mon: set_healthy_stretch_mode in update_from_paxos, not random leader calls!

Add header comment describing how this works now.

Fixes: https://tracker.ceph.com/issues/50308

Signed-off-by: Greg Farnum <gfarnum@redhat.com>

src/mon/Monitor.cc

@@ -6655,7 +6655,6 @@ void Monitor::go_recovery_stretch_mode()
   if (!osdmon()->is_writeable()) {
     osdmon()->wait_for_writeable_ctx(new CMonGoRecovery(this));
   }
-  set_recovery_stretch_mode();
   osdmon()->trigger_recovery_stretch_mode();
 }
 
@@ -6749,7 +6748,6 @@ void Monitor::trigger_healthy_stretch_mode()
   }
 
   ceph_assert(osdmon()->osdmap.recovering_stretch_mode);
-  set_healthy_stretch_mode();
   osdmon()->trigger_healthy_stretch_mode();
   monmon()->trigger_healthy_stretch_mode();
 }

src/mon/Monitor.h

@@ -266,6 +266,19 @@ public:
   bool is_stretch_mode() { return stretch_mode_engaged; }
   bool is_degraded_stretch_mode() { return degraded_stretch_mode; }
   bool is_recovering_stretch_mode() { return recovering_stretch_mode; }
+
+  /**
+   * This set of functions maintains the in-memory stretch state
+   * and sets up transitions of the map states by calling in to
+   * MonmapMonitor and OSDMonitor.
+   *
+   * The [maybe_]go_* functions are called on the leader to
+   * decide if transitions should happen; the trigger_* functions
+   * set up the map transitions; and the set_* functions actually
+   * change the memory state -- but these are only called
+   * via OSDMonitor::update_from_paxos, to guarantee consistent
+   * updates across the entire cluster.
+   */
   void try_engage_stretch_mode();
   void maybe_go_degraded_stretch_mode();
   void trigger_degraded_stretch_mode(const set<string>& dead_mons,

src/mon/OSDMonitor.cc

@@ -959,6 +959,8 @@ void OSDMonitor::update_from_paxos(bool *need_bootstrap)
       } else {
 	mon.set_recovery_stretch_mode();
       }
+    } else {
+      mon.set_healthy_stretch_mode();
     }
     if (marked_osd_down &&
 	(!osdmap.degraded_stretch_mode || osdmap.recovering_stretch_mode)) {