fix: SIGSEGV on dedicated server when HTTP requests are in flight during shutdown

Source/FicsitRemoteMonitoring/Private/FicsitRemoteMonitoring.cpp

@@ -1,5 +1,6 @@
 #include "FicsitRemoteMonitoring.h"
 
+#include <atomic>
 #include <sstream>
 
 #include "Runtime/Core/Public/Logging/LogCategory.h"
@@ -37,6 +38,36 @@
 us_listen_socket_t* SocketListener;
 bool SocketRunning = false;
 
+// Set as soon as teardown begins, cleared when a server starts. Deliberately a
+// file-scope atomic rather than a member: once teardown starts, the subsystem
+// actor and the world are being destroyed, so the uWS worker thread cannot
+// safely touch `this`, the World, or any UObject — not even to call IsValid(),
+// which must dereference the object to read its flags. A dangling pointer is
+// not null, so the IsValid() guards inside CallEndpoint() are reached and then
+// fault. This flag is the only state a request handler can consult without
+// dereferencing anything that may already be freed.
+std::atomic<bool> GFRMShuttingDown{false};
+
+// Shutdown barrier for uWS request handlers. Answers 503 and reports true when
+// teardown has begun, so the caller returns before touching the subsystem, the
+// world, or any other UObject. Touches nothing but the response socket, which
+// uWS owns and keeps alive for the duration of the callback.
+static bool FRMRejectIfShuttingDown(uWS::HttpResponse<false>* Res)
+{
+	if (!GFRMShuttingDown.load(std::memory_order_acquire))
+	{
+		return false;
+	}
+
+	if (Res)
+	{
+		Res->writeStatus("503 Service Unavailable");
+		Res->end("{\"error\":\"Server is shutting down\"}");
+	}
+
+	return true;
+}
+
 AFicsitRemoteMonitoring* AFicsitRemoteMonitoring::Get(UWorld* WorldContext)
 {
 	for (TActorIterator<AFicsitRemoteMonitoring> It(WorldContext, StaticClass(), EActorIteratorFlags::AllActors); It; ++It) {
@@ -65,23 +96,33 @@ void AFicsitRemoteMonitoring::BeginPlay()
 	// Load FRM's API Endpoints
 	InitAPIRegistry();
 
+	// Read the auth token ONCE here, on the game thread, and cache it in the
+	// AuthenticationToken member. The uWS request handlers must not read config
+	// per-request: that runs on the uWS worker thread and reaches GEngine, which
+	// is unsafe while the engine is tearing down (observed SIGSEGV).
 	const FString AuthToken = UFRMConfigManager::GetConfigOrDefault<FString>(TEXT("uWS.AuthenticationToken"), "");
-	
+
 	// Debug log to verify token retrieval -Porisius
 	// UE_LOGFMT(LogHttpServer, Log, "DEBUG: AuthToken - {AuthToken}", *AuthToken);
-	
+
 	if (AuthToken.IsEmpty())
 	{
-		if (!UFRMConfigManager::SetConfigFromInput(TEXT("uWS.AuthenticationToken"), GenerateAuthToken(32), false))
+		const FString GeneratedToken = GenerateAuthToken(32);
+		if (!UFRMConfigManager::SetConfigFromInput(TEXT("uWS.AuthenticationToken"), GeneratedToken, false))
 		{
 			UE_LOG(LogTemp, Warning, TEXT("Failed to apply setting"));
 			return;
 		}
 
-		UE_LOG(LogTemp, Log, TEXT("Generated and saved new token: %s"), *AuthToken);
+		AuthenticationToken = GeneratedToken;
+
+		// Log the token that was actually generated. This previously logged
+		// AuthToken, which is empty on this branch by definition.
+		UE_LOG(LogTemp, Log, TEXT("Generated and saved new token: %s"), *GeneratedToken);
 	}
 	else
 	{
+		AuthenticationToken = AuthToken;
 		UE_LOG(LogTemp, Log, TEXT("Token already exists."));
 	}
 
@@ -128,6 +169,11 @@ void AFicsitRemoteMonitoring::EndPlay(const EEndPlayReason::Type EndPlayReason)
 
 void AFicsitRemoteMonitoring::StopWebSocketServer()
 {
+	// Raise the shutdown barrier FIRST, before anything is torn down, so any
+	// request already inside the uWS worker thread bails out with 503 instead
+	// of walking into a half-destroyed world.
+	GFRMShuttingDown.store(true, std::memory_order_release);
+
 	bShouldStop = true;
 
