Make async_logger::flush() synchronous and wait for the flush to complete (#3049)

include/spdlog/async_logger-inl.h

@@ -43,13 +43,15 @@ SPDLOG_LOGGER_CATCH(msg.source)
 }
 
 // send flush request to the thread pool
-SPDLOG_INLINE void spdlog::async_logger::flush_(){
-    SPDLOG_TRY{if (auto pool_ptr = thread_pool_.lock()){
-        pool_ptr->post_flush(shared_from_this(), overflow_policy_);
-}
-else {
+SPDLOG_INLINE void spdlog::async_logger::flush_(){SPDLOG_TRY{auto pool_ptr = thread_pool_.lock();
+if (!pool_ptr) {
     throw_spdlog_ex("async flush: thread pool doesn't exist anymore");
 }
+
+std::future<void> future = pool_ptr->post_flush(shared_from_this(), overflow_policy_);
+// Wait for the flush operation to complete.
+// This might throw exception if the flush message get dropped because of overflow.
+future.get();
 }
 SPDLOG_LOGGER_CATCH(source_loc())
 }

include/spdlog/details/thread_pool-inl.h

@@ -62,9 +62,13 @@ void SPDLOG_INLINE thread_pool::post_log(async_logger_ptr &&worker_ptr,
     post_async_msg_(std::move(async_m), overflow_policy);
 }
 
-void SPDLOG_INLINE thread_pool::post_flush(async_logger_ptr &&worker_ptr,
-                                           async_overflow_policy overflow_policy) {
-    post_async_msg_(async_msg(std::move(worker_ptr), async_msg_type::flush), overflow_policy);
+std::future<void> SPDLOG_INLINE thread_pool::post_flush(async_logger_ptr &&worker_ptr,
+                                                        async_overflow_policy overflow_policy) {
+    std::promise<void> promise;
+    std::future<void> future = promise.get_future();
+    post_async_msg_(async_msg(std::move(worker_ptr), async_msg_type::flush, std::move(promise)),
+                    overflow_policy);
+    return future;
 }
 
 size_t SPDLOG_INLINE thread_pool::overrun_counter() { return q_.overrun_counter(); }
@@ -108,6 +112,7 @@ bool SPDLOG_INLINE thread_pool::process_next_msg_() {
         }
         case async_msg_type::flush: {
             incoming_async_msg.worker_ptr->backend_flush_();
+            incoming_async_msg.flush_promise.set_value();
             return true;
         }
 

include/spdlog/details/thread_pool.h

@@ -9,6 +9,7 @@
 
 #include <chrono>
 #include <functional>
+#include <future>
 #include <memory>
 #include <thread>
 #include <vector>
@@ -27,6 +28,7 @@ enum class async_msg_type { log, flush, terminate };
 struct async_msg : log_msg_buffer {
     async_msg_type msg_type{async_msg_type::log};
     async_logger_ptr worker_ptr;
+    std::promise<void> flush_promise;
 
     async_msg() = default;
     ~async_msg() = default;
@@ -56,12 +58,22 @@ struct async_msg : log_msg_buffer {
     async_msg(async_logger_ptr &&worker, async_msg_type the_type, const details::log_msg &m)
         : log_msg_buffer{m},
           msg_type{the_type},
-          worker_ptr{std::move(worker)} {}
+          worker_ptr{std::move(worker)},
+          flush_promise{} {}
 
     async_msg(async_logger_ptr &&worker, async_msg_type the_type)
         : log_msg_buffer{},
           msg_type{the_type},
-          worker_ptr{std::move(worker)} {}
+          worker_ptr{std::move(worker)},
+          flush_promise{} {}
+
+    async_msg(async_logger_ptr &&worker,
+              async_msg_type the_type,
+              std::promise<void> &&promise)
+        : log_msg_buffer{},
+          msg_type{the_type},
+          worker_ptr{std::move(worker)},
+          flush_promise{std::move(promise)} {}
 
     explicit async_msg(async_msg_type the_type)
         : async_msg{nullptr, the_type} {}
@@ -88,7 +100,8 @@ public:
     void post_log(async_logger_ptr &&worker_ptr,
                   const details::log_msg &msg,
                   async_overflow_policy overflow_policy);
-    void post_flush(async_logger_ptr &&worker_ptr, async_overflow_policy overflow_policy);
+    std::future<void> post_flush(async_logger_ptr &&worker_ptr,
+                                 async_overflow_policy overflow_policy);
     size_t overrun_counter();
     void reset_overrun_counter();
     size_t discard_counter();

tests/test_async.cpp

@@ -93,6 +93,49 @@ TEST_CASE("flush", "[async]") {
     REQUIRE(test_sink->flush_counter() == 1);
 }
 
+TEST_CASE("multithread flush", "[async]") {
+    auto test_sink = std::make_shared<spdlog::sinks::test_sink_mt>();
+    size_t queue_size = 2;
+    size_t messages = 10;
+    size_t n_threads = 10;
+    size_t flush_count = 1024;
+    std::mutex mtx;
+    std::vector<std::string> errmsgs;
+    {
+        auto tp = std::make_shared<spdlog::details::thread_pool>(queue_size, 1);
+        auto logger = std::make_shared<spdlog::async_logger>(
+            "as", test_sink, tp, spdlog::async_overflow_policy::discard_new);
+
+        logger->set_error_handler([&](const std::string &) {
+            std::unique_lock<std::mutex> lock(mtx);
+            errmsgs.push_back("Broken promise");
+        });
+
+        for (size_t i = 0; i < messages; i++) {
+            logger->info("Hello message #{}", i);
+        }
+
+        std::vector<std::thread> threads;
+        for (size_t i = 0; i < n_threads; i++) {
+            threads.emplace_back([logger, flush_count] {
+                for (size_t j = 0; j < flush_count; j++) {
+                    // flush does not throw exception even if failed.
+                    // Instead, the error handler is invoked.
+                    logger->flush();
+                }
+            });
+        }
+
+        for (auto &t : threads) {
+            t.join();
+        }
+    }
+    REQUIRE(test_sink->flush_counter() >= 1);
+    REQUIRE(test_sink->flush_counter() + errmsgs.size() == n_threads * flush_count);
+    REQUIRE(errmsgs.size() >= 1);
+    REQUIRE(errmsgs[0] == "Broken promise");
+}
+
 TEST_CASE("async periodic flush", "[async]") {
     auto logger = spdlog::create_async<spdlog::sinks::test_sink_mt>("as");
     auto test_sink = std::static_pointer_cast<spdlog::sinks::test_sink_mt>(logger->sinks()[0]);