From 2ff513f0cd9ce9852a7aea889fd1673c29723dce Mon Sep 17 00:00:00 2001
From: Jonathan G Rennison <j.g.rennison@gmail.com>
Date: Sun, 9 Apr 2023 01:34:04 +0100
Subject: [PATCH] Worker thread pool: Fix worker wake CV notify condition

The worker wake CV was only notified when the job queue was empty.
Now notify if the number of queued jobs is less than the number
of waiting workers.
---
 src/worker_thread.cpp | 10 ++++++----
 src/worker_thread.h   |  3 ++-
 2 files changed, 8 insertions(+), 5 deletions(-)

diff --git a/src/worker_thread.cpp b/src/worker_thread.cpp
index 64ef0325d1..a138973a76 100644
--- a/src/worker_thread.cpp
+++ b/src/worker_thread.cpp
@@ -42,7 +42,7 @@ void WorkerThreadPool::Stop()
 {
 	std::unique_lock<std::mutex> lk(this->lock);
 	this->exit = true;
-	this->empty_cv.notify_all();
+	this->worker_wait_cv.notify_all();
 	this->done_cv.wait(lk, [this]() { return this->workers == 0; });
 }
 
@@ -55,10 +55,10 @@ void WorkerThreadPool::EnqueueJob(WorkerJobFunc *func, void *data1, void *data2,
 		func(data1, data2, data3);
 		return;
 	}
-	bool notify = this->jobs.empty();
+	bool notify = this->jobs.size() < (size_t)this->workers_waiting;
 	this->jobs.push({ func, data1, data2, data3 });
 	lk.unlock();
-	if (notify) this->empty_cv.notify_one();
+	if (notify) this->worker_wait_cv.notify_one();
 }
 
 void WorkerThreadPool::Run(WorkerThreadPool *pool)
@@ -66,7 +66,9 @@ void WorkerThreadPool::Run(WorkerThreadPool *pool)
 	std::unique_lock<std::mutex> lk(pool->lock);
 	while (!pool->exit || !pool->jobs.empty()) {
 		if (pool->jobs.empty()) {
-			pool->empty_cv.wait(lk);
+			pool->workers_waiting++;
+			pool->worker_wait_cv.wait(lk);
+			pool->workers_waiting--;
 		} else {
 			WorkerJob job = pool->jobs.front();
 			pool->jobs.pop();
diff --git a/src/worker_thread.h b/src/worker_thread.h
index 2a2b2134a6..6000af3c78 100644
--- a/src/worker_thread.h
+++ b/src/worker_thread.h
@@ -30,10 +30,11 @@ private:
 	};
 
 	uint workers = 0;
+	uint workers_waiting = 0;
 	bool exit = false;
 	std::mutex lock;
 	std::queue<WorkerJob> jobs;
-	std::condition_variable empty_cv;
+	std::condition_variable worker_wait_cv;
 	std::condition_variable done_cv;
 
 	static void Run(WorkerThreadPool *pool);