test: fix data race in the recording server's readers (logs --watch tests)

internal/cmd/helpers_test.go

@@ -91,18 +91,28 @@ type recordedRequest struct {
 	Body   []byte
 }
 
+// recordedMu orders the servers' appends against recordedSoFar. A reader that
+// runs while a cancelled request is still inside its handler (a stopped watch)
+// has no other happens-before edge to that append.
+var recordedMu sync.Mutex
+
+// recordedSoFar is *reqs for a reader that may overlap a handler still running.
+// Appends only write past len, so the elements of the returned slice are stable.
+func recordedSoFar(reqs *[]recordedRequest) []recordedRequest {
+	recordedMu.Lock()
+	defer recordedMu.Unlock()
+	return *reqs
+}
+
 func newRecordingServer(t *testing.T, handler http.HandlerFunc) (*httptest.Server, *[]recordedRequest) {
 	t.Helper()
-	var (
-		reqs []recordedRequest
-		mu   sync.Mutex
-	)
+	var reqs []recordedRequest
 	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		var b bytes.Buffer
 		_, _ = b.ReadFrom(r.Body)
-		mu.Lock()
+		recordedMu.Lock()
 		reqs = append(reqs, recordedRequest{r.Method, r.URL.Path, r.URL.RawQuery, r.Header.Clone(), b.Bytes()})
-		mu.Unlock()
+		recordedMu.Unlock()
 		r.Body = http.NoBody
 		handler(w, r)
 	}))

internal/cmd/logs_watch_test.go

@@ -39,7 +39,7 @@ func logsScripted(t *testing.T, bodies ...string) (string, func() []recordedRequ
 		i := min(int(n.Add(1))-1, len(bodies)-1)
 		_, _ = w.Write([]byte(bodies[i]))
 	})
-	return srv.URL, func() []recordedRequest { return *reqs }
+	return srv.URL, func() []recordedRequest { return recordedSoFar(reqs) }
 }
 
 var (
@@ -319,7 +319,7 @@ func logsTUIServer(t *testing.T) (string, func() []recordedRequest) {
 		}
 		_, _ = w.Write([]byte(logsList(false, e)))
 	})
-	return srv.URL, func() []recordedRequest { return *reqs }
+	return srv.URL, func() []recordedRequest { return recordedSoFar(reqs) }
 }
 
 func TestLogsWatchFullScreenQuitsOnQ(t *testing.T) {