Ensure SIGCHLD is unblocked

Thanks to @hjelmn for debugging it and providing the patch

Signed-off-by: Ralph Castain <rhc@open-mpi.org>
(cherry picked from commit efa8bcc17078c89f1c9d6aabed35c90973a469bf)
(cherry picked from commit 647a760b7e24194b37571a8245d8d39ed202e75b)
Signed-off-by: Nathan Hjelm <hjelmn@lanl.gov>

orte/mca/odls/base/odls_base_frame.c

@@ -28,6 +28,7 @@
 #include "orte/constants.h"
 
 #include <string.h>
+#include <signal.h>
 
 #include "opal/class/opal_ring_buffer.h"
 #include "orte/mca/mca.h"
@@ -225,6 +226,7 @@ static int orte_odls_base_open(mca_base_open_flag_t flags)
     int rc, i, rank;
     orte_namelist_t *nm;
     bool xterm_hold;
+    sigset_t unblock;
 
     ORTE_CONSTRUCT_LOCK(&orte_odls_globals.lock);
     orte_odls_globals.lock.active = false;   // start with nobody having the thread
@@ -243,6 +245,17 @@ static int orte_odls_base_open(mca_base_open_flag_t flags)
     OBJ_CONSTRUCT(&orte_odls_globals.xterm_ranks, opal_list_t);
     orte_odls_globals.xtermcmd = NULL;
 
+    /* ensure that SIGCHLD is unblocked as we need to capture it */
+    if (0 != sigemptyset(&unblock)) {
+        return ORTE_ERROR;
+    }
+    if (0 != sigaddset(&unblock, SIGCHLD)) {
+        return ORTE_ERROR;
+    }
+    if (0 != sigprocmask(SIG_UNBLOCK, &unblock, NULL)) {
+        return ORTE_ERR_NOT_SUPPORTED;
+    }
+
     /* check if the user requested that we display output in xterms */
     if (NULL != orte_xterm) {
         /* construct a list of ranks to be displayed */