     // Signal the WebSocket server to stop
@@ -168,10 +214,15 @@ FArduinoConfig AFicsitRemoteMonitoring::GetSerialConfig()
 	return Config;
 }
 
-void AFicsitRemoteMonitoring::StartWebSocketServer(bool bSkipIfRunning) 
+void AFicsitRemoteMonitoring::StartWebSocketServer(bool bSkipIfRunning)
 {
     UE_LOGFMT(LogHttpServer, Log, "Initializing WebSocket Service");
 
+	// Lower the shutdown barrier: a previous StopWebSocketServer() raised it, and
+	// without this a server restarted in the same session (e.g. `/frm http start`
+	// after a stop) would answer 503 to every request forever.
+	GFRMShuttingDown.store(false, std::memory_order_release);
+
     if (SocketRunning)
     {
 	    if (bSkipIfRunning)
@@ -290,13 +341,14 @@ void AFicsitRemoteMonitoring::StartWebSocketServer(bool bSkipIfRunning)
                 });
 
                 app.get("/api/:APIEndpoint", [this, World](auto* res, auto* req) {
+                    if (FRMRejectIfShuttingDown(res)) return;
                     std::string url(req->getParameter("APIEndpoint"));
                     FString Endpoint = FString(url.c_str());
 
                     // Log the request URL
                     //UE_LOGFMT(LogHttpServer, Log, "Request URL: {0}", Endpoint);
 
-                	const FString AuthToken = UFRMConfigManager::GetConfigOrDefault<FString>(TEXT("uWS.AuthenticationToken"), "");
+                	const FString AuthToken = AuthenticationToken;
 
                 	FRequestData RequestData;
                 	RequestData.bIsAuthorized = IsAuthorizedRequest(req, AuthToken);
@@ -313,11 +365,13 @@ void AFicsitRemoteMonitoring::StartWebSocketServer(bool bSkipIfRunning)
 
             	app.post("/*", [this, World](auto* res, uWS::HttpRequest* req)
             	{
+            		if (FRMRejectIfShuttingDown(res)) return;
 		            const std::string URL(req->getUrl().begin(), req->getUrl().end());
 					FString RelativePath = FString(URL.c_str()).Mid(1);
 
             		res->onData([this, res, req, World, RelativePath](const std::string_view data, bool)
             		{
+            			if (FRMRejectIfShuttingDown(res)) return;
 			            try
 			            {
 				            const std::string PostData(data);
@@ -330,7 +384,7 @@ void AFicsitRemoteMonitoring::StartWebSocketServer(bool bSkipIfRunning)
 			            		return UFRM_RequestLibrary::SendErrorMessage(res, "400 Bad Request", FString("Invalid Request Body"));
 			            	}
 
-			            	const FString AuthToken = UFRMConfigManager::GetConfigOrDefault<FString>(TEXT("uWS.AuthenticationToken"), "");
+			            	const FString AuthToken = AuthenticationToken;
 
 			            	FRequestData RequestData;
 			            	RequestData.Method = "POST";
@@ -363,11 +417,12 @@ void AFicsitRemoteMonitoring::StartWebSocketServer(bool bSkipIfRunning)
             	});
 
                 app.get("/*", [this, UIPath, World](auto* res, uWS::HttpRequest* req) {
+                    if (FRMRejectIfShuttingDown(res)) return;
                     if (!res) return;
 
                     std::string url(req->getUrl().begin(), req->getUrl().end());
 
-                	const FString AuthToken = UFRMConfigManager::GetConfigOrDefault<FString>(TEXT("uWS.AuthenticationToken"), "");
+                	const FString AuthToken = AuthenticationToken;
 
                     bool bFileExists = false;
                     // Remove initial '/'

Source/FicsitRemoteMonitoring/Public/Libraries/FRMConfigManager.h

@@ -22,6 +22,17 @@ class FICSITREMOTEMONITORING_API UFRMConfigManager : public UBlueprintFunctionLi
 	template<typename T>
 	static bool GetConfig(const FString& StrID, T& OutValue)
 	{
+		// GetFGGameUserSettings() dereferences GEngine internally, so GEngine must
+		// be checked BEFORE the call — the UserSettings null-check below happens
+		// too late to help. During engine teardown GEngine is destroyed while the
+		// uWS worker thread may still be serving a request, and the dereference
+		// faults (SIGSEGV reading a small member offset). Degrade to the caller's
+		// default instead of crashing the server.
+		if (!GEngine)
+		{
+			return false;
+		}
+
 		UFGGameUserSettings* UserSettings = UFGGameUserSettings::GetFGGameUserSettings();
 		if (!UserSettings)
 		{