fix(linux): re-apply signal handlers periodically to survive JSC lazy init (#5507)

* fix(linux): re-apply signal handlers periodically to survive JSC lazy init

WebKit's JavaScriptCore (JSC) lazily installs signal handlers (SIGSEGV for
JIT crash recovery, SIGUSR1 for GC thread sync) when JavaScript first
executes - not at WebView creation. These handlers are registered without
SA_ONSTACK, overwriting Go's handler which requires it.

The existing one-shot fix runs before any JS executes, so JSC wipes it out
immediately when it initialises. A 50ms periodic timer (100 iterations =
5s) running on the GTK main thread re-applies install_signal_handlers()
throughout the JSC initialisation window, ensuring Go's SA_ONSTACK
requirement is always satisfied.

Fixes #5506

* fix(linux): add SIGUSR1, WEBKIT_LOAD_FINISHED hook, and anchor GTK4 timer

Addresses code review feedback on #5507:

- Add SIGUSR1 to install_signal_handlers(): JSC uses SIGUSR1 for GC
  thread synchronisation and installs it without SA_ONSTACK (visible as
  "Overriding existing handler for signal 10" in stderr). The fix_signal
  helper only adds SA_ONSTACK to whatever handler is already registered,
  so this is safe for both Go's and JSC's handler.

- Hook WEBKIT_LOAD_FINISHED (v3 GTK4/GTK3) and DomReady (v2): by
  page-load completion JSC is guaranteed to have initialised and
  installed all its signal handlers. A targeted call to
  install_signal_handlers() here provides a deterministic, event-driven
  fix that complements the periodic timer.

- Anchor the GTK4 periodic timer to first WebView creation (sync.Once
  inside windowNewWebview) rather than appNew(), mirroring the GTK3
  pattern. This ensures the 5s coverage window starts when JSC can
  actually initialise, not before any WebView exists.

* fix(linux): immediate install_signal_handlers in GTK4 fixSignalHandlers.Do

Author: leaanthony

v2/internal/frontend/desktop/linux/frontend.go

@@ -65,6 +65,9 @@ static void install_signal_handlers()
 #if defined(SIGSEGV)
     fix_signal(SIGSEGV);
 #endif
+#if defined(SIGUSR1)
+    fix_signal(SIGUSR1);
+#endif
 #if defined(SIGXCPU)
     fix_signal(SIGXCPU);
 #endif
@@ -79,8 +82,24 @@ static gboolean install_signal_handlers_idle(gpointer data) {
     return G_SOURCE_REMOVE;
 }
 
+// WebKit's JSC lazily installs signal handlers without SA_ONSTACK when
+// JavaScript first executes. This timer re-applies the fix every 50ms
+// for the first 5 seconds, covering the JSC initialization window.
+static gboolean install_signal_handlers_timeout(gpointer data) {
+    install_signal_handlers();
+    int *remaining = (int *)data;
+    (*remaining)--;
+    if (*remaining <= 0) {
+        return G_SOURCE_REMOVE;
+    }
+    return G_SOURCE_CONTINUE;
+}
+
 static void fix_signal_handlers_after_gtk_init() {
     g_idle_add(install_signal_handlers_idle, NULL);
+    int *remaining = (int *)g_malloc(sizeof(int));
+    *remaining = 100;
+    g_timeout_add_full(G_PRIORITY_DEFAULT, 50, install_signal_handlers_timeout, remaining, g_free);
 }
 
 */
@@ -442,6 +461,9 @@ var edgeMap = map[string]uintptr{
 
 func (f *Frontend) processMessage(message string) {
 	if message == "DomReady" {
+		// JSC is guaranteed to have initialised by page-load completion, so
+		// re-apply SA_ONSTACK now to cover any handlers it installed during load.
+		C.install_signal_handlers()
 		if f.frontendOptions.OnDomReady != nil {
 			f.frontendOptions.OnDomReady(f.ctx)
 		}

v3/pkg/application/linux_cgo.c

@@ -114,6 +114,9 @@ void install_signal_handlers(void) {
 #if defined(SIGSEGV)
     fix_signal(SIGSEGV);
 #endif
+#if defined(SIGUSR1)
+    fix_signal(SIGUSR1);
+#endif
 #if defined(SIGXCPU)
     fix_signal(SIGXCPU);
 #endif
@@ -122,6 +125,25 @@ void install_signal_handlers(void) {
 #endif
 }
 
