As it turned out, "if (interrupted) break;" is needed in both loops,

in "collect tasks" loop and in enclosing "collect/process" loop.
Before this patch, "strace cat" + "killall -INT strace" wasn't
making strace stop and exit. This patch restores 4.5.18 behavior
in this regard.

strace.c

@@ -2789,6 +2789,8 @@ trace()
 		 * To fix it, we collect *all* waitable tasks, then handle
 		 * them all, then repeat.
 		 */
+		if (interrupted)
+			return 0;
 		tcbs = collect_stopped_tcbs();
 		if (!tcbs)
 			break;