fix(single-instance): limit second instance's wait on Windows (#3645)

* Update windows.rs

* Create single-instance-forward-timeout.md

* use SMTO_NORMAL

* nit

---------

Co-authored-by: Lucas Nogueira <lucas@tauri.app>

Author: justanotheranonymoususer

.changes/single-instance-forward-timeout.md

@@ -0,0 +1,5 @@
+---
+"single-instance": patch
+---
+
+On Windows, a second instance no longer waits indefinitely for a first instance that stopped processing messages: it stops waiting after 10 seconds and exits. The first instance may still receive the arguments once it resumes processing messages.

plugins/single-instance/src/platform_impl/windows.rs

@@ -21,14 +21,17 @@ use windows_sys::Win32::{
     UI::WindowsAndMessaging::{
         self as w32wm, AllowSetForegroundWindow, CREATESTRUCTW, CreateWindowExW, DefWindowProcW,
         DestroyWindow, FindWindowW, GWL_STYLE, GWLP_USERDATA, GetWindowThreadProcessId,
-        RegisterClassExW, SendMessageW, WINDOW_LONG_PTR_INDEX, WM_COPYDATA, WM_CREATE, WM_DESTROY,
-        WNDCLASSEXW, WS_EX_LAYERED, WS_EX_NOACTIVATE, WS_EX_TOOLWINDOW, WS_EX_TRANSPARENT,
-        WS_OVERLAPPED, WS_POPUP, WS_VISIBLE,
+        RegisterClassExW, SMTO_NORMAL, SendMessageTimeoutW, WINDOW_LONG_PTR_INDEX, WM_COPYDATA,
+        WM_CREATE, WM_DESTROY, WNDCLASSEXW, WS_EX_LAYERED, WS_EX_NOACTIVATE, WS_EX_TOOLWINDOW,
+        WS_EX_TRANSPARENT, WS_OVERLAPPED, WS_POPUP, WS_VISIBLE,
     },
 };
 
 const WMCOPYDATA_SINGLE_INSTANCE_DATA: usize = 1542;
 
+/// How long the second instance waits for the first instance to take its arguments.
+const FORWARD_TIMEOUT_MS: u32 = 10_000;
+
 struct MutexHandle(isize);
 
 struct TargetWindowHandle(isize);
@@ -102,7 +105,21 @@ pub fn init<R: Runtime>(callback: Box<SingleInstanceCallback<R>>) -> TauriPlugin
                             lpData: bytes.as_ptr() as _,
                         };
 
-                        SendMessageW(hwnd, WM_COPYDATA, 0, &cds as *const _ as _);
+                        let sent = SendMessageTimeoutW(
+                            hwnd,
+                            WM_COPYDATA,
+                            0,
+                            &cds as *const _ as _,
+                            SMTO_NORMAL,
+                            FORWARD_TIMEOUT_MS,
+                            std::ptr::null_mut(),
+                        );
+                        if sent == 0 {
+                            tracing::warn!(
+                                "single_instance failed to forward arguments to the running instance within {FORWARD_TIMEOUT_MS}ms (error {})",
+                                GetLastError()
+                            );
+                        }
 
                         app.cleanup_before_exit();
                         std::process::exit(0);