Allow manual rotation of rotating_file_sink (#3269)

* Allow manual rotation of rotating_file_sink

* Rename rotation method

* Attempted fix for tests on Windows

* Apply review mark-ups

include/spdlog/sinks/rotating_file_sink.h

@@ -27,6 +27,7 @@ public:
 
     static filename_t calc_filename(const filename_t &filename, std::size_t index);
     filename_t filename();
+    void rotate_now();
 
 protected:
     void sink_it_(const details::log_msg &msg) override;

tests/test_file_logging.cpp

@@ -100,3 +100,23 @@ TEST_CASE("rotating_file_logger3", "[rotating_logger]") {
     spdlog::filename_t basename = SPDLOG_FILENAME_T(ROTATING_LOG);
     REQUIRE_THROWS_AS(spdlog::rotating_logger_mt("logger", basename, max_size, 0), spdlog::spdlog_ex);
 }
+
+// test on-demand rotation of logs
+TEST_CASE("rotating_file_logger4", "[rotating_logger]") {
+    prepare_logdir();
+    size_t max_size = 1024 * 10;
+    spdlog::filename_t basename = SPDLOG_FILENAME_T(ROTATING_LOG);
+    auto sink = std::make_shared<spdlog::sinks::rotating_file_sink_st>(basename, max_size, 2);
+    auto logger = std::make_shared<spdlog::logger>("rotating_sink_logger", sink);
+
+    logger->info("Test message - pre-rotation");
+    logger->flush();
+
+    sink->rotate_now();
+
+    logger->info("Test message - post-rotation");
+    logger->flush();
+
+    REQUIRE(get_filesize(ROTATING_LOG) > 0);
+    REQUIRE(get_filesize(ROTATING_LOG ".1") > 0);
+}