timer: cancel all timers during reactor destruction

If a timer is not canceled it will try to cancel itself during
destruction which may happen after engine is already destroyed.

apps/memcached/memcache.cc

@@ -290,6 +290,11 @@ public:
         }
     }
 
+    // needed by timer_set
+    bool cancel() {
+        return false;
+    }
+
     friend bool operator==(const item_type &a, const item_type &b) {
          return a._key == b._key;
     }

core/reactor.hh

@@ -645,6 +645,16 @@ public:
     static boost::program_options::options_description get_options_description();
     reactor();
     reactor(const reactor&) = delete;
+    ~reactor() {
+        auto eraser = [](auto& list) {
+            while (!list.empty()) {
+                auto timer = *list.begin();
+                timer.cancel();
+            }
+        };
+        eraser(_expired_timers);
+        eraser(_expired_lowres_timers);
+    }
     void operator=(const reactor&) = delete;
 
     void configure(boost::program_options::variables_map config);

core/timer-set.hh

@@ -17,6 +17,7 @@
 #include <chrono>
 #include <limits>
 #include <bitset>
+#include <array>
 #include <boost/intrusive/list.hpp>
 #include "bitset-iter.hh"
 
@@ -48,7 +49,7 @@ private:
     // The last bucket is reserved for active timers with timeout <= _last.
     static constexpr int n_buckets = timestamp_bits + 1;
 
-    timer_list_t _buckets[n_buckets];
+    std::array<timer_list_t, n_buckets> _buckets;
     timestamp_t _last;
     timestamp_t _next;
 
@@ -92,6 +93,15 @@ public:
     {
     }
 
+    ~timer_set() {
+        for (auto&& list : _buckets) {
+            while (!list.empty()) {
+                auto& timer = *list.begin();
+                timer.cancel();
+            }
+        }
+    }
+
     /**
      * Adds timer to the active set.
      *