Exchange promise for condition_variable when flushing (fixes #3221) (#3228)

std::promise and std::future use std::call_once under the hood, which requires
the tls-model to be at least initial_exec, excluding local_exec.

Furthermore, gcc has a bug regarding exceptions in std::call_once that
is best avoided. See https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66146
for more info.

Signed-off-by: Michael de Lang <kingoipo@gmail.com>

include/spdlog/async_logger-inl.h

@@ -32,28 +32,30 @@ SPDLOG_INLINE spdlog::async_logger::async_logger(std::string logger_name,
 
 // send the log message to the thread pool
 SPDLOG_INLINE void spdlog::async_logger::sink_it_(const details::log_msg &msg){
-    SPDLOG_TRY{if (auto pool_ptr = thread_pool_.lock()){
+    SPDLOG_TRY {
+        if (auto pool_ptr = thread_pool_.lock()){
             pool_ptr->post_log(shared_from_this(), msg, overflow_policy_);
-}
-else {
+        }
+        else {
             throw_spdlog_ex("async log: thread pool doesn't exist anymore");
-}
-}
-SPDLOG_LOGGER_CATCH(msg.source)
+        }
+    }
+    SPDLOG_LOGGER_CATCH(msg.source)
 }
 
 // send flush request to the thread pool
-SPDLOG_INLINE void spdlog::async_logger::flush_(){SPDLOG_TRY{auto pool_ptr = thread_pool_.lock();
-if (!pool_ptr) {
+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())
+        // Wait for the flush operation to complete.
+        // This might throw exception if the flush message get dropped because of overflow.
+        pool_ptr->post_and_wait_for_flush(shared_from_this(), overflow_policy_);
+    }
+    SPDLOG_LOGGER_CATCH(source_loc())
 }
 
 //

include/spdlog/details/thread_pool-inl.h

@@ -62,13 +62,25 @@ void SPDLOG_INLINE thread_pool::post_log(async_logger_ptr &&worker_ptr,
     post_async_msg_(std::move(async_m), overflow_policy);
 }
 
-std::future<void> SPDLOG_INLINE thread_pool::post_flush(async_logger_ptr &&worker_ptr,
+void SPDLOG_INLINE thread_pool::post_and_wait_for_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;
+    std::mutex m;
+    std::unique_lock<std::mutex> l(m);
+    std::condition_variable cv;
+    std::atomic<async_msg_flush> cv_flag{async_msg_flush::not_synced};
+    post_async_msg_(async_msg(std::move(worker_ptr), async_msg_type::flush, [&cv, &cv_flag](async_msg_flush flushed) {
+        cv_flag.store(flushed, std::memory_order_relaxed);
+        cv.notify_all();
+    }), overflow_policy);
+    while(cv_flag.load(std::memory_order_relaxed) == async_msg_flush::not_synced) {
+        cv.wait_for(l, std::chrono::milliseconds(100), [&cv_flag]() {
+            return cv_flag.load(std::memory_order_relaxed) != async_msg_flush::not_synced;
+        });
+    }
+
+    if(cv_flag.load(std::memory_order_relaxed) == async_msg_flush::synced_not_flushed) {
+        throw spdlog_ex("Request for flushing got dropped.");
+    }
 }
 
 size_t SPDLOG_INLINE thread_pool::overrun_counter() { return q_.overrun_counter(); }
@@ -112,7 +124,10 @@ 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();
+            if(incoming_async_msg.flush_callback) {
+                incoming_async_msg.flush_callback(async_msg_flush::synced_flushed);
+                incoming_async_msg.flush_callback = nullptr;
+            }
             return true;
         }
 

include/spdlog/details/thread_pool.h

@@ -9,7 +9,6 @@
 
 #include <chrono>
 #include <functional>
-#include <future>
 #include <memory>
 #include <thread>
 #include <vector>
@@ -23,55 +22,60 @@ using async_logger_ptr = std::shared_ptr<spdlog::async_logger>;
 
 enum class async_msg_type { log, flush, terminate };
 
+enum class async_msg_flush { not_synced, synced_flushed, synced_not_flushed };
+
 // Async msg to move to/from the queue
 // Movable only. should never be copied
 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;
+    std::function<void(async_msg_flush)> flush_callback;
 
     async_msg() = default;
-    ~async_msg() = default;
+    ~async_msg() {
+        if (flush_callback) {
+            flush_callback(async_msg_flush::synced_not_flushed);
+            flush_callback = nullptr;
+        }
+    }
 
     // should only be moved in or out of the queue..
     async_msg(const async_msg &) = delete;
 
-// support for vs2013 move
-#if defined(_MSC_VER) && _MSC_VER <= 1800
-    async_msg(async_msg &&other)
+    async_msg(async_msg &&other) SPDLOG_NOEXCEPT
         : log_msg_buffer(std::move(other)),
           msg_type(other.msg_type),
-          worker_ptr(std::move(other.worker_ptr)) {}
-
-    async_msg &operator=(async_msg &&other) {
-        *static_cast<log_msg_buffer *>(this) = std::move(other);
+          worker_ptr(std::move(other.worker_ptr)),
+          flush_callback(std::move(other.flush_callback)) {
+        other.flush_callback = nullptr;
+    }
+    async_msg &operator=(async_msg &&other) SPDLOG_NOEXCEPT {
+        *static_cast<log_msg_buffer *>(this) = static_cast<log_msg_buffer&&>(other);
         msg_type = other.msg_type;
         worker_ptr = std::move(other.worker_ptr);
+        flush_callback = std::move(other.flush_callback);
+        other.flush_callback = nullptr;
         return *this;
     }
-#else  // (_MSC_VER) && _MSC_VER <= 1800
-    async_msg(async_msg &&) = default;
-    async_msg &operator=(async_msg &&) = default;
-#endif
 
     // construct from log_msg with given type
     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)},
-          flush_promise{} {}
+          flush_callback{} {}
 
     async_msg(async_logger_ptr &&worker, async_msg_type the_type)
         : log_msg_buffer{},
           msg_type{the_type},
           worker_ptr{std::move(worker)},
-          flush_promise{} {}
+          flush_callback{} {}
 
-    async_msg(async_logger_ptr &&worker, async_msg_type the_type, std::promise<void> &&promise)
+    async_msg(async_logger_ptr &&worker, async_msg_type the_type, std::function<void(async_msg_flush)> &&callback)
         : log_msg_buffer{},
           msg_type{the_type},
           worker_ptr{std::move(worker)},
-          flush_promise{std::move(promise)} {}
+          flush_callback{std::move(callback)} {}
 
     explicit async_msg(async_msg_type the_type)
         : async_msg{nullptr, the_type} {}
@@ -98,7 +102,7 @@ public:
     void post_log(async_logger_ptr &&worker_ptr,
                   const details::log_msg &msg,
                   async_overflow_policy overflow_policy);
-    std::future<void> post_flush(async_logger_ptr &&worker_ptr,
+    void post_and_wait_for_flush(async_logger_ptr &&worker_ptr,
                                  async_overflow_policy overflow_policy);
     size_t overrun_counter();
     void reset_overrun_counter();