Repository: go
Issue: net/http/httptest: (*Server).Close races with HTTP/2 request handler
Labels: NeedsInvestigation

```
~/go/src$ go version
go version devel +24ea77559f Thu Apr 9 11:09:00 2020 -0400 linux/amd64

~/go/src$ go doc httptest.Server.Close
package httptest // import "net/http/httptest"

func (s *Server) Close()
    Close shuts down the server and blocks until all outstanding requests on
    this server have completed.
```

While investigating #37669, I saw a reported race that led me to believe that, contrary to its documentation, `(*httptest.Server).Close` was not blocking for requests to complete.

To check that hypothesis, I added the following test function:

```diff
diff --git c/src/net/http/httptest/server_test.go w/src/net/http/httptest/server_test.go
index 0aad15c5ed..8b0e0f137f 100644
--- c/src/net/http/httptest/server_test.go
+++ w/src/net/http/httptest/server_test.go
@@ -6,9 +6,12 @@ package httptest

 import (
 	"bufio"
+	"context"
+	"io"
 	"io/ioutil"
 	"net"
 	"net/http"
+	"strings"
 	"testing"
 )

@@ -238,3 +241,51 @@ func TestTLSServerWithHTTP2(t *testing.T) {
 		})
 	}
 }
+
+func TestCloseBlocksUntilRequestsCompleted(t *testing.T) {
+	t.Parallel()
+
+	modes := []string{
+		"http1",
+		"http2",
+	}
+
+	for _, mode := range modes {
+		mode := mode
+		t.Run(mode, func(t *testing.T) {
+			t.Parallel()
+
+			ctx, cancel := context.WithCancel(context.Background())
+			closed := false
+
+			cst := NewUnstartedServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				cancel()
+
+				io.Copy(w, r.Body)
+
+				<-r.Context().Done()
+				if closed {
+					panic("Close did not wait for handler to return")
+				}
+			}))
+
+			switch mode {
+			case "http1":
+				cst.Start()
+			case "http2":
+				cst.EnableHTTP2 = true
+				cst.StartTLS()
+			}
+			defer func() {
+				cst.Close()
+				closed = true
+			}()
+
+			req, _ := http.NewRequestWithContext(ctx, "POST", cst.URL, strings.NewReader("Hello, server!"))
+			resp, err := cst.Client().Do(req)
+			if err == nil {
+				resp.Body.Close()
+			}
+		})
+	}
+}
```

