net/http: data race in test

[bradfitz]

Test-only data race I saw once in a trybot:

==================
WARNING: DATA RACE
Write by goroutine 127:
  net/http_test.TestTransportConcurrency()
      /tmp/workdir/go/src/net/http/transport_test.go:1104 +0x345
  testing.tRunner()
      /tmp/workdir/go/src/testing/testing.go:455 +0xdc

Previous read by goroutine 146:
  net/http.(*Transport).getConn.func2.1()
      /tmp/workdir/go/src/net/http/transport.go:534 +0xa0

Goroutine 127 (running) created at:
  testing.RunTests()
      /tmp/workdir/go/src/testing/testing.go:563 +0xcac
  testing.(*M).Run()
      /tmp/workdir/go/src/testing/testing.go:493 +0xe4
  net/http_test.TestMain()
      /tmp/workdir/go/src/net/http/main_test.go:19 +0x2e
  main.main()
      net/http/_test/_testmain.go:538 +0x209

Goroutine 146 (finished) created at:
  net/http.(*Transport).getConn.func2()
      /tmp/workdir/go/src/net/http/transport.go:537 +0x9c
  net/http.(*Transport).getConn()
      /tmp/workdir/go/src/net/http/transport.go:559 +0x45e
  net/http.(*Transport).RoundTrip()
      /tmp/workdir/go/src/net/http/transport.go:228 +0x62a
  net/http.send()
      /tmp/workdir/go/src/net/http/client.go:220 +0x6e4
  net/http.(*Client).send()
      /tmp/workdir/go/src/net/http/client.go:143 +0x1f7
  net/http.(*Client).doFollowingRedirects()
      /tmp/workdir/go/src/net/http/client.go:380 +0x1007
  net/http.(*Client).Get()
      /tmp/workdir/go/src/net/http/client.go:306 +0xc8
  net/http_test.TestChunkedNoContent()
      /tmp/workdir/go/src/net/http/transport_test.go:1071 +0x32a
  testing.tRunner()
      /tmp/workdir/go/src/testing/testing.go:455 +0xdc
==================
PASS
Found 1 data race(s)
FAIL    net/http    13.506s

Looks like TestChunkedNoContent isn't waiting for its Transport connections to die before a later test changes hooks.

[tw4452852]

I think race condition is between the goroutine in the handlePendingDial and the next new transport. We didn't wait goroutine in the handlePendingDial to exit.
I'm sorry I can't access go.googlesource.com so I just upload an alternative patch here:

diff --git a/src/net/http/transport.go b/src/net/http/transport.go
index 5de5d94..30e6469 100644
--- a/src/net/http/transport.go
+++ b/src/net/http/transport.go
@@ -523,7 +523,7 @@ func (t *Transport) getConn(req *Request, cm connectMethod) (*persistConn, error
    }
    dialc := make(chan dialRes)

-   handlePendingDial := func() {
+   handlePendingDial := func(prePendingDial, postPendingDial func()) {
        if prePendingDial != nil {
            prePendingDial()
        }
@@ -556,10 +556,10 @@ func (t *Transport) getConn(req *Request, cm connectMethod) (*persistConn, error
        // else's dial that they didn't use.
        // But our dial is still going, so give it away
        // when it finishes:
-       handlePendingDial()
+       handlePendingDial(prePendingDial, postPendingDial)
        return pc, nil
    case <-cancelc:
-       handlePendingDial()
+       handlePendingDial(prePendingDial, postPendingDial)
        return nil, errors.New("net/http: request canceled while waiting for connection")
    }
 }

[bradfitz]

I wish I could reproduce this.

[tzneal]

I can reproduce it reliably with this change:

diff --git a/src/net/http/transport.go b/src/net/http/transport.go
index 5de5d94..0c305cf 100644
--- a/src/net/http/transport.go
+++ b/src/net/http/transport.go
@@ -531,6 +531,11 @@ func (t *Transport) getConn(req *Request, cm connectMethod) (*persistConn, error
            if v := <-dialc; v.err == nil {
                t.putIdleConn(v.pc)
            }
+           for i := 0; i < 10; i++ {
+               prePendingDial, postPendingDial = postPendingDial, prePendingDial
+               time.Sleep(10 * time.Millisecond)
+               prePendingDial, postPendingDial = postPendingDial, prePendingDial
+           }
            if postPendingDial != nil {
                postPendingDial()
            }

https://go-review.googlesource.com/#/c/11250/

[gopherbot]

CL https://golang.org/cl/11250 mentions this issue.