net/http: fix scheduling race resulting in flaky test

The test was measuring something, assuming other goroutines had
already scheduled.

Fixes #10427

Change-Id: I2a4d3906f9d4b5ea44b57d972e303bbe2b0b1cde
Reviewed-on: https://go-review.googlesource.com/9561Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/export_test.go

@@ -10,9 +10,17 @@ package http
 import (
 	"net"
 	"net/url"
+	"sync"
 	"time"
 )
 
+func init() {
+	// We only want to pay for this cost during testing.
+	// When not under test, these values are always nil
+	// and never assigned to.
+	testHookMu = new(sync.Mutex)
+}
+
 func NewLoggingConn(baseName string, c net.Conn) net.Conn {
 	return newLoggingConn(baseName, c)
 }
@@ -86,6 +94,12 @@ func SetEnterRoundTripHook(f func()) {
 	testHookEnterRoundTrip = f
 }
 
+func SetReadLoopBeforeNextReadHook(f func()) {
+	testHookMu.Lock()
+	defer testHookMu.Unlock()
+	testHookReadLoopBeforeNextRead = f
+}
+
 func NewTestTimeoutHandler(handler Handler, ch <-chan time.Time) Handler {
 	f := func() <-chan time.Time {
 		return ch

src/net/http/transport.go

@@ -877,6 +877,11 @@ func (pc *persistConn) readLoop() {
 	eofc := make(chan struct{})
 	defer close(eofc) // unblock reader on errors
 
+	// Read this once, before loop starts. (to avoid races in tests)
+	testHookMu.Lock()
+	testHookReadLoopBeforeNextRead := testHookReadLoopBeforeNextRead
+	testHookMu.Unlock()
+
 	alive := true
 	for alive {
 		pb, err := pc.br.Peek(1)
@@ -993,6 +998,10 @@ func (pc *persistConn) readLoop() {
 				pc.wroteRequest() &&
 				pc.t.putIdleConn(pc)
 		}
+
+		if hook := testHookReadLoopBeforeNextRead; hook != nil {
+			hook()
+		}
 	}
 	pc.close()
 }
@@ -1090,6 +1099,8 @@ var errRequestCanceled = errors.New("net/http: request canceled")
 var (
 	testHookPersistConnClosedGotRes func()
 	testHookEnterRoundTrip          func()
+	testHookMu                      sync.Locker = fakeLocker{} // guards following
+	testHookReadLoopBeforeNextRead  func()
 )
 
 func (pc *persistConn) roundTrip(req *transportRequest) (resp *Response, err error) {
@@ -1344,3 +1355,11 @@ func (nr noteEOFReader) Read(p []byte) (n int, err error) {
 	}
 	return
 }
+
+// fakeLocker is a sync.Locker which does nothing. It's used to guard
+// test-only fields when not under test, to avoid runtime atomic
+// overhead.
+type fakeLocker struct{}
+
+func (fakeLocker) Lock()   {}
+func (fakeLocker) Unlock() {}

src/net/http/transport_test.go

@@ -1827,6 +1827,11 @@ func TestIdleConnChannelLeak(t *testing.T) {
 	}))
 	defer ts.Close()
 
+	const nReqs = 5
+	didRead := make(chan bool, nReqs)
+	SetReadLoopBeforeNextReadHook(func() { didRead <- true })
+	defer SetReadLoopBeforeNextReadHook(nil)
+
 	tr := &Transport{
 		Dial: func(netw, addr string) (net.Conn, error) {
 			return net.Dial(netw, ts.Listener.Addr().String())
@@ -1839,12 +1844,28 @@ func TestIdleConnChannelLeak(t *testing.T) {
 	// First, without keep-alives.
 	for _, disableKeep := range []bool{true, false} {
 		tr.DisableKeepAlives = disableKeep
-		for i := 0; i < 5; i++ {
+		for i := 0; i < nReqs; i++ {
 			_, err := c.Get(fmt.Sprintf("http://foo-host-%d.tld/", i))
 			if err != nil {
 				t.Fatal(err)
 			}
+			// Note: no res.Body.Close is needed here, since the
+			// response Content-Length is zero. Perhaps the test
+			// should be more explicit and use a HEAD, but tests
+			// elsewhere guarantee that zero byte responses generate
+			// a "Content-Length: 0" instead of chunking.
+		}
+
+		// At this point, each of the 5 Transport.readLoop goroutines
+		// are scheduling noting that there are no response bodies (see
+		// earlier comment), and are then calling putIdleConn, which
+		// decrements this count. Usually that happens quickly, which is
+		// why this test has seemed to work for ages. But it's still
+		// racey: we have wait for them to finish first. See Issue 10427
+		for i := 0; i < nReqs; i++ {
+			<-didRead
 		}
+
 		if got := tr.IdleConnChMapSizeForTesting(); got != 0 {
 			t.Fatalf("ForDisableKeepAlives = %v, map size = %d; want 0", disableKeep, got)
 		}