flux-shell: use f58plain in shell service name

Problem: the job-exec module reconstructs the shell's service
name to send the rank 0 shell an exception message when one of
the other shells goes offline, but when f58 encoding is used,
the reconstruction could be different when the shell node and
the rank 0 node have different locale configurations.

Use the f58plain jobid encoding to avoid this potential mismatch.

Fixes #5257

Author: garlick

src/modules/job-exec/job-exec.c

@@ -171,12 +171,14 @@ flux_future_t *jobinfo_shell_rpc_pack (struct jobinfo *job,
     char *shell_topic = NULL;
     flux_future_t *f = NULL;
     uint32_t rank;
-
-    if (asprintf (&shell_topic,
-                  "%ju-shell-%s.%s",
-                  (uintmax_t) job->userid,
-                  idf58 (job->id),
-                  topic) < 0
+    char idbuf[21];
+
+    if (flux_job_id_encode (job->id, "f58plain", idbuf, sizeof (idbuf)) < 0
+        || asprintf (&shell_topic,
+                     "%ju-shell-%s.%s",
+                     (uintmax_t) job->userid,
+                     idbuf,
+                     topic) < 0
         || ((rank = resource_set_nth_rank (job->R, 0)) == IDSET_INVALID_ID))
         goto out;
     va_start (ap, fmt);

src/shell/svc.c

@@ -61,16 +61,25 @@ static int lookup_rank (struct shell_svc *svc, int shell_rank, int *rank)
     return 0;
 }
 
+/* Avoid locale-specific encoding for the shell service name.
+ * See flux-framework/flux-core#5257.
+ */
 static int build_topic (struct shell_svc *svc,
                         const char *method,
                         char *buf,
                         int len)
 {
+    char idbuf[21];
+    if (flux_job_id_encode (svc->shell->info->jobid,
+                            "f58plain",
+                            idbuf,
+                            sizeof (idbuf)) < 0)
+        return -1;
     if (snprintf (buf,
                   len,
                   "%ju-shell-%s%s%s",
                   (uintmax_t)svc->uid,
-                  idf58 (svc->shell->info->jobid),
+                  idbuf,
                   method ? "." : "",
                   method ? method : "") >= len) {
         errno = EINVAL;