net/http: pause briefly after closing Server connection when body remains

From https://github.com/golang/go/issues/11745#issuecomment-123555313
this implements option (b), having the server pause slightly after
sending the final response on a TCP connection when we're about to close
it when we know there's a request body outstanding. This biases the
client (which might not be Go) to prefer our response header over the
request body write error.

Updates #11745

Change-Id: I07cb0b74519d266c8049d9e0eb23a61304eedbf8
Reviewed-on: https://go-review.googlesource.com/12658Reviewed-by: Russ Cox <rsc@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/serve_test.go

@@ -3034,6 +3034,30 @@ Host: foo
 	}
 }
 
+// If a Handler finishes and there's an unread request body,
+// verify the server try to do implicit read on it before replying.
+func TestHandlerFinishSkipBigContentLengthRead(t *testing.T) {
+	conn := &testConn{closec: make(chan bool)}
+	conn.readBuf.Write([]byte(fmt.Sprintf(
+		"POST / HTTP/1.1\r\n" +
+			"Host: test\r\n" +
+			"Content-Length: 9999999999\r\n" +
+			"\r\n" + strings.Repeat("a", 1<<20))))
+
+	ls := &oneConnListener{conn}
+	var inHandlerLen int
+	go Serve(ls, HandlerFunc(func(rw ResponseWriter, req *Request) {
+		inHandlerLen = conn.readBuf.Len()
+		rw.WriteHeader(404)
+	}))
+	<-conn.closec
+	afterHandlerLen := conn.readBuf.Len()
+
+	if afterHandlerLen != inHandlerLen {
+		t.Errorf("unexpected implicit read. Read buffer went from %d -> %d", inHandlerLen, afterHandlerLen)
+	}
+}
+
 func BenchmarkClientServer(b *testing.B) {
 	b.ReportAllocs()
 	b.StopTimer()

src/net/http/server.go

@@ -874,8 +874,14 @@ func (cw *chunkWriter) writeHeader(p []byte) {
 	if w.req.ContentLength != 0 && !w.closeAfterReply {
 		ecr, isExpecter := w.req.Body.(*expectContinueReader)
 		if !isExpecter || ecr.resp.wroteContinue {
-			n, _ := io.CopyN(ioutil.Discard, w.req.Body, maxPostHandlerReadBytes+1)
-			if n >= maxPostHandlerReadBytes {
+			var tooBig bool
+			if reqBody, ok := w.req.Body.(*body); ok && reqBody.unreadDataSize() >= maxPostHandlerReadBytes {
+				tooBig = true
+			} else {
+				n, _ := io.CopyN(ioutil.Discard, w.req.Body, maxPostHandlerReadBytes+1)
+				tooBig = n >= maxPostHandlerReadBytes
+			}
+			if tooBig {
 				w.requestTooLarge()
 				delHeader("Connection")
 				setHeader.connection = "close"
@@ -1144,13 +1150,18 @@ func (w *response) shouldReuseConnection() bool {
 		return false
 	}
 
-	if body, ok := w.req.Body.(*body); ok && body.didEarlyClose() {
+	if w.closedRequestBodyEarly() {
 		return false
 	}
 
 	return true
 }
 
+func (w *response) closedRequestBodyEarly() bool {
+	body, ok := w.req.Body.(*body)
+	return ok && body.didEarlyClose()
+}
+
 func (w *response) Flush() {
 	if !w.wroteHeader {
 		w.WriteHeader(StatusOK)
@@ -1318,7 +1329,7 @@ func (c *conn) serve() {
 		}
 		w.finishRequest()
 		if !w.shouldReuseConnection() {
-			if w.requestBodyLimitHit {
+			if w.requestBodyLimitHit || w.closedRequestBodyEarly() {
 				c.closeWriteAndWait()
 			}
 			break

src/net/http/transfer.go

@@ -737,6 +737,17 @@ func mergeSetHeader(dst *Header, src Header) {
 	}
 }
 
+// unreadDataSize returns the number of bytes of unread input.
+// It returns -1 if unknown.
+func (b *body) unreadDataSize() int64 {
+	b.mu.Lock()
+	defer b.mu.Unlock()
+	if lr, ok := b.src.(*io.LimitedReader); ok {
+		return lr.N
+	}
+	return -1
+}
+
 func (b *body) Close() error {
 	b.mu.Lock()
 	defer b.mu.Unlock()

src/net/http/transport.go

@@ -1164,7 +1164,7 @@ WaitResponse:
 	for {
 		select {
 		case err := <-writeErrCh:
-			if isSyscallWriteError(err) {
+			if isNetWriteError(err) {
 				// Issue 11745. If we failed to write the request
 				// body, it's possible the server just heard enough
 				// and already wrote to us. Prioritize the server's
@@ -1383,14 +1383,12 @@ type fakeLocker struct{}
 func (fakeLocker) Lock()   {}
 func (fakeLocker) Unlock() {}
 
-func isSyscallWriteError(err error) bool {
+func isNetWriteError(err error) bool {
 	switch e := err.(type) {
 	case *url.Error:
-		return isSyscallWriteError(e.Err)
+		return isNetWriteError(e.Err)
 	case *net.OpError:
-		return e.Op == "write" && isSyscallWriteError(e.Err)
-	case *os.SyscallError:
-		return e.Syscall == "write"
+		return e.Op == "write"
 	default:
 		return false
 	}