include/seastar/gate: re-enable SEASTAR_GATE_HOLDER_DEBUG and fix race condition during gate debug

Author: NitzanMordhai

include/seastar/core/gate.hh

@@ -29,13 +29,13 @@
 #include <seastar/util/modules.hh>
 #include <cassert>
 #include <exception>
+#include <mutex>
 #include <optional>
 #include <utility>
 #endif
 
 #ifdef SEASTAR_DEBUG
-// See: https://tracker.ceph.com/issues/64332
-// #define SEASTAR_GATE_HOLDER_DEBUG
+#define SEASTAR_GATE_HOLDER_DEBUG
 #endif
 
 namespace seastar {
@@ -174,14 +174,17 @@ public:
 #ifdef SEASTAR_GATE_HOLDER_DEBUG
         using member_hook_t = boost::intrusive::list_member_hook<boost::intrusive::link_mode<boost::intrusive::auto_unlink>>;
         member_hook_t _link;
+        std::mutex _debug_mutex;
 
         void debug_hold_gate() noexcept {
             if (_g) {
+                std::lock_guard<std::mutex> lock(_debug_mutex);
                 _g->_holders.push_back(*this);
             }
         }
 
         void debug_release_gate() noexcept {
+            seastar::lock_guard<seastar::mutex> lock(_debug_mutex);
             _link.unlink();
         }
 #else   // SEASTAR_GATE_HOLDER_DEBUG