From c7d033755c1566f7908e3a66bced6b3c3d062d70 Mon Sep 17 00:00:00 2001
From: Stefan Boberg <stefan.boberg@epicgames.com>
Date: Fri, 20 Feb 2026 14:43:32 +0100
Subject: added new I/O thread pool implementation

the new variant manages a dynamically growing/shrinking set of threads manually instead of relying on the built-in Windows thread pool

the benefit of this is that we're in charge of setup and teardown so can make better guarantees about lifetimes of threads which can help with shutdown issues
---
 src/zenhttp/servers/httpsys.cpp | 53 +++++++++++++++++++++++++----------------
 1 file changed, 33 insertions(+), 20 deletions(-)

(limited to 'src/zenhttp/servers/httpsys.cpp')

diff --git a/src/zenhttp/servers/httpsys.cpp b/src/zenhttp/servers/httpsys.cpp
index c640ba90b..6809c280a 100644
--- a/src/zenhttp/servers/httpsys.cpp
+++ b/src/zenhttp/servers/httpsys.cpp
@@ -128,6 +128,7 @@ private:
 	bool m_IsAsyncResponseEnabled  = true;
 
 	std::unique_ptr<WinIoThreadPool> m_IoThreadPool;
+	bool							 m_IoThreadPoolIsWinTp = true;
 
 	RwLock			  m_AsyncWorkPoolInitLock;
 	WorkerThreadPool* m_AsyncWorkPool = nullptr;
@@ -374,7 +375,8 @@ public:
 	void IssueInitialRequest(std::error_code& ErrorCode);
 	bool IssueNextRequest(HttpSysRequestHandler* NewCompletionHandler);
 
-	PTP_IO				  Iocp();
+	void				  StartIo();
+	void				  CancelIo();
 	HANDLE				  RequestQueueHandle();
 	inline OVERLAPPED*	  Overlapped() { return &m_HttpOverlapped; }
 	inline HttpSysServer& Server() { return m_HttpServer; }
@@ -614,9 +616,8 @@ HttpMessageResponseRequest::IssueRequest(std::error_code& ErrorCode)
 	ZEN_TRACE_CPU("httpsys::Response::IssueRequest");
 	HttpSysTransaction& Tx		= Transaction();
 	HTTP_REQUEST* const HttpReq = Tx.HttpRequest();
-	PTP_IO const		Iocp	= Tx.Iocp();
 
-	StartThreadpoolIo(Iocp);
+	Tx.StartIo();
 
 	// Split payload into batches to play well with the underlying API
 
@@ -822,7 +823,7 @@ HttpMessageResponseRequest::IssueRequest(std::error_code& ErrorCode)
 
 		// An error occurred, no completion will be posted to IOCP
 
-		CancelThreadpoolIo(Iocp);
+		Tx.CancelIo();
 
 		// Emit diagnostics
 
@@ -1001,7 +1002,8 @@ HttpSysServer::HttpSysServer(const HttpSysConfig& InConfig)
 		MaxThreadCount *= 2;
 	}
 
-	m_IoThreadPool = std::make_unique<WinIoThreadPool>(MinThreadCount, MaxThreadCount);
+	m_IoThreadPoolIsWinTp = !m_InitialConfig.UseExplicitIoThreadPool;
+	m_IoThreadPool		  = WinIoThreadPool::Create(!m_IoThreadPoolIsWinTp, MinThreadCount, MaxThreadCount);
 
 	if (m_InitialConfig.AsyncWorkThreadCount == 0)
 	{
@@ -1018,10 +1020,11 @@ HttpSysServer::HttpSysServer(const HttpSysConfig& InConfig)
 	m_IsHttpInitialized = true;
 	m_IsOk				= true;
 
-	ZEN_INFO("http.sys server started in {} mode, using {}-{} I/O threads and {} async worker threads",
+	ZEN_INFO("http.sys server started in {} mode, using {}-{} I/O threads ({}) and {} async worker threads",
 			 m_InitialConfig.IsDedicatedServer ? "DEDICATED" : "NORMAL",
 			 MinThreadCount,
 			 MaxThreadCount,
+			 m_InitialConfig.UseExplicitIoThreadPool ? "explicit IOCP" : "Windows Thread Pool",
 			 m_InitialConfig.AsyncWorkThreadCount);
 }
 
@@ -1497,10 +1500,16 @@ HttpSysTransaction::~HttpSysTransaction()
 {
 }
 
-PTP_IO
-HttpSysTransaction::Iocp()
+void
+HttpSysTransaction::StartIo()
+{
+	m_HttpServer.m_IoThreadPool->StartIo();
+}
+
+void
+HttpSysTransaction::CancelIo()
 {
-	return m_HttpServer.m_IoThreadPool->Iocp();
+	m_HttpServer.m_IoThreadPool->CancelIo();
 }
 
 HANDLE
@@ -1521,7 +1530,6 @@ static std::atomic<int> HttpSysThreadIndex	   = 0;
 static void
 NameCurrentHttpSysThread()
 {
-	t_IsHttpSysThreadNamed						 = true;
 	const int						 ThreadIndex = ++HttpSysThreadIndex;
 	zen::ExtendableStringBuilder<16> ThreadName;
 	ThreadName << "httpio_" << ThreadIndex;
@@ -1538,19 +1546,25 @@ HttpSysTransaction::IoCompletionCallback(PTP_CALLBACK_INSTANCE Instance,
 {
 	ZEN_UNUSED(Io, Instance);
 
-	// Assign names to threads for context
-
-	if (!t_IsHttpSysThreadNamed)
-	{
-		NameCurrentHttpSysThread();
-	}
-
 	// Note that for a given transaction we may be in this completion function on more
 	// than one thread at any given moment. This means we need to be careful about what
 	// happens in here
 
 	HttpSysTransaction* Transaction = CONTAINING_RECORD(pOverlapped, HttpSysTransaction, m_HttpOverlapped);
 
+	// Assign names to threads for context (only needed when using Windows' native
+	// thread pool)
+
+	if (Transaction->Server().m_IoThreadPoolIsWinTp)
+	{
+		if (!t_IsHttpSysThreadNamed)
+		{
+			t_IsHttpSysThreadNamed = true;
+
+			NameCurrentHttpSysThread();
+		}
+	}
+
 	if (Transaction->HandleCompletion(IoResult, NumberOfBytesTransferred) == HttpSysTransaction::Status::kDone)
 	{
 		delete Transaction;
@@ -2020,10 +2034,9 @@ InitialRequestHandler::IssueRequest(std::error_code& ErrorCode)
 	ZEN_TRACE_CPU("httpsys::Request::IssueRequest");
 
 	HttpSysTransaction& Tx		= Transaction();
-	PTP_IO				Iocp	= Tx.Iocp();
 	HTTP_REQUEST*		HttpReq = Tx.HttpRequest();
 
-	StartThreadpoolIo(Iocp);
+	Tx.StartIo();
 
 	ULONG HttpApiResult;
 
@@ -2057,7 +2070,7 @@ InitialRequestHandler::IssueRequest(std::error_code& ErrorCode)
 
 	if (HttpApiResult != ERROR_IO_PENDING && HttpApiResult != NO_ERROR)
 	{
-		CancelThreadpoolIo(Iocp);
+		Tx.CancelIo();
 
 		ErrorCode = MakeErrorCode(HttpApiResult);
 
-- 
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