It reliably fails under the race detector with even a very modest count, indicating that the server is not waiting for the handler to complete when HTTP/2 is enabled:

```
~/go/src$ go test -race net/http/httptest -run=TestCloseBlocksUntilRequestsCompleted -count=10
==================
WARNING: DATA RACE
Write at 0x00c000202018 by goroutine 10:
  net/http/httptest.TestCloseBlocksUntilRequestsCompleted.func1.2()
      /usr/local/google/home/bcmills/go/src/net/http/httptest/server_test.go:281 +0x46
  net/http/httptest.TestCloseBlocksUntilRequestsCompleted.func1()
      /usr/local/google/home/bcmills/go/src/net/http/httptest/server_test.go:289 +0x508
  testing.tRunner()
      /usr/local/google/home/bcmills/go/src/testing/testing.go:1022 +0x1eb

Previous read at 0x00c000202018 by goroutine 31:
  net/http/httptest.TestCloseBlocksUntilRequestsCompleted.func1.1()
      /usr/local/google/home/bcmills/go/src/net/http/httptest/server_test.go:267 +0x18b
  net/http.HandlerFunc.ServeHTTP()
      /usr/local/google/home/bcmills/go/src/net/http/server.go:2012 +0x51
  net/http.serverHandler.ServeHTTP()
      /usr/local/google/home/bcmills/go/src/net/http/server.go:2808 +0xc4
  net/http.initALPNRequest.ServeHTTP()
      /usr/local/google/home/bcmills/go/src/net/http/server.go:3380 +0xfc
  net/http.(*initALPNRequest).ServeHTTP()
      <autogenerated>:1 +0xa6
  net/http.Handler.ServeHTTP-fm()
      /usr/local/google/home/bcmills/go/src/net/http/server.go:87 +0x64
  net/http.(*http2serverConn).runHandler()
      /usr/local/google/home/bcmills/go/src/net/http/h2_bundle.go:5712 +0xc0

Goroutine 10 (running) created at:
  testing.(*T).Run()
      /usr/local/google/home/bcmills/go/src/testing/testing.go:1073 +0x6a3
  net/http/httptest.TestCloseBlocksUntilRequestsCompleted()
      /usr/local/google/home/bcmills/go/src/net/http/httptest/server_test.go:255 +0x13e
  testing.tRunner()
      /usr/local/google/home/bcmills/go/src/testing/testing.go:1022 +0x1eb

Goroutine 31 (finished) created at:
  net/http.(*http2serverConn).processHeaders()
      /usr/local/google/home/bcmills/go/src/net/http/h2_bundle.go:5446 +0x8fb
  net/http.(*http2serverConn).processFrame()
      /usr/local/google/home/bcmills/go/src/net/http/h2_bundle.go:4975 +0x44a
  net/http.(*http2serverConn).processFrameFromReader()
      /usr/local/google/home/bcmills/go/src/net/http/h2_bundle.go:4933 +0x750
  net/http.(*http2serverConn).serve()
      /usr/local/google/home/bcmills/go/src/net/http/h2_bundle.go:4432 +0x1485
  net/http.(*http2Server).ServeConn()
      /usr/local/google/home/bcmills/go/src/net/http/h2_bundle.go:4033 +0xd88
  net/http.http2ConfigureServer.func1()
      /usr/local/google/home/bcmills/go/src/net/http/h2_bundle.go:3859 +0x117
  net/http.(*conn).serve()
      /usr/local/google/home/bcmills/go/src/net/http/server.go:1805 +0x1c77
==================
--- FAIL: TestCloseBlocksUntilRequestsCompleted (0.00s)
    --- FAIL: TestCloseBlocksUntilRequestsCompleted/http2 (0.02s)
        testing.go:937: race detected during execution of test
2020/04/10 16:35:14 http2: panic serving 127.0.0.1:46044: Close did not wait for handler to return
goroutine 132 [running]:
net/http.(*http2serverConn).runHandler.func1(0xc0000ae068, 0xc00022af4f, 0xc00031a180)
        /usr/local/google/home/bcmills/go/src/net/http/h2_bundle.go:5705 +0x231
panic(0x8b5220, 0x9c14f0)
        /usr/local/google/home/bcmills/go/src/runtime/panic.go:975 +0x3b3
net/http/httptest.TestCloseBlocksUntilRequestsCompleted.func1.1(0x9cd0c0, 0xc0000ae068, 0xc0000ac600)
        /usr/local/google/home/bcmills/go/src/net/http/httptest/server_test.go:268 +0x1e2
net/http.HandlerFunc.ServeHTTP(0xc000096740, 0x9cd0c0, 0xc0000ae068, 0xc0000ac600)
        /usr/local/google/home/bcmills/go/src/net/http/server.go:2012 +0x52
net/http.serverHandler.ServeHTTP(0xc0000a8460, 0x9cd0c0, 0xc0000ae068, 0xc0000ac600)
        /usr/local/google/home/bcmills/go/src/net/http/server.go:2808 +0xc5
net/http.initALPNRequest.ServeHTTP(0x9cd940, 0xc000282b70, 0xc00029a380, 0xc0000a8460, 0x9cd0c0, 0xc0000ae068, 0xc0000ac600)
        /usr/local/google/home/bcmills/go/src/net/http/server.go:3380 +0xfd
net/http.(*http2serverConn).runHandler(0xc00031a180, 0xc0000ae068, 0xc0000ac600, 0xc0000977c0)
        /usr/local/google/home/bcmills/go/src/net/http/h2_bundle.go:5712 +0xc1
created by net/http.(*http2serverConn).processHeaders
        /usr/local/google/home/bcmills/go/src/net/http/h2_bundle.go:5446 +0x8fc
FAIL
FAIL    net/http/httptest       0.202s
FAIL
```

CC @bradfitz @tombergan @nerd2