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

sandbox/linux/suid/init_process.c

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #define _GNU_SOURCE
 #include "init_process.h"
 
 #include <dirent.h>
 #include <fcntl.h>
 #include <signal.h>
@@ skipping 122 matching lines @@
   sigemptyset(&mask);
   sigaddset(&mask, SIGCHLD);
   sigprocmask(SIG_BLOCK, &mask, NULL);
 
   // Notify other processes that we are done initializing
   if (write(init_fd, " ", 1)) { }
   close(init_fd);
 
   // 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 (;;) {
         // Reap all exit codes of our child processes. This includes both
         // processes that originally were our immediate children, and processes
         // that have since been re-parented to be our children.
         int status;
         pid_t pid = waitpid(0, &status, __WALL | WNOHANG);
         if (pid <= 0) {
           break;
@@ skipping 33 matching lines @@
           // particularly true, because scanning "/proc" cannot possibly be
           // an atomic operation.
           // If we find that we no longer appear to have any children, we check
           // one more time whether there are any children we can now reap.
           // They might have died while we were scanning "/proc" and if so,
           // they should now show up.
           retry = true;
         }
       }
     } while (retry);
+
+    // Wait until we receive a SIGCHLD signal. Our signal handler doesn't
+    // actually need to do anything, though
+    sigwaitinfo(&mask, NULL);
   }
 }