shell: correct HOSTNAME env var if set in job environment

Problem: If the HOSTNAME variable is set in the job environment, it
is almost certainly incorrect. This can cause confusing behavior
for job tasks that make use of this variable.

If HOSTNAME is set in the job environment, correct it by using the
current hostname.

Fixes #5258

Author: grondo

src/shell/task.c

@@ -22,6 +22,7 @@
  *    . FLUX_JOB_NNODES
  *    . FLUX_JOB_ID
  *    . FLUX_URI (if not running standalone)
+ *    . correct HOSTNAME if set in job environment
  *
  * Current working directory
  *    Ignore - shell should already be in it.
@@ -154,6 +155,17 @@ struct shell_task *shell_task_create (flux_shell_t *shell,
                               getenv ("FLUX_KVS_NAMESPACE")) < 0)
             goto error;
     }
+
+    /* If HOSTNAME is set in job environment it is almost certain to be
+     * incorrect. Overwrite with the correct hostname.
+     */
+    if (flux_cmd_getenv (task->cmd, "HOSTNAME")
+        && flux_cmd_setenvf (task->cmd,
+                             1,
+                             "HOSTNAME",
+                             "%s",
+                             shell->hostname) < 0)
+        goto error;
     return task;
 error:
     shell_task_destroy (task);