rproxy: read the rest of a Connection: close visitor's body after an early answer (closes #373)

Go's server reads the body left unread once the handler returns, but not
for a request with Connection: close: it closes the connection with the
rest unread, and the visitor, still sending, gets a reset. rproxy now
reads that rest itself once it has answered, up to the 256 KiB Go's
server reads, until the deadline already set on the connection.

Model: opus-5-5

internal/rproxy/request.go

@@ -38,6 +38,10 @@ const plainText = "text/plain; charset=utf-8"
 // and the visitor then gets a closed connection and no answer.
 const minSendTime = 10 * time.Second
 
+// maxBodyReadAfterAnswer is the most of a body left unread that Go's server
+// reads once the handler returns, for a visitor without Connection: close.
+const maxBodyReadAfterAnswer = 256 << 10
+
 // errResponseTooLate refuses an app's answer that arrives once
 // SWW_RPROXY_MAX_RESPONSE_TIME_MS has run out.
 var errResponseTooLate = errors.New(
@@ -338,8 +342,8 @@ func (rq *request) answer(status int) {
 	}
 
 	// A visitor found too slow is read no more; any other may go on sending until
-	// its time is up, so that Go's server can read the rest of the body and end
-	// the request cleanly.
+	// its time is up, so that the rest of the body is read and the request ends
+	// cleanly: by Go's server, or by readRestOfBody.
 	deadline := rq.requestDeadline()
 	if status == http.StatusRequestTimeout {
 		deadline = time.Now()
@@ -523,6 +527,22 @@ func (rq *request) stopReadingBody(deadline time.Time) {
 	_ = rq.rc.SetReadDeadline(deadline)
 }
 
+// readRestOfBody reads, once the visitor has been answered, the rest of a body
+// that has not arrived whole, when the visitor sent Connection: close. Go's
+// server reads it for any other visitor, but closes this visitor's connection
+// with the rest unread: the visitor, still sending, then gets a reset and may
+// never read the answer. The read ends at the deadline already set on the
+// connection, and after as much as Go's server reads.
+func (rq *request) readRestOfBody() {
+	if !rq.in.Close || rq.body == nil || rq.body.received.Load() ||
+		rq.out.status == 0 ||
+		rq.out.status == http.StatusSwitchingProtocols {
+		return
+	}
+
+	_, _ = io.CopyN(io.Discard, rq.in.Body, maxBodyReadAfterAnswer)
+}
+
 // 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 deadline. Go's
 // server reads it once the handler returns, but with no deadline at all if