rproxy: give its own answers at least 10 seconds to be sent (closes #295)

The write deadline of an answer rproxy gives itself was
SWRP_MAX_RESPONSE_TIME_MS from when the answer started. With a limit
under a second, a host too busy to run rproxy for that long passes the
deadline before the first byte is written. Go then closes the
connection and the visitor gets no answer.

Model: opus-5-5

internal/rproxy/request.go

@@ -32,6 +32,11 @@ const reportStatusMax = 599
 // plainText is the Content-Type of the answers rproxy gives itself.
 const plainText = "text/plain; charset=utf-8"
 
+// ownAnswerMinSendTime is the least time an answer rproxy gives itself has to be
+// sent. A shorter time can run out on a busy host before the first byte is
+// written, and the visitor then gets a closed connection and no answer.
+const ownAnswerMinSendTime = 10 * time.Second
+
 // request is one request on its way through rproxy, from the moment its headers
 // have been read to its log line and report.
 type request struct {
@@ -326,14 +331,14 @@ func (rq *request) send(status int, body []byte) {
 // 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
-// used that time up first.
+// used that time up first, and at least ownAnswerMinSendTime.
 func (rq *request) prepareOwnAnswer(deadline time.Time) {
 	rq.stopReadingBody(deadline)
 	rq.endRequest()
 
 	limit := rq.h.config.MaxResponseTime
 	if limit > 0 {
-		_ = rq.rc.SetWriteDeadline(time.Now().Add(limit))
+		_ = rq.rc.SetWriteDeadline(time.Now().Add(max(limit, ownAnswerMinSendTime)))
 	}
 }