rproxy tests: a cut-off answer closes the connection, also after its first part reached the visitor

A new case fails on the current code: the app sends the first part of its
body at once and then stalls. The visitor gets that part, and the
connection stays open past SWRP_MAX_RESPONSE_TIME_MS. That is the failure
seen at random on next, where a busy host let the first part through.

The app in the two older cases sent its body after a sleep timed against
the limit, so a busy host could let it through too. The app now sends its
late parts only once rproxy has ended the request to it, which happens
when rproxy cuts off the answer. Those two cases fail on the current code
as well. The limit is 2 seconds, so that a busy host still passes on a
part sent at once.

Model: opus-5-5

internal/rproxy/limits_test.go

@@ -12,7 +12,6 @@ import (
 	"os"
 	"strconv"
 	"strings"
-	"sync"
 	"sync/atomic"
 	"testing"
 	"time"
@@ -918,49 +917,48 @@ func TestAVisitorThatStopsALittleShortHasTheConnectionClosedWhenTheAnswerIsCutOf
 ) {
 	t.Parallel()
 
-	const limit = 300 * time.Millisecond
+	// A busy host passes on a part sent at once well within this limit.
+	const limit = 2 * time.Second
 
-	// With two parts, rproxy fails to send the first and then waits for the
-	// second, which the app sends only once rproxy has ended the request to it.
+	// The app sends the parts in atOnce with its status and headers. It sends the
+	// parts in late only once rproxy has ended the request to it, which rproxy
+	// does when it cuts off the answer, so that those never reach the visitor.
 	for _, tc := range []struct {
-		name  string
-		parts []string
+		name         string
+		atOnce, late []string
 	}{
-		{"with the body in one part", []string{greeting}},
-		{"with the body in two parts", []string{greeting[:5], greeting[5:]}},
+		{"with the body in one part", nil, []string{greeting}},
+		{"with the body in two parts", nil, []string{greeting[:5], greeting[5:]}},
+		{
+			"with the first part of the body at once",
+			[]string{greeting[:5]}, []string{greeting[5:]},
+		},
 	} {
 		t.Run(tc.name, func(t *testing.T) {
 			t.Parallel()
 
-			headersRead := make(chan struct{})
-			// letAppSendBody also runs when the test ends, as the app's server waits
-			// for the app.
-			letAppSendBody := sync.OnceFunc(func() { close(headersRead) })
 			standIn := startWAF(t, allow)
-			// The app sends its body once the visitor has the status and headers and
-			// the response time limit has run out since, so that rproxy cannot send
-			// it. After each part, it reads the request's body, which ends when rproxy
-			// ends the request to it.
 			app := startApp(t, func(w http.ResponseWriter, r *http.Request) {
 				rc := http.NewResponseController(w)
 				_ = rc.EnableFullDuplex()
 
 				w.Header().Set(contentLengthHeader, strconv.Itoa(len(greeting)))
 
+				for _, part := range tc.atOnce {
+					_, _ = io.WriteString(w, part)
+				}
+
 				_ = rc.Flush()
 
-				<-headersRead
-				time.Sleep(limit)
+				// The request's body ends when rproxy ends the request to the app.
+				_, _ = io.Copy(io.Discard, r.Body)
 
-				for _, part := range tc.parts {
+				for _, part := range tc.late {
 					_, _ = io.WriteString(w, part)
 					_ = rc.Flush()
-					_, _ = io.Copy(io.Discard, r.Body)
 				}
 			})
 
-			t.Cleanup(letAppSendBody)
-
 			// The request time limit is far longer than dial's deadline.
 			addr, _ := startRProxy(t, standIn.URL, app.URL, map[string]string{
 				maxRequestTime:  "120000",
@@ -975,15 +973,14 @@ func TestAVisitorThatStopsALittleShortHasTheConnectionClosedWhenTheAnswerIsCutOf
 				t.Fatalf("read the answer: %v", err)
 			}
 
-			letAppSendBody()
-
 			// If the connection stays open, dial's deadline ends this read with a
 			// timeout.
 			body, err := io.ReadAll(res.Body)
 			_ = res.Body.Close()
 
-			if len(body) != 0 || !errors.Is(err, io.ErrUnexpectedEOF) {
-				t.Errorf("read %q (%v), want the connection closed", body, err)
+			want := strings.Join(tc.atOnce, "")
+			if string(body) != want || !errors.Is(err, io.ErrUnexpectedEOF) {
+				t.Errorf("read %q (%v), want %q and the connection closed", body, err, want)
 			}
 		})
 	}