDeadline.Round(0).Before(now))
+
 			if tooOld {
 				// Async cleanup. Launch in its own goroutine (as if a
 				// time.AfterFunc called it); it acquires idleMu, which we're
@@ -1616,6 +1630,11 @@ func (t *Transport) dialConn(ctx context.Context, cm connectMethod) (pconn *pers
 		}
 	}
 
+	var reuseDeadline time.Time
+	if t.MaxConnLifespan > 0 {
+		reuseDeadline = time.Now().Add(t.MaxConnLifespan)
+	}
+
 	// Proxy setup.
 	switch {
 	case cm.proxyURL == nil:
@@ -1736,10 +1755,11 @@ func (t *Transport) dialConn(ctx context.Context, cm connectMethod) (pconn *pers
 				// pconn.conn was closed by next (http2configureTransports.upgradeFn).
 				return nil, e.RoundTripErr()
 			}
-			return &persistConn{t: t, cacheKey: pconn.cacheKey, alt: alt}, nil
+			return &persistConn{t: t, cacheKey: pconn.cacheKey, alt: alt, reuseDeadline: reuseDeadline}, nil
 		}
 	}
 
+	pconn.reuseDeadline = reuseDeadline
 	pconn.br = bufio.NewReaderSize(pconn, t.readBufferSize())
 	pconn.bw = bufio.NewWriterSize(persistConnWriter{pconn}, t.writeBufferSize())
 
@@ -1892,6 +1912,8 @@ type persistConn struct {
 
 	writeLoopDone chan struct{} // closed when write loop ends
 
+	reuseDeadline time.Time // time when this connection can no longer be reused
+
 	// Both guarded by Transport.idleMu:
 	idleAt    time.Time   // time it last become idle
 	idleTimer *time.Timer // holding an AfterFunc to close it
@@ -1908,6 +1930,30 @@ type persistConn struct {
 	mutateHeaderFunc func(Header)
 }
 
+// timeToLive checks if a persistent connection has been initialized
+// from a transport with MaxConnLifespan > 0 and returns the time
+// remaining for this connection to be reusable. The second response
+// would be true in this case.
+//
+// If the connection has a zero-value reuseDeadline set then
+// it returns (0, false)
+//
+// The returned duration will never be less than zero and the connection's
+// idle time is NOT taken into account.
+func (pc *persistConn) timeToLive() (time.Duration, bool) {
+
+	if pc.reuseDeadline.IsZero() {
+		return 0, false
+	}
+
+	ttl := time.Until(pc.reuseDeadline)
+	if ttl < 0 {
+		return 0, true
+	}
+
+	return ttl, true
+}
+
 func (pc *persistConn) maxHeaderResponseSize() int64 {
 	if v := pc.t.MaxResponseHeaderBytes; v != 0 {
 		return v

src/net/http/transport_test.go

@@ -4895,6 +4895,70 @@ func TestTransportMaxIdleConns(t *testing.T) {
 	}
 }
 
+func TestTransportMaxConnLifespan_h1(t *testing.T) { testTransportMaxConnLifespan(t, h1Mode) }
+func TestTransportMaxConnLifespan_h2(t *testing.T) { testTransportMaxConnLifespan(t, h2Mode) }
+func testTransportMaxConnLifespan(t *testing.T) {
+	if testing.Short() {
+		t.Skip("skipping in short mode")
+	}
+	defer afterTest(t)
+
+	const timeout = 1 * time.Second
+
+	cst := newClientServerTest(t, h2, HandlerFunc(func(w ResponseWriter, r *Request) {
+		// No body for convenience.
+	}))
+	defer cst.close()
+	tr := cst.tr
+	tr.MaxConnLifespan = timeout
+	tr.IdleConnTimeout = timeout * 3
+	defer tr.CloseIdleConnections()
+	c := &Client{Transport: tr}
+
+	idleConns := func() []string {
+		if h2 {
+			return tr.IdleConnStrsForTesting_h2()
+		} else {
+			return tr.IdleConnStrsForTesting()
+		}
+	}
+
+	var conn string
+	doReq := func(n int) {
+		req, _ := NewRequest("GET", cst.ts.URL, nil)
+		req = req.WithContext(httptrace.WithClientTrace(context.Background(), &httptrace.ClientTrace{
+			PutIdleConn: func(err error) {
+				if err != nil {
+					t.Errorf("failed to keep idle conn: %v", err)
+				}
+			},
+		}))
+		res, err := c.Do(req)
+		if err != nil {
+			t.Fatal(err)
+		}
+		res.Body.Close()
+		conns := idleConns()
+		if len(conns) != 1 {
+			t.Fatalf("req %v: unexpected number of idle conns: %q", n, conns)
+		}
+		if conn == "" {
+			conn = conns[0]
+		}
+		if conn != conns[0] {
+			t.Fatalf("req %v: cached connection changed; expected the same one throughout the test", n)
+		}
+	}
+	for i := 0; i < 3; i++ {
+		doReq(i)
+		time.Sleep(timeout / 4)
+	}
+	time.Sleep(timeout / 2)
+	if got := idleConns(); len(got) != 0 {
+		t.Errorf("idle conns = %q; want none", got)
+	}
+}
+
 func TestTransportIdleConnTimeout_h1(t *testing.T) { testTransportIdleConnTimeout(t, h1Mode) }
 func TestTransportIdleConnTimeout_h2(t *testing.T) { testTransportIdleConnTimeout(t, h2Mode) }
 func testTransportIdleConnTimeout(t *testing.T, h2 bool) {