shell: make task list available before shell.init

Problem: It would be convenient to have access to the shell task
list in the shell.init plugin callback, but currenty the shell
creates, initializes, and starts tasks in a single loop.

Split task creation and start into separate functions:

 - shell_create_tasks() which creates all local tasks and adds
   them to the shell->tasks list.
 - shell_start_tasks() which completes task initialization and
   starts the tasks.

Move shell_create_tasks() before the shell.init callback is invoked.

src/shell/shell.c

@@ -1771,11 +1771,86 @@ static int frob_command (flux_shell_t *shell, flux_cmd_t *cmd)
     return 0;
 }
 
+static int shell_create_tasks (flux_shell_t *shell)
+{
+    int i = 0;
+    int taskid;
+
+    if (!(shell->tasks = zlist_new ()))
+        shell_die (1, "zlist_new failed");
+
+    taskid = idset_first (shell->info->taskids);
+    while (taskid != IDSET_INVALID_ID) {
+        struct shell_task *task;
+
+        if (!(task = shell_task_create (shell, i, taskid)))
+            shell_die (1, "shell_task_create index=%d", i);
+
+        task->pre_exec_cb = shell_task_exec;
+        task->pre_exec_arg = shell;
+
+        if (zlist_append (shell->tasks, task) < 0)
+            shell_die (1, "zlist_append failed");
+        i++;
+        taskid = idset_next (shell->info->taskids, taskid);
+    }
+    return 0;
+}
+
+static int shell_start_tasks (flux_shell_t *shell)
+{
+    flux_shell_task_t *task;
+
+    task = zlist_first (shell->tasks);
+    while (task) {
+        shell->current_task = task;
+
+        /*  Call all plugin task_init callbacks:
+         */
+        if (shell_task_init (shell) < 0)
+            shell_die (1, "failed to initialize taskid=%d", task->rank);
+
+        /*  Render any mustache templates in command args
+         */
+        if (frob_command (shell, task->cmd))
+            shell_die (1, "failed rendering of mustachioed command args");
+
+        if (shell_task_start (shell, task, task_completion_cb, shell) < 0) {
+            int ec = 1;
+            /* bash standard, 126 for permission/access denied, 127
+             * for command not found.  Note that shell only launches
+             * local tasks, therefore no need to check for
+             * EHOSTUNREACH.
+             */
+            if (errno == EPERM || errno == EACCES)
+                ec = 126;
+            else if (errno == ENOENT)
+                ec = 127;
+            shell_die (ec,
+                       "task %d (host %s): start failed: %s: %s",
+                       task->rank,
+                       shell->hostname,
+                       flux_cmd_arg (task->cmd, 0),
+                       strerror (errno));
+        }
+
+        if (flux_shell_add_completion_ref (shell, "task%d", task->rank) < 0)
+            shell_die (1, "flux_shell_add_completion_ref");
+
+        /*  Call all plugin task_fork callbacks:
+         */
+        if (shell_task_forked (shell) < 0)
+            shell_die (1, "shell_task_forked");
+
+        task = zlist_next (shell->tasks);
+    }
+    shell->current_task = NULL;
+    return 0;
+}
+
 int main (int argc, char *argv[])
 {
     flux_shell_t shell;
-    int i;
-    unsigned int taskid;
 
     /* Initialize locale from environment
      */
@@ -1848,6 +1923,11 @@ int main (int argc, char *argv[])
     if (shell_register_event_context (&shell) < 0)
         shell_die (1, "failed to add standard shell event context");
 
+    /* Create all tasks
+     */
+    if (shell_create_tasks (&shell) < 0)
+        shell_die_errno (1, "shell_create_tasks");
+
     /* Call "shell_init" plugins.
      */
     if (shell_init (&shell) < 0)
@@ -1874,70 +1954,11 @@ int main (int argc, char *argv[])
     if (shell_post_init (&shell) < 0)
         shell_die_errno (1, "shell_post_init");
 
-    /* Create tasks
+    /* Start all tasks
      */
-    if (!(shell.tasks = zlist_new ()))
+    if (shell_start_tasks (&shell))
         shell_die (1, "zlist_new failed");
 
-    i = 0;
-    taskid = idset_first (shell.info->taskids);
-    while (taskid != IDSET_INVALID_ID) {
-        struct shell_task *task;
-
-        if (!(task = shell_task_create (&shell, i, taskid)))
-            shell_die (1, "shell_task_create index=%d", i);
-
-        task->pre_exec_cb = shell_task_exec;
-        task->pre_exec_arg = &shell;
-        shell.current_task = task;
-
-        /*  Call all plugin task_init callbacks:
-         */
-        if (shell_task_init (&shell) < 0)
-            shell_die (1, "failed to initialize taskid=%d", i);
-
-        /*  Render any mustache templates in command args
-         */
-        if (frob_command (&shell, task->cmd))
-            shell_die (1, "failed rendering of mustachioed command args");
-
-        if (shell_task_start (&shell, task, task_completion_cb, &shell) < 0) {
-            int ec = 1;
-            /* bash standard, 126 for permission/access denied, 127
-             * for command not found.  Note that shell only launches
-             * local tasks, therefore no need to check for
-             * EHOSTUNREACH.
-             */
-            if (errno == EPERM || errno == EACCES)
-                ec = 126;
-            else if (errno == ENOENT)
-                ec = 127;
-            shell_die (ec,
-                       "task %d (host %s): start failed: %s: %s",
-                       task->rank,
-                       shell.hostname,
-                       flux_cmd_arg (task->cmd, 0),
-                       strerror (errno));
-        }
-
-        if (zlist_append (shell.tasks, task) < 0)
-            shell_die (1, "zlist_append failed");
-
-        if (flux_shell_add_completion_ref (&shell, "task%d", task->rank) < 0)
-            shell_die (1, "flux_shell_add_completion_ref");
-
-        /*  Call all plugin task_fork callbacks:
-         */
-        if (shell_task_forked (&shell) < 0)
-            shell_die (1, "shell_task_forked");
-
-        i++;
-        taskid = idset_next (shell.info->taskids, taskid);
-    }
-    /*  Reset current task since we've left task-specific context:
-     */
-    shell.current_task = NULL;
-
     if (shell_start (&shell) < 0)
         shell_die_errno (1, "shell.start callback(s) failed");