If using the suid sandbox, but not using the seccomp sandbox, there is a

sandbox/linux/suid/init_process.c

Index: sandbox/linux/suid/init_process.c
===================================================================
--- sandbox/linux/suid/init_process.c	(revision 125700)
+++ sandbox/linux/suid/init_process.c	(working copy)
@@ -140,10 +140,6 @@
 
   // Handle dying processes that have been re-parented to the "init" process
   for (;;) {
-    // Wait until we receive a SIGCHLD signal. Our signal handler doesn't
-    // actually need to do anything, though
-    sigwaitinfo(&mask, NULL);
-
     bool retry = false;
     do {
       for (;;) {
@@ -197,5 +193,9 @@
         }
       }
     } while (retry);
+
+    // Wait until we receive a SIGCHLD signal. Our signal handler doesn't
+    // actually need to do anything, though
+    sigwaitinfo(&mask, NULL);
   }
 }