resource: fetch local hwloc XML from parent if possible

Problem: The resource module always loads local topology via
hwloc_topology_load(3), an expensive operation.

When there is a parent, attempt to fetch the topology XML from the
parent resource module. This is not only more efficient (much more so
on some systems), but also ensures that the initializing subinstance is
using the same hwloc topology used by the parent when a fake topology
is loaded for testing purposes.

If the request takes longer than 100ms or fails, or there is no parent
instance detected, then the old behavior is preserved.

Fixes flux-framework#5611

src/modules/resource/topo.c

@@ -275,19 +275,62 @@ void topo_destroy (struct topo *topo)
     }
 }
 
+static char *topo_get_local_xml (struct resource_ctx *ctx, bool no_restrict)
+{
+    flux_t *parent_h;
+    flux_future_t *f = NULL;
+    char *result = NULL;
+    const char *xml;
+
+    errno = 0;
+    if (!(parent_h = resource_parent_handle_open (ctx))
+        || !(f = flux_rpc (parent_h,
+                           "resource.topo-get",
+                           NULL,
+                           FLUX_NODEID_ANY,
+                           0))
+        || flux_future_wait_for (f, 0.1) < 0
+        || flux_rpc_get (f, &xml) < 0) {
+        rhwloc_flags_t flags = no_restrict ? RHWLOC_NO_RESTRICT : 0;
+        /*  ENOENT just means there is no parent instance.
+         *  No need for an error.
+         */
+        if (errno && errno != ENOENT)
+            flux_log (ctx->h,
+                      LOG_DEBUG,
+                      "resource.topo-get to parent failed: %s",
+                      strerror (errno));
+        result = rhwloc_local_topology_xml (flags);
+        goto out;
+    }
+    flux_log (ctx->h,
+              LOG_INFO,
+              "retrieved local hwloc XML from parent");
+    if (no_restrict) {
+        result = strdup (xml);
+        goto out;
+    }
+    /*  restrict topology to current CPU binding
+     */
+    result = rhwloc_topology_xml_restrict (xml);
+out:
+    flux_future_destroy (f);
+    resource_parent_handle_close (ctx);
+    return result;
+}
+
 struct topo *topo_create (struct resource_ctx *ctx,
                           bool no_verify,
                           bool no_restrict)
 {
     struct topo *topo;
     json_t *R;
-    int flags = no_restrict ? RHWLOC_NO_RESTRICT : 0;
 
     if (!(topo = calloc (1, sizeof (*topo))))
         return NULL;
     topo->ctx = ctx;
-    if (!(topo->xml = rhwloc_local_topology_xml (flags))) {
-        flux_log_error (ctx->h, "error loading hwloc topology");
+    if (!(topo->xml = topo_get_local_xml (ctx, no_restrict))) {
+        flux_log (ctx->h, LOG_ERR, "error loading hwloc topology");
         goto error;
     }
     if (!(topo->r_local = rlist_from_hwloc (ctx->rank, topo->xml))) {