+// WebKit's JSC lazily installs signal handlers without SA_ONSTACK when
+// JavaScript first executes. This timer re-applies the fix every 50ms
+// for the first 5 seconds, covering the JSC initialization window.
+static gboolean install_signal_handlers_timeout(gpointer data) {
+    install_signal_handlers();
+    int *remaining = (int *)data;
+    (*remaining)--;
+    if (*remaining <= 0) {
+        return G_SOURCE_REMOVE;
+    }
+    return G_SOURCE_CONTINUE;
+}
+
+void schedule_signal_handler_fix(void) {
+    int *remaining = (int *)g_malloc(sizeof(int));
+    *remaining = 100;
+    g_timeout_add_full(G_PRIORITY_DEFAULT, 50, install_signal_handlers_timeout, remaining, g_free);
+}
+
 // ============================================================================
 // Object data helpers
 // ============================================================================

v3/pkg/application/linux_cgo.go

@@ -81,7 +81,10 @@ var (
 	mainThreadId        *C.GThread
 )
 
-var registerURIScheme sync.Once
+var (
+	registerURIScheme sync.Once
+	fixSignalHandlers sync.Once
+)
 
 func init() {
 	gtkSignalToMenuItem = map[uint]*MenuItem{}
@@ -1190,6 +1193,14 @@ func windowNewWebview(parentId uint, gpuPolicy WebviewGpuPolicy) pointer {
 			(*[0]byte)(C.onProcessRequest), nil, nil)
 	})
 
+	// Start the periodic signal-handler fix now that a WebView exists and JSC
+	// can actually initialise. Anchoring to first webview creation (not appNew)
+	// ensures the 5s window covers the JSC lazy-init race window.
+	fixSignalHandlers.Do(func() {
+		C.install_signal_handlers()
+		C.schedule_signal_handler_fix()
+	})
+
 	_ = networkSession
 	return pointer(webView)
 }
@@ -1492,6 +1503,9 @@ func handleLoadChanged(wv *C.WebKitWebView, event C.WebKitLoadEvent, data C.uint
 	case C.WEBKIT_LOAD_COMMITTED:
 		processWindowEvent(C.uint(data), C.uint(events.Linux.WindowLoadCommitted))
 	case C.WEBKIT_LOAD_FINISHED:
+		// JSC is guaranteed to have initialised by page-load completion, so
+		// re-apply SA_ONSTACK now to cover any handlers it installed during load.
+		C.install_signal_handlers()
 		processWindowEvent(C.uint(data), C.uint(events.Linux.WindowLoadFinished))
 	}
 }

v3/pkg/application/linux_cgo.h

@@ -83,6 +83,7 @@ void dispatchOnMainThread(unsigned int id);
 // ============================================================================
 
 void install_signal_handlers(void);
+void schedule_signal_handler_fix(void);
 
 // ============================================================================
 // Object data helpers

v3/pkg/application/linux_cgo_gtk3.go

@@ -222,6 +222,9 @@ static void install_signal_handlers() {
 	#if defined(SIGSEGV)
 		fix_signal(SIGSEGV);
 	#endif
+	#if defined(SIGUSR1)
+		fix_signal(SIGUSR1);
+	#endif
 	#if defined(SIGXCPU)
 		fix_signal(SIGXCPU);
 	#endif
@@ -230,6 +233,25 @@ static void install_signal_handlers() {
 	#endif
 }
 
+// WebKit's JSC lazily installs signal handlers without SA_ONSTACK when
+// JavaScript first executes. This timer re-applies the fix every 50ms
+// for the first 5 seconds, covering the JSC initialization window.
+static gboolean install_signal_handlers_timeout(gpointer data) {
+    install_signal_handlers();
+    int *remaining = (int *)data;
+    (*remaining)--;
+    if (*remaining <= 0) {
+        return G_SOURCE_REMOVE;
+    }
+    return G_SOURCE_CONTINUE;
+}
+
+static void schedule_signal_handler_fix() {
+    int *remaining = (int *)g_malloc(sizeof(int));
+    *remaining = 100;
+    g_timeout_add_full(G_PRIORITY_DEFAULT, 50, install_signal_handlers_timeout, remaining, g_free);
+}
+
 static int GetNumScreens(){
     return 0;
 }
@@ -1419,6 +1441,7 @@ func windowNewWebview(parentId uint, gpuPolicy WebviewGpuPolicy) pointer {
 
 	fixSignalHandlers.Do(func() {
 		C.install_signal_handlers()
+		C.schedule_signal_handler_fix()
 	})
 
 	C.save_webview_to_content_manager(unsafe.Pointer(manager), unsafe.Pointer(webView))
@@ -1731,6 +1754,9 @@ func handleLoadChanged(webview *C.WebKitWebView, event C.WebKitLoadEvent, data C
 	case C.WEBKIT_LOAD_COMMITTED:
 		processWindowEvent(C.uint(data), C.uint(events.Linux.WindowLoadCommitted))
 	case C.WEBKIT_LOAD_FINISHED:
+		// JSC is guaranteed to have initialised by page-load completion, so
+		// re-apply SA_ONSTACK now to cover any handlers it installed during load.
+		C.install_signal_handlers()
 		processWindowEvent(C.uint(data), C.uint(events.Linux.WindowLoadFinished))
 	}
 }