job-ingest: don't fail on rank > 16383

Problem: fluid_init() fails on rank > 16383 due to a design
limitation on the number of concurrent fluid generators.

If fluid_init() fails on one of these high ranks, log a message at
debug level and return 0, so the instance can continue with ingest
requests on that rank being handled upstream.

Fixes #5708

Author: garlick

src/modules/job-ingest/job-ingest.c

@@ -852,8 +852,18 @@ int mod_main (flux_t *h, int argc, char **argv)
         }
         flux_future_destroy (f);
         if (fluid_init (&ctx.gen, 0, fluid_get_timestamp (max_jobid) + 1) < 0) {
-            flux_log (h, LOG_ERR, "fluid_init failed");
-            errno = EINVAL;
+            // See flux-framework/flux-core#5708
+            if (rank > 16383) {
+                flux_log (h,
+                          LOG_DEBUG,
+                          "job-ingest cannot allocate job IDs on ranks > 16383."
+                          " Exiting - upstream will handle ingest requests.");
+                rc = 0;
+            }
+            else {
+                flux_log (h, LOG_ERR, "fluid_init failed");
+                errno = EINVAL;
+            }
             goto done;
         }
     }