osd/PG: wait for scan_snaps() mapper repair

Wait for our repair to apply before continuing to avoid reading the
broken state again.

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit ea531df)

Conflicts:
	src/osd/PG.cc (luminous uses osd->store->apply_transaction() where
                      master has osd->store->queue_transaction())

src/osd/PG.cc

@@ -4159,10 +4159,25 @@ void PG::_scan_snaps(ScrubMap &smap)
 			    << "...repaired";
 	}
 	snap_mapper.add_oid(hoid, obj_snaps, &_t);
-	r = osd->store->apply_transaction(osr.get(), std::move(t));
-	if (r != 0) {
-	  derr << __func__ << ": apply_transaction got " << cpp_strerror(r)
-	       << dendl;
+
+	// wait for repair to apply to avoid confusing other bits of the system.
+	{
+	  Cond my_cond;
+	  Mutex my_lock("PG::_scan_snaps my_lock");
+	  int r = 0;
+	  bool done;
+	  t.register_on_applied_sync(
+	    new C_SafeCond(&my_lock, &my_cond, &done, &r));
+	  r = osd->store->apply_transaction(osr.get(), std::move(t));
+	  if (r != 0) {
+	    derr << __func__ << ": apply_transaction got " << cpp_strerror(r)
+		 << dendl;
+	  } else {
+	    my_lock.Lock();
+	    while (!done)
+	      my_cond.Wait(my_lock);
+	    my_lock.Unlock();
+	  }
 	}
       }
     }