add argument to rotating file sink for rotate_on_open

when true, the log file will be rotated when it is opened so the newly constructed file will start off being empty

include/spdlog/sinks/rotating_file_sink.h

@@ -27,12 +27,14 @@ template<typename Mutex>
 class rotating_file_sink SPDLOG_FINAL : public base_sink<Mutex>
 {
 public:
-    rotating_file_sink(filename_t base_filename, std::size_t max_size, std::size_t max_files)
+    rotating_file_sink(filename_t base_filename, std::size_t max_size, std::size_t max_files, bool rotate_on_open=false)
         : base_filename_(std::move(base_filename))
         , max_size_(max_size)
         , max_files_(max_files)
     {
         file_helper_.open(calc_filename(base_filename_, 0));
+        if (rotate_on_open)
+            rotate_();
         current_size_ = file_helper_.size(); // expensive. called only once
     }
 
@@ -130,15 +132,15 @@ using rotating_file_sink_st = rotating_file_sink<details::null_mutex>;
 
 template<typename Factory = default_factory>
 inline std::shared_ptr<logger> rotating_logger_mt(
-    const std::string &logger_name, const filename_t &filename, size_t max_file_size, size_t max_files)
+    const std::string &logger_name, const filename_t &filename, size_t max_file_size, size_t max_files, bool rotate_on_open=false)
 {
-    return Factory::template create<sinks::rotating_file_sink_mt>(logger_name, filename, max_file_size, max_files);
+    return Factory::template create<sinks::rotating_file_sink_mt>(logger_name, filename, max_file_size, max_files, rotate_on_open);
 }
 
 template<typename Factory = default_factory>
 inline std::shared_ptr<logger> rotating_logger_st(
-    const std::string &logger_name, const filename_t &filename, size_t max_file_size, size_t max_files)
+    const std::string &logger_name, const filename_t &filename, size_t max_file_size, size_t max_files, bool rotate_on_open = false)
 {
-    return Factory::template create<sinks::rotating_file_sink_st>(logger_name, filename, max_file_size, max_files);
+    return Factory::template create<sinks::rotating_file_sink_st>(logger_name, filename, max_file_size, max_files, rotate_on_open);
 }
 } // namespace spdlog

tests/file_log.cpp

@@ -60,7 +60,7 @@ TEST_CASE("rotating_file_logger2", "[rotating_logger]]")
     prepare_logdir();
     size_t max_size = 1024 * 10;
     std::string basename = "logs/rotating_log";
-    auto logger = spdlog::rotating_logger_mt("logger", basename, max_size, 1);
+    auto logger = spdlog::rotating_logger_mt("logger", basename, max_size, 1, true);
     for (int i = 0; i < 10; ++i)
         logger->info("Test message {}", i);