rproxy: send its own answers at once to a visitor that stops sending its body (closes #303)

An answer rproxy gives itself, such as a deny or a refusal, waited while
Go's server read up to 256 KiB of a body that had not arrived whole. A
visitor that stopped sending got nothing until SWRP_MAX_REQUEST_TIME_MS
ran out, and no bytes at all once that was past the 10 seconds the
answer has to be sent.

send now sets Connection: close in that case, as modifyResponse does for
the app's answer; both call closeAfterAnswer. requestTimedOut no longer
ends that read of Go's server, which no longer happens, so the time the
app's answer started is no longer noted.

Model: opus-5-5

internal/rproxy/request.go

@@ -83,8 +83,8 @@ type request struct {
 	upgraded *upgradedConn
 
 	// mu guards what follows. The time limits run on goroutines of their own,
-	// and the transport starts and stops them, and notes the times below, from
-	// its own; once timersStopped is set, neither time limit acts any more.
+	// and the transport starts and stops them, and notes requestEnd, from its
+	// own; once timersStopped is set, neither time limit acts any more.
 	mu            sync.Mutex
 	timersStopped bool
 	requestTimer  *time.Timer
@@ -92,8 +92,6 @@ type request struct {
 	// requestEnd is when the app had been sent the whole request, or the answer
 	// started, if that was sooner.
 	requestEnd time.Time
-	// answerStarted is when the first byte of the app's answer arrived.
-	answerStarted time.Time
 }
 
 // newRequest starts handling r: it notes the time, works out the visitor from
@@ -177,8 +175,7 @@ func (rq *request) forward(ctx context.Context) {
 
 	rq.cancel = cancel
 	ctx = httptrace.WithClientTrace(ctx, &httptrace.ClientTrace{
-		WroteRequest:         rq.wroteRequest,
-		GotFirstResponseByte: rq.gotFirstResponseByte,
+		WroteRequest: rq.wroteRequest,
 	})
 
 	out := rq.in.WithContext(ctx)
@@ -273,12 +270,7 @@ func (rq *request) modifyResponse(res *http.Response) error {
 		rq.out.Header()["Content-Type"] = nil
 	}
 
-	// Go's server reads up to 256 KiB of a body left unread before it sends an
-	// answer, and so waits on a visitor that has stopped sending. With this
-	// header it sends the answer at once, and closes the connection after it.
-	if rq.body != nil && !rq.body.received.Load() {
-		rq.out.Header().Set("Connection", "close")
-	}
+	rq.closeAfterAnswer()
 
 	res.Body = &responseBody{body: limitBody(res.Body, maxBytes), rq: rq}
 	rq.out.afterStatus = rq.setResponseDeadline
@@ -347,11 +339,22 @@ func (rq *request) send(status int, body []byte) {
 		rq.out.Header().Set("Content-Length", strconv.Itoa(len(body)))
 	}
 
+	rq.closeAfterAnswer()
 	rq.out.WriteHeader(status)
 	_, _ = rq.out.Write(body)
 	_ = rq.rc.Flush()
 }
 
+// closeAfterAnswer has Go's server send the answer at once, and close the
+// connection after it, when the visitor's body has not arrived whole. Without
+// it, Go's server reads up to 256 KiB of the body left before it sends an
+// answer, and so waits on a visitor that has stopped sending.
+func (rq *request) closeAfterAnswer() {
+	if rq.body != nil && !rq.body.received.Load() {
+		rq.out.Header().Set("Connection", "close")
+	}
+}
+
 // prepareOwnAnswer readies the connection for an answer rproxy gives itself: the
 // reading of a body that has not arrived whole ends at deadline, and the answer
 // has SWRP_MAX_RESPONSE_TIME_MS from now to reach the visitor, even when the app
@@ -517,16 +520,6 @@ func (rq *request) requestTimedOut() {
 	}
 
 	if rq.body == nil || !rq.body.waiting.Load() {
-		// Once the app has answered, Go's server can read what is left of a body
-		// under 256 KiB without requestBody.Read, before an answer rproxy gives
-		// itself, so waiting is false while that read waits for the visitor.
-		// That read ends now, before refuse:
-		// should rproxy still answer itself, answer then sets the deadline it
-		// gives its own answers.
-		if !rq.answerStarted.IsZero() {
-			rq.stopReadingBody(time.Now())
-		}
-
 		rq.refuse(http.StatusGatewayTimeout)
 
 		return
@@ -539,15 +532,6 @@ func (rq *request) requestTimedOut() {
 	_ = rq.rc.SetReadDeadline(time.Now())
 }
 
-// gotFirstResponseByte is called once the first byte of the app's answer has
-// arrived.
-func (rq *request) gotFirstResponseByte() {
-	rq.mu.Lock()
-	defer rq.mu.Unlock()
-
-	rq.answerStarted = time.Now()
-}
-
 // wroteRequest is called once the app has been sent the whole request:
 // SWRP_MAX_REQUEST_TIME_MS ends and SWRP_MAX_RESPONSE_TIME_MS starts.
 func (rq *request) wroteRequest(info httptrace.WroteRequestInfo) {