net/http: fix wantConnQueue memory leaks in Transport

I'm trying to keep the code changes minimal for backporting to Go 1.13,
so it is still possible for a handful of entries to leak,
but the leaks are now O(1) instead of O(N) in the steady state.

Longer-term, I think it would be a good idea to coalesce idleMu with
connsPerHostMu and clear entries out of both queues as soon as their
goroutines are done waiting.

Fixes #33849
Fixes #33850

Change-Id: Ia66bc64671eb1014369f2d3a01debfc023b44281
Reviewed-on: https://go-review.googlesource.com/c/go/+/191964
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/transport.go

@@ -953,6 +953,7 @@ func (t *Transport) queueForIdleConn(w *wantConn) (delivered bool) {
 		t.idleConnWait = make(map[connectMethodKey]wantConnQueue)
 	}
 	q := t.idleConnWait[w.key]
+	q.cleanFront()
 	q.pushBack(w)
 	t.idleConnWait[w.key] = q
 	return false
@@ -1137,7 +1138,7 @@ func (q *wantConnQueue) pushBack(w *wantConn) {
 	q.tail = append(q.tail, w)
 }
 
-// popFront removes and returns the w at the front of the queue.
+// popFront removes and returns the wantConn at the front of the queue.
 func (q *wantConnQueue) popFront() *wantConn {
 	if q.headPos >= len(q.head) {
 		if len(q.tail) == 0 {
@@ -1152,6 +1153,30 @@ func (q *wantConnQueue) popFront() *wantConn {
 	return w
 }
 
+// peekFront returns the wantConn at the front of the queue without removing it.
+func (q *wantConnQueue) peekFront() *wantConn {
+	if q.headPos < len(q.head) {
+		return q.head[q.headPos]
+	}
+	if len(q.tail) > 0 {
+		return q.tail[0]
+	}
+	return nil
+}
+
+// cleanFront pops any wantConns that are no longer waiting from the head of the
+// queue, reporting whether any were popped.
+func (q *wantConnQueue) cleanFront() (cleaned bool) {
+	for {
+		w := q.peekFront()
+		if w == nil || w.waiting() {
+			return cleaned
+		}
+		q.popFront()
+		cleaned = true
+	}
+}
+
 // getConn dials and creates a new persistConn to the target as
 // specified in the connectMethod. This includes doing a proxy CONNECT
 // and/or setting up TLS.  If this doesn't return an error, the persistConn
@@ -1261,6 +1286,7 @@ func (t *Transport) queueForDial(w *wantConn) {
 		t.connsPerHostWait = make(map[connectMethodKey]wantConnQueue)
 	}
 	q := t.connsPerHostWait[w.key]
+	q.cleanFront()
 	q.pushBack(w)
 	t.connsPerHostWait[w.key] = q
 }

src/net/http/transport_test.go

@@ -1658,6 +1658,176 @@ func TestTransportPersistConnLeakShortBody(t *testing.T) {
 	}
 }
 
+// A countedConn is a net.Conn that decrements an atomic counter when finalized.
+type countedConn struct {
+	net.Conn
+}
+
+// A countingDialer dials connections and counts the number that remain reachable.
+type countingDialer struct {
+	dialer      net.Dialer
+	mu          sync.Mutex
+	total, live int64
+}
+
+func (d *countingDialer) DialContext(ctx context.Context, network, address string) (net.Conn, error) {
+	conn, err := d.dialer.DialContext(ctx, network, address)
+	if err != nil {
+		return nil, err
+	}
+
+	counted := new(countedConn)
+	counted.Conn = conn
+
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	d.total++
+	d.live++
+
+	runtime.SetFinalizer(counted, d.decrement)
+	return counted, nil
+}
+
+func (d *countingDialer) decrement(*countedConn) {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	d.live--
+}
+
+func (d *countingDialer) Read() (total, live int64) {
+	d.mu.Lock()
+	defer d.mu.Unlock()
+	return d.total, d.live
+}
+
+func TestTransportPersistConnLeakNeverIdle(t *testing.T) {
+	defer afterTest(t)
+
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		// Close every connection so that it cannot be kept alive.
+		conn, _, err := w.(Hijacker).Hijack()
+		if err != nil {
+			t.Errorf("Hijack failed unexpectedly: %v", err)
+			return
+		}
+		conn.Close()
+	}))
+	defer ts.Close()
+
+	var d countingDialer
+	c := ts.Client()
+	c.Transport.(*Transport).DialContext = d.DialContext
+
+	body := []byte("Hello")
+	for i := 0; ; i++ {
+		total, live := d.Read()
+		if live < total {
+			break
+		}
+		if i >= 1<<12 {
+			t.Fatalf("Count of live client net.Conns (%d) not lower than total (%d) after %d Do / GC iterations.", live, total, i)
+		}
+
+		req, err := NewRequest("POST", ts.URL, bytes.NewReader(body))
+		if err != nil {
+			t.Fatal(err)
+		}
+		_, err = c.Do(req)
+		if err == nil {
+			t.Fatal("expected broken connection")
+		}
+
+		runtime.GC()
+	}
+}
+
+type countedContext struct {
+	context.Context
+}
+
+type contextCounter struct {
+	mu   sync.Mutex
+	live int64
+}
+
+func (cc *contextCounter) Track(ctx context.Context) context.Context {
+	counted := new(countedContext)
+	counted.Context = ctx
+	cc.mu.Lock()
+	defer cc.mu.Unlock()
+	cc.live++
+	runtime.SetFinalizer(counted, cc.decrement)
+	return counted
+}
+
+func (cc *contextCounter) decrement(*countedContext) {
+	cc.mu.Lock()
+	defer cc.mu.Unlock()
+	cc.live--
+}
+
+func (cc *contextCounter) Read() (live int64) {
+	cc.mu.Lock()
+	defer cc.mu.Unlock()
+	return cc.live
+}
+
+func TestTransportPersistConnContextLeakMaxConnsPerHost(t *testing.T) {
+	defer afterTest(t)
+
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		runtime.Gosched()
+		w.WriteHeader(StatusOK)
+	}))
+	defer ts.Close()
+
+	c := ts.Client()
+	c.Transport.(*Transport).MaxConnsPerHost = 1
+
+	ctx := context.Background()
+	body := []byte("Hello")
+	doPosts := func(cc *contextCounter) {
+		var wg sync.WaitGroup
+		for n := 64; n > 0; n-- {
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
+
+				ctx := cc.Track(ctx)
+				req, err := NewRequest("POST", ts.URL, bytes.NewReader(body))
+				if err != nil {
+					t.Error(err)
+				}
+
+				_, err = c.Do(req.WithContext(ctx))
+				if err != nil {
+					t.Errorf("Do failed with error: %v", err)
+				}
+			}()
+		}
+		wg.Wait()
+	}
+
+	var initialCC contextCounter
+	doPosts(&initialCC)
+
+	// flushCC exists only to put pressure on the GC to finalize the initialCC
+	// contexts: the flushCC allocations should eventually displace the initialCC
+	// allocations.
+	var flushCC contextCounter
+	for i := 0; ; i++ {
+		live := initialCC.Read()
+		if live == 0 {
+			break
+		}
+		if i >= 100 {
+			t.Fatalf("%d Contexts still not finalized after %d GC cycles.", live, i)
+		}
+		doPosts(&flushCC)
+		runtime.GC()
+	}
+}
+
 // This used to crash; https://golang.org/issue/3266
 func TestTransportIdleConnCrash(t *testing.T) {
 	defer afterTest(t)