net/http: Transport leaks context.Contexts when limited by MaxConnsPerHost

[bcmills]

The following test case fails at go1.13rc1 due to a leak introduced in CL 184262. The leak occurs due to persistConn pointers entering the connsPerHostWait queue and remaining reachable via that queue until a connection to the host is broken: if connections are never broken, the queue is never cleared.

type countedContext struct {
	context.Context
}

type contextCounter struct {
	atomicCount int64
}

func (cc *contextCounter) Track(ctx context.Context) context.Context {
	counted := new(countedContext)
	counted.Context = ctx
	atomic.AddInt64(&cc.atomicCount, 1)
	runtime.SetFinalizer(counted, cc.decrement)
	return counted
}

func (cc *contextCounter) decrement(*countedContext) {
	atomic.AddInt64(&cc.atomicCount, -1)
}

func (cc *contextCounter) Count() int64 {
	return atomic.LoadInt64(&cc.atomicCount)
}

func TestTransportPersistConnContextLeakWithMaxConnsPerHost(t *testing.T) {
	defer afterTest(t)

	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
		time.Sleep(time.Millisecond)
		w.WriteHeader(StatusOK)
	}))
	defer ts.Close()

	c := ts.Client()
	c.Transport.(*Transport).MaxConnsPerHost = 1

	running := make(chan bool, 2)
	defer func() {
		for n := cap(running); n > 0; n-- {
			running <- true
		}
	}()

	body := []byte("Hello")
	var cc contextCounter
	prevCount := cc.Count()
	for i := 0; ; i++ {
		n := cc.Count()
		if n < prevCount {
			break
		}
		if i >= 1<<12 {
			t.Fatalf("Count of allocated client context.Contexts (%d) did not decrease after %d Do / GC iterations.", n, i)
		}
		prevCount = n

		running <- true
		go func() {
			defer func() { <-running }()
			ctx := cc.Track(context.Background())
			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)
			}
		}()

		if t.Failed() {
			return
		}
		runtime.GC()
	}
}

The test passes with that CL reverted, but since that CL fixed a significant deadlock (#32336), I intend to fix the leak (hopefully today) rather than reverting the CL.

CC @bradfitz

[gopherbot]

Change https://golang.org/cl/191964 mentions this issue: net/http: fix wantConnQueue memory leaks in Transport

[gopherbot]

Change https://golang.org/cl/191967 mentions this issue: [release-branch.go1.13] net/http: fix wantConnQueue memory leaks in Transport