rproxy: close the connection when a short answer is cut off (closes #318)

When SWRP_MAX_RESPONSE_TIME_MS ran out before the last of a short
answer was written, the flush failed and ReverseProxy ignored it. If
the visitor had stopped short of its announced body, Go's server then
read the rest until SWRP_MAX_REQUEST_TIME_MS ran out, keeping the
connection open with the answer cut off. Go does that read even after
an aborting panic.

forward now checks for a failed flush once ReverseProxy returns. It
then records a 504 refusal, ends the reading of the body at once, and
panics so that Go's server closes the connection, as SPEC.md says a
cut-off answer does.

Model: opus-5-5

internal/rproxy/request.go

@@ -195,6 +195,20 @@ func (rq *request) forward(ctx context.Context) {
 	rq.startRequestTimer()
 	rq.upstreamStart = time.Now()
 	reverseProxy.ServeHTTP(rq.out, out)
+
+	// ReverseProxy ignores a failed flush, so it misses a short answer cut off when
+	// SWRP_MAX_RESPONSE_TIME_MS runs out; once a flush has failed, every later one
+	// fails too. Go's server would then keep the connection open while it reads
+	// the rest of a body that has not arrived whole. So that reading ends now, and
+	// the refusal keeps finish from extending it to SWRP_MAX_REQUEST_TIME_MS. The
+	// panic has Go's server close the connection, and log nothing; the deferred
+	// finish still runs.
+	if rq.out.status != http.StatusSwitchingProtocols &&
+		errors.Is(rq.rc.Flush(), os.ErrDeadlineExceeded) {
+		rq.refuse(http.StatusGatewayTimeout)
+		rq.stopPassingOnBody(time.Now())
+		panic(http.ErrAbortHandler)
+	}
 }
 
 // rewrite makes the request the app receives: the visitor's request, unchanged,
@@ -385,7 +399,7 @@ func (rq *request) finish(ctx context.Context) {
 	rq.stopTimers()
 
 	if rq.refused.Load() == 0 {
-		rq.stopPassingOnBody()
+		rq.stopPassingOnBody(rq.requestDeadline())
 	}
 
 	requestEnd := rq.endRequest()
@@ -473,10 +487,10 @@ func (rq *request) stopReadingBody(deadline time.Time) {
 }
 
 // stopPassingOnBody ends the transport's reading of a visitor's body that has
-// not arrived whole, and lets Go's server read the rest until
-// SWRP_MAX_REQUEST_TIME_MS runs out. Go's server reads it once the handler
-// returns, but with no deadline at all if another read of it is still waiting.
-func (rq *request) stopPassingOnBody() {
+// not arrived whole, and lets Go's server read the rest until deadline. Go's
+// server reads it once the handler returns, but with no deadline at all if
+// another read of it is still waiting.
+func (rq *request) stopPassingOnBody(deadline time.Time) {
 	if rq.body == nil || rq.body.received.Load() {
 		return
 	}
@@ -489,7 +503,7 @@ func (rq *request) stopPassingOnBody() {
 	rq.body.stopped = true
 	rq.body.mu.Unlock()
 
-	rq.stopReadingBody(rq.requestDeadline())
+	rq.stopReadingBody(deadline)
 }
 
 // startRequestTimer starts SWRP_MAX_REQUEST_TIME_MS, which runs from the