rktio: fix signal mask for subprocesses

... by making `centralized_unblock_child_signal` actually unblock SIGCHLD.

Closes #2176

pkgs/racket-test-core/tests/racket/subprocess.rktl

@@ -543,6 +543,22 @@
     (close-input-port (car p))
     (close-input-port (cadddr p))))
 
+;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
+;; Check the process state for subprocesses to ensure that, for example,
+;; SIGCHLD is not blocked
+
+(unless (eq? (system-type) 'windows)
+  (let* ([dir (make-temporary-file "sub~a" 'directory)]
+         [exe (build-path dir "check")])
+    (when (system* (or (find-executable-path "cc")
+                       (find-executable-path "gcc"))
+                   "-o"
+                   exe
+                   (path->complete-path "unix_check.c" (or (current-load-relative-directory)
+                                                           (current-directory))))
+      (test #t 'subprocess-state (system* exe)))
+    (delete-directory/files dir)))
+
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 
 (for ([f (list tmpfile tmpfile2)] #:when (file-exists? f)) (delete-file f))

pkgs/racket-test-core/tests/racket/unix_check.c

@@ -0,0 +1,17 @@
+#include <stdio.h>
+#include <signal.h>
+
+/* Make sure a child process is started with normal properties, such
+   as no blocked signals */
+
+int main()
+{
+  sigset_t set, old_set;
+  sigemptyset(&set);
+  sigprocmask(SIG_BLOCK, &set, &old_set);
+
+  return (sigismember(&old_set, SIGCHLD)
+          || sigismember(&old_set, SIGINT)
+          || sigismember(&old_set, SIGHUP)
+          || sigismember(&old_set, SIGQUIT));
+}

racket/src/rktio/rktio_process.c

@@ -392,6 +392,10 @@ void centralized_block_child_signal()
 
 void centralized_unblock_child_signal()
 {
+  sigset_t set;
+  sigemptyset(&set);
+  sigaddset(&set, SIGCHLD);
+  sigprocmask(SIG_UNBLOCK, &set, NULL);
 }
 
 void centralized_start_child_signal_handler()