Fixed copy ctor of err_helper to be thread safe

include/spdlog/details/err_helper.h

@@ -15,9 +15,10 @@ namespace details {
 class SPDLOG_API err_helper {
     err_handler custom_err_handler_;
     std::chrono::steady_clock::time_point last_report_time_;
-    std::mutex mutex_;
+    mutable std::mutex mutex_;
 public:
     err_helper() = default;
+    ~err_helper() = default;
     err_helper(const err_helper& other);
     err_helper(err_helper&& other) noexcept;
     void handle_ex(const std::string& origin, const source_loc& loc, const std::exception& ex) noexcept;

src/details/err_helper.cpp

@@ -8,13 +8,16 @@
 namespace spdlog {
 namespace details {
 
-err_helper::err_helper(const err_helper &other)
-    : custom_err_handler_(other.custom_err_handler_),
-      last_report_time_(other.last_report_time_) {}
+err_helper::err_helper(const err_helper &other) {
+    std::lock_guard lock(other.mutex_);
+    custom_err_handler_ = other.custom_err_handler_;
+    last_report_time_ = other.last_report_time_;
+}
 
-err_helper::err_helper(err_helper &&other) noexcept
-    : custom_err_handler_(std::move(other.custom_err_handler_)),
-      last_report_time_(other.last_report_time_) {}
+err_helper::err_helper(err_helper &&other) noexcept {
+    custom_err_handler_ = std::move(other.custom_err_handler_);
+    last_report_time_ = std::move(other.last_report_time_);
+}
 
 // Prints error to stderr with source location (if available). A stderr sink is not used because reaching
 // this point might indicate a problem with the logging system itself so we use fputs() directly.