Fix for false-positive memory leak when using leak detection methods (such as CRT dump memory leaks).

Since we cannot rely on static de-initialisation for freeing the registry::s_instance, this is now lazily allocated instead and freed during spdlog::shutdown() function.
In my test case, saves ~15 reported memory leaks.

include/spdlog/details/registry-inl.h

@@ -30,6 +30,9 @@
 namespace spdlog {
 namespace details {
 
+registry* registry::s_instance = nullptr;
+std::mutex registry::s_instanceMutex;
+
 SPDLOG_INLINE registry::registry()
     : formatter_(new pattern_formatter()) {
 #ifndef SPDLOG_DISABLE_DEFAULT_LOGGER
@@ -237,9 +240,19 @@ SPDLOG_INLINE void registry::set_levels(log_levels levels, level::level_enum *gl
     }
 }
 
-SPDLOG_INLINE registry &registry::instance() {
-    static registry s_instance;
-    return s_instance;
+SPDLOG_INLINE void registry::free() {
+    std::lock_guard<std::mutex> lock(s_instanceMutex);
+
+    delete s_instance;
+    s_instance = nullptr;
+}
+
+SPDLOG_INLINE registry &registry::instance() {    
+    std::lock_guard<std::mutex> lock(s_instanceMutex);
+    if (s_instance == nullptr) {
+        s_instance = new registry();
+    }
+    return *s_instance;
 }
 
 SPDLOG_INLINE void registry::apply_logger_env_levels(std::shared_ptr<logger> new_logger) {

include/spdlog/details/registry.h

@@ -90,7 +90,10 @@ public:
 
     static registry &instance();
 
-    void apply_logger_env_levels(std::shared_ptr<logger> new_logger);
+    // explicitely free the singleton instance.
+    static void free();
+
+    void apply_logger_env_levels(std::shared_ptr<logger> new_logger);   
 
 private:
     registry();
@@ -112,6 +115,9 @@ private:
     std::shared_ptr<logger> default_logger_;
     bool automatic_registration_ = true;
     size_t backtrace_n_messages_ = 0;
+
+    static registry* s_instance;
+    static std::mutex s_instanceMutex;
 };
 
 }  // namespace details

include/spdlog/spdlog-inl.h

@@ -67,7 +67,10 @@ SPDLOG_INLINE void drop(const std::string &name) { details::registry::instance()
 
 SPDLOG_INLINE void drop_all() { details::registry::instance().drop_all(); }
 
-SPDLOG_INLINE void shutdown() { details::registry::instance().shutdown(); }
+SPDLOG_INLINE void shutdown() {
+    details::registry::instance().shutdown();
+    details::registry::free();
+}
 
 SPDLOG_INLINE void set_automatic_registration(bool automatic_registration) {
     details::registry::instance().set_automatic_registration(automatic_registration);