WebServer: Don't make shutdown wait for a stuck server registration

Registering with report.ppsspp.org starts with a DNS lookup that can't be
cancelled, and it ran on the server thread, so ShutdownWebServer() from
onDestroy waited for it on the main thread (an ANR on Play). It now runs on its
own thread, which the server leaves behind if it's still in the lookup.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Core/WebServer.cpp

@@ -65,6 +65,10 @@ static std::atomic<bool> serverRequireExactPort;
 // Set when the server is asked to stop, so a registration in progress gives up instead of making
 // shutdown wait for its timeouts. A plain bool, since that's what the HTTP client polls.
 static bool registerCancelled;
+// Registration runs on its own thread, since its DNS lookup can't be cancelled and can take
+// many seconds on a bad network. The server thread mustn't wait for it, or shutdown does too.
+static std::thread registerThread;
+static std::atomic<bool> registerRunning;
 
 std::mutex g_webServerLock;
 static Path g_uploadPath;  // TODO: Supply this through registration instead.
@@ -822,6 +826,43 @@ void OpenWebDebugger() {
 	System_LaunchUrl(LaunchUrlType::BROWSER_URL, "http://localhost:" + std::to_string(g_Config.iRemoteISOPort) + "/debugger/index.html");
 }
 
+// Called from the server thread.
+static void StartRegisterServer(int port) {
+	if (registerRunning) {
+		// The previous attempt is still stuck. Try again next time.
+		return;
+	}
+	if (registerThread.joinable()) {
+		registerThread.join();
+	}
+	registerRunning = true;
+	registerThread = std::thread([port]() {
+		SetCurrentThreadName("RegisterServer");
+		AndroidJNIThreadContext context;
+		RegisterServer(port);
+		registerRunning = false;
+	});
+}
+
+// Called from the server thread on its way out. Cancelling makes connecting and the request give
+// up, but not a DNS lookup in progress, so leave a thread that's still in one to finish alone.
+//
+// Detaching is safe because the thread only uses its own http::Client, the two statics above and
+// the log (which drops messages after LogManager::Shutdown(), and is never freed while the app
+// runs). If the server is restarted before it finishes, it may still register the old port, and
+// StartRegisterServer() skips registering until it's done. Only a process exit during the lookup
+// can catch it, the same as the download threads.
+static void FinishRegisterServer() {
+	if (!registerThread.joinable()) {
+		return;
+	}
+	if (registerRunning) {
+		registerThread.detach();
+	} else {
+		registerThread.join();
+	}
+}
+
 static void WebServerThread() {
 	SetCurrentThreadName("HTTPServer");
 
@@ -870,7 +911,7 @@ static void WebServerThread() {
 
 	double lastRegister = time_now_d();
 	if (running) {
-		RegisterServer(http->Port());
+		StartRegisterServer(http->Port());
 		lastRegister = time_now_d();
 
 		// NOTICE rather than INFO on purpose: with --debugger=0 the port is picked for us, and this
@@ -890,7 +931,7 @@ static void WebServerThread() {
 		http->RunSlice(webServerSliceSeconds);
 		double now = time_now_d();
 		if (now > lastRegister + 540.0) {
-			RegisterServer(http->Port());
+			StartRegisterServer(http->Port());
 			lastRegister = now;
 		}
 	}
@@ -899,6 +940,7 @@ static void WebServerThread() {
 	http->Stop();
 	StopAllDebuggers();
 	delete http;
+	FinishRegisterServer();
 
 	UpdateStatus(ServerStatus::FINISHED);
 	INFO_LOG(Log::HTTP, "Left web server loop.");