From 5e487e28988f5dec8acb1e3be51afc2ebfdc1374 Mon Sep 17 00:00:00 2001
From: Stefan Boberg <stefan@boberg.org>
Date: Sun, 15 Mar 2026 20:56:12 +0100
Subject: Fix thread vector growth in dynamic thread pools

Exiting threads now register their ID before returning, and
PruneExitedThreads() joins and removes them on the next scale-up,
preventing unbounded vector growth through repeated scale cycles.
---
 src/zencore/workthreadpool.cpp | 55 +++++++++++++++++++++++++++++++++---------
 1 file changed, 44 insertions(+), 11 deletions(-)

(limited to 'src/zencore/workthreadpool.cpp')

diff --git a/src/zencore/workthreadpool.cpp b/src/zencore/workthreadpool.cpp
index 25a1d982a..b179527d7 100644
--- a/src/zencore/workthreadpool.cpp
+++ b/src/zencore/workthreadpool.cpp
@@ -11,6 +11,7 @@
 #include <zencore/thread.h>
 #include <zencore/trace.h>
 
+#include <algorithm>
 #include <thread>
 #include <vector>
 
@@ -195,17 +196,19 @@ struct WorkerThreadPool::ThreadStartInfo
 
 struct ExplicitImpl : WorkerThreadPool::Impl
 {
-	const int				  m_MinThreads;
-	const int				  m_MaxThreads;
-	std::atomic<int>		  m_TotalThreads{0};
-	std::atomic<int>		  m_ActiveCount{0};
-	void					  WorkerThreadFunction(WorkerThreadPool::ThreadStartInfo Info);
-	void					  SpawnWorkerThread();
-	std::string				  m_WorkerThreadBaseName;
-	RwLock					  m_ThreadListLock;
-	std::vector<std::thread>  m_WorkerThreads;
-	BlockingQueue<Ref<IWork>> m_WorkQueue;
-	std::atomic<int>		  m_FreeWorkerCount{0};
+	const int					 m_MinThreads;
+	const int					 m_MaxThreads;
+	std::atomic<int>			 m_TotalThreads{0};
+	std::atomic<int>			 m_ActiveCount{0};
+	void						 WorkerThreadFunction(WorkerThreadPool::ThreadStartInfo Info);
+	void						 SpawnWorkerThread();
+	void						 PruneExitedThreads();
+	std::string					 m_WorkerThreadBaseName;
+	RwLock						 m_ThreadListLock;
+	std::vector<std::thread>	 m_WorkerThreads;
+	std::vector<std::thread::id> m_ExitedThreadIds;
+	BlockingQueue<Ref<IWork>>	 m_WorkQueue;
+	std::atomic<int>			 m_FreeWorkerCount{0};
 
 	bool ScalingEnabled() const { return m_MinThreads != m_MaxThreads; }
 
@@ -286,6 +289,31 @@ struct ExplicitImpl : WorkerThreadPool::Impl
 	}
 };
 
+void
+ExplicitImpl::PruneExitedThreads()
+{
+	// Must be called under m_ThreadListLock
+	if (m_ExitedThreadIds.empty())
+	{
+		return;
+	}
+
+	for (auto It = m_WorkerThreads.begin(); It != m_WorkerThreads.end();)
+	{
+		auto IdIt = std::find(m_ExitedThreadIds.begin(), m_ExitedThreadIds.end(), It->get_id());
+		if (IdIt != m_ExitedThreadIds.end())
+		{
+			It->join();
+			It = m_WorkerThreads.erase(It);
+			m_ExitedThreadIds.erase(IdIt);
+		}
+		else
+		{
+			++It;
+		}
+	}
+}
+
 void
 ExplicitImpl::SpawnWorkerThread()
 {
@@ -293,6 +321,7 @@ ExplicitImpl::SpawnWorkerThread()
 	const int				ThreadNumber = ++s_DynamicThreadIndex;
 
 	RwLock::ExclusiveLockScope _(m_ThreadListLock);
+	PruneExitedThreads();
 	m_WorkerThreads.emplace_back(&ExplicitImpl::WorkerThreadFunction, this, WorkerThreadPool::ThreadStartInfo{ThreadNumber, nullptr});
 }
 
@@ -361,6 +390,10 @@ ExplicitImpl::WorkerThreadFunction(WorkerThreadPool::ThreadStartInfo Info)
 							  m_WorkerThreadBaseName,
 							  CurrentTotal - 1);
 					m_FreeWorkerCount--;
+					{
+						RwLock::ExclusiveLockScope _(m_ThreadListLock);
+						m_ExitedThreadIds.push_back(std::this_thread::get_id());
+					}
 					return;	 // Thread exits
 				}
 			}
-- 
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