ns: Call notify_networkstatus_changed() after the new consensus is set globally

In 0.3.2.1-alpha, we've added this function in order to have a way to notify
other subsystems that the consensus just changed. The old consensus and the
new one are passed to it.

Before this patch, this was done _before_ the new consensus was set globally
(thus NOT accessible by getting the latest consensus). The scheduler
notification was assuming that it was set and select_scheduler() is looking at
the latest consensus to get the parameters it might needs. This was very wrong
because at that point it is still the old consensus set globally.

With this commit, notify_networkstatus_changed() has been moved _after_ the
new consensus is set globally. The main obvious reasons is to fix the bug
described above and in #24975. The other reason is that this notify function
doesn't return anything which could be allowing the possibility of refusing to
set the new consensus on error. In other words, the new consensus is set right
after the notification whatever happens.

It does no harm or change in behavior to set the new consensus first and then
notify the subsystems. The two functions currently used are for the control
port using the old and new consensus and sending the diff. The second is the
scheduler that needs the new consensus to be set globally before being called.

Of course, the function has been documented accordinly to clearly state it is
done _after_ the new consensus is set.

Fixes #24975

Signed-off-by: David Goulet <dgoulet@torproject.org>

changes/bug24975

@@ -0,0 +1,6 @@
+  o Major bugfixes (scheduler, consensus):
+    - A logic in the code was preventing the scheduler subystem to properly
+      make a decision based on the latest consensus when it arrives. This lead
+      to the scheduler failing to notice any consensus parameters that might
+      change from one consensus to another. Fixes bug 24975; bugfix on
+      0.3.2.1-alpha.

src/or/networkstatus.c

@@ -1564,7 +1564,11 @@ notify_control_networkstatus_changed(const networkstatus_t *old_c,
   smartlist_free(changed);
 }
 
-/* Called when the consensus has changed from old_c to new_c. */
+/* Called when the consensus has changed from old_c to new_c.
+ *
+ * IMPORTANT: This is called _after_ the new consensus has been set in the
+ * global state so this is safe for anything getting the latest consensus from
+ * that state. */
 static void
 notify_networkstatus_changed(const networkstatus_t *old_c,
                              const networkstatus_t *new_c)
@@ -1897,9 +1901,6 @@ networkstatus_set_current_consensus(const char *consensus,
 
   const int is_usable_flavor = flav == usable_consensus_flavor();
 
-  if (is_usable_flavor) {
-    notify_networkstatus_changed(networkstatus_get_latest_consensus(), c);
-  }
   if (flav == FLAV_NS) {
     if (current_ns_consensus) {
       networkstatus_copy_old_consensus_info(c, current_ns_consensus);
@@ -1922,6 +1923,13 @@ networkstatus_set_current_consensus(const char *consensus,
     free_consensus = 0; /* avoid free */
   }
 
+  /* Called _after_ the consensus is set in its global variable so any
+   * functions called from this notification can safely get the latest
+   * consensus being the new one. */
+  if (is_usable_flavor) {
+    notify_networkstatus_changed(networkstatus_get_latest_consensus(), c);
+  }
+
   waiting = &consensus_waiting_for_certs[flav];
   if (waiting->consensus &&
       waiting->consensus->valid_after <= c->valid_after) {