Add ringbuffer sink (requires boost::circular_buffer)

Ringbuffer sink keeps user-given number of most recent log messages in
memory and returns them upon request (using the ringbuffer_sink::last
method). This can be useful for e.g. remote debugging of a running app.

include/spdlog/sinks/ringbuffer_sink-inl.h

+// Copyright(c) 2015-present, Gabi Melman & spdlog contributors.
+// Distributed under the MIT License (http://opensource.org/licenses/MIT)
+
+#pragma once
+
+#ifndef SPDLOG_HEADER_ONLY
+#include "spdlog/sinks/ringbuffer_sink.h"
+#endif
+
+#include "spdlog/common.h"
+#include "spdlog/details/os.h"
+
+#include<boost/circular_buffer.hpp>
+
+namespace spdlog {
+namespace sinks {
+
+template<typename Mutex>
+SPDLOG_INLINE ringbuffer_sink<Mutex>::ringbuffer_sink(size_t buf_size)
+{
+    buf.set_capacity(buf_size);
+}
+
+template<typename Mutex>
+SPDLOG_INLINE void ringbuffer_sink<Mutex>::sink_it_(const details::log_msg &msg)
+{
+    memory_buf_t formatted;
+    base_sink<Mutex>::formatter_->format(msg, formatted);
+    buf.push_front(fmt::to_string(formatted));
+}
+
+template<typename Mutex>
+SPDLOG_INLINE std::vector<std::string> ringbuffer_sink<Mutex>::last(size_t lim)
+{
+    std::lock_guard<Mutex> lock(base_sink<Mutex>::mutex_);
+    std::vector<std::string> ret;
+    ret.reserve(lim);
+    size_t num=0;
+    for(const std::string& msg: buf){
+        num++;
+        ret.push_back(msg);
+        if(lim>0 && num==lim) break;
+    }
+    return ret;
+}
+
+} // namespace sinks
+} // namespace spdlog

include/spdlog/sinks/ringbuffer_sink.h

+// Copyright(c) 2015-present, Gabi Melman & spdlog contributors.
+// Distributed under the MIT License (http://opensource.org/licenses/MIT)
+
+#pragma once
+
+#include "spdlog/details/null_mutex.h"
+#include "spdlog/sinks/base_sink.h"
+#include "spdlog/details/synchronous_factory.h"
+
+#include <mutex>
+#include <string>
+#include <vector>
+
+#include<boost/circular_buffer.hpp>
+
+namespace spdlog {
+namespace sinks {
+/*
+ * Ring buffer sink
+ */
+template<typename Mutex>
+class ringbuffer_sink final : public base_sink<Mutex>
+{
+public:
+    explicit ringbuffer_sink(size_t buf_size);
+    std::vector<std::string> last(size_t lim=0);
+
+protected:
+    void sink_it_(const details::log_msg &msg) override;
+    void flush_() override {};
+
+private:
+    boost::circular_buffer<std::string> buf;
+};
+
+using ringbuffer_sink_mt = ringbuffer_sink<std::mutex>;
+using ringbuffer_sink_st = ringbuffer_sink<details::null_mutex>;
+
+} // namespace sinks
+
+//
+// factory functions
+//
+template<typename Factory = spdlog::synchronous_factory>
+inline std::shared_ptr<logger> basic_logger_mt(const std::string &logger_name, size_t buf_size)
+{
+    return Factory::template create<sinks::ringbuffer_sink_mt>(logger_name, buf_size);
+}
+
+template<typename Factory = spdlog::synchronous_factory>
+inline std::shared_ptr<logger> basic_logger_st(const std::string &logger_name, size_t buf_size)
+{
+    return Factory::template create<sinks::ringbuffer_sink_st>(logger_name, buf_size);
+}
+
+} // namespace spdlog
+
+#ifdef SPDLOG_HEADER_ONLY
+#include "ringbuffer_sink-inl.h"
+#endif