shell: affinity: use cached hwloc XML

Problem: Loading hwloc topology can be very slow, especially on a
system with many cores and when possibly many processes are trying
to simultaneously call hwloc_topology_load(3). This can occur when
many short running jobs are being launched by Flux, since the job
shell loads topology by default in the affinity plugin.

Since the job shell now caches the hwloc XML in the shell info object,
fetch this XML and use it to load topology, avoiding redundant scans
of ths sytem. This may greatly improve job throughput on many core
systems.

Fixes flux-framework#4365

src/shell/affinity.c

@@ -158,10 +158,30 @@ static void shell_affinity_destroy (void *arg)
 
 /*  Initialize topology object for affinity processing.
  */
-static int shell_affinity_topology_init (struct shell_affinity *sa)
+static int shell_affinity_topology_init (flux_shell_t *shell,
+                                         struct shell_affinity *sa)
 {
+    const char *xml;
+
+    /*  Fetch hwloc XML cached in job shell to avoid heavyweight
+     *   hwloc topology load (Issue #4365)
+     */
+    if (flux_shell_info_unpack (shell, "{s:s}", "hwloc", &xml) < 0)
+        return shell_log_errno ("failed to unpack hwloc object");
+
     if (hwloc_topology_init (&sa->topo) < 0)
         return shell_log_errno ("hwloc_topology_init");
+
+    if (hwloc_topology_set_xmlbuffer (sa->topo, xml, strlen (xml)) < 0)
+        return shell_log_errno ("hwloc_topology_set_xmlbuffer");
+
+    /*  Tell hwloc that our XML loaded topology is from this system,
+     *   O/w hwloc CPU binding will not work.
+     */
+    if (hwloc_topology_set_flags (sa->topo,
+                                  HWLOC_TOPOLOGY_FLAG_IS_THISSYSTEM) < 0)
+        return shell_log_errno ("hwloc_topology_set_flags");
+
     if (hwloc_topology_load (sa->topo) < 0)
         return shell_log_errno ("hwloc_topology_load");
     if (topology_restrict_current (sa->topo) < 0)
@@ -178,7 +198,7 @@ static struct shell_affinity * shell_affinity_create (flux_shell_t *shell)
     struct shell_affinity *sa = calloc (1, sizeof (*sa));
     if (!sa)
         return NULL;
-    if (shell_affinity_topology_init (sa) < 0)
+    if (shell_affinity_topology_init (shell, sa) < 0)
         goto err;
     if (flux_shell_rank_info_unpack (shell,
                                      -1,