slurmstepd: Move wait-for-parent code into fork_all_tasks

Move the code that waits for parent signal before exec(2) out of
exec_task() and into fork_all_tasks() directly. This makes all
the code that handles the fork-and-wait into slurmstepd/mgr.c,
and allows the exec_wait_child_wait_for_parent() function to
be used in place of explicit read().

src/slurmd/slurmstepd/mgr.c

@@ -1319,7 +1319,15 @@ _fork_all_tasks(slurmd_job_t *job)
 			 */
 			prepare_tty (job, job->task[i]);
 
-			exec_task(job, i, ei->childfd);
+			/*
+			 *  Block until parent notifies us that it is ok to
+			 *   proceed. This allows the parent to place all
+			 *   children in any process groups or containers
+			 *   before they make a call to exec(2).
+			 */
+			exec_wait_child_wait_for_parent (ei);
+
+			exec_task(job, i);
 		}
 
 		/*

src/slurmd/slurmstepd/task.c

@@ -329,27 +329,15 @@ _setup_mpi(slurmd_job_t *job, int ltaskid)
  *  Current process is running as the user when this is called.
  */
 void
-exec_task(slurmd_job_t *job, int i, int waitfd)
+exec_task(slurmd_job_t *job, int i)
 {
-	char c;
 	uint32_t *gtids;		/* pointer to arrary of ranks */
 	int fd, j;
-	int rc;
 	slurmd_task_info_t *task = job->task[i];
 
 	if (i == 0)
 		_make_tmpdir(job);
 
-	/*
-	 * Stall exec until all tasks have joined the same process group
-	 */
-	if ((rc = read (waitfd, &c, sizeof (c))) != 1) {
-		error ("_exec_task read failed, fd = %d, rc=%d: %m", waitfd, rc);
-		log_fini();
-		exit(1);
-	}
-	close(waitfd);
-
 	gtids = xmalloc(job->node_tasks * sizeof(uint32_t));
 	for (j = 0; j < job->node_tasks; j++)
 		gtids[j] = job->task[j]->gtid;

src/slurmd/slurmstepd/task.h

@@ -52,6 +52,6 @@
 
 #include "src/slurmd/slurmstepd/slurmstepd_job.h"
 
-void exec_task(slurmd_job_t *job, int i, int waitfd);
+void exec_task(slurmd_job_t *job, int i);
 
 #endif /* !_TASK_H */