fix(linux): stop forcing SA_ONSTACK on SIGUSR1 to prevent WebKit freeze (#5530)

WebKit's JavaScriptCore uses SIGUSR1 to suspend/resume threads for
conservative GC stack scanning. The signal-handler fix added in
alpha.97 (#5507) re-applied SA_ONSTACK to every signal it touched,
including SIGUSR1. Once JSC installs its own SIGUSR1 handler it owns the
signal (Go no longer handles it), so forcing SA_ONSTACK makes that
handler run on Go's alternate signal stack, breaking GC thread
synchronisation. This froze the WebKit UI during idle garbage
collection (most visibly with the inspector open) while the Go backend
kept running.

Remove fix_signal(SIGUSR1) from install_signal_handlers in v3 (GTK4 +
GTK3) and v2. The genuine #5506 fix for SIGSEGV/SIGBUS (which Go needs
SA_ONSTACK for) is unaffected.

Fixes #5527

https://claude.ai/code/session_01AtT1CjDNRArXv1eddXte2a

Co-authored-by: Claude <noreply@anthropic.com>

Author: leaanthony

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

@@ -65,9 +65,12 @@ static void install_signal_handlers()
 #if defined(SIGSEGV)
     fix_signal(SIGSEGV);
 #endif
-#if defined(SIGUSR1)
-    fix_signal(SIGUSR1);
-#endif
+    // NOTE: Do NOT add SA_ONSTACK to SIGUSR1. WebKit's JavaScriptCore uses
+    // SIGUSR1 to suspend/resume threads for conservative GC stack scanning.
+    // Once JSC installs its own SIGUSR1 handler it owns the signal (Go no
+    // longer handles it), and forcing SA_ONSTACK makes that handler run on
+    // Go's alternate signal stack, breaking GC thread synchronisation and
+    // freezing WebKit during idle collection. See issue #5527.
 #if defined(SIGXCPU)
     fix_signal(SIGXCPU);
 #endif

v3/UNRELEASED_CHANGELOG.md

@@ -23,6 +23,7 @@ After processing, the content will be moved to the main changelog and this file
 
 ## Fixed
 <!-- Bug fixes -->
+- Fix WebKit UI freeze on Linux when idle (e.g. with the inspector open) by no longer forcing `SA_ONSTACK` on `SIGUSR1`, which broke JavaScriptCore's GC thread synchronisation (#5527)
 
 ## Deprecated
 <!-- Soon-to-be removed features -->

v3/pkg/application/linux_cgo.c

@@ -114,9 +114,12 @@ void install_signal_handlers(void) {
 #if defined(SIGSEGV)
     fix_signal(SIGSEGV);
 #endif
-#if defined(SIGUSR1)
-    fix_signal(SIGUSR1);
-#endif
+    // NOTE: Do NOT add SA_ONSTACK to SIGUSR1. WebKit's JavaScriptCore uses
+    // SIGUSR1 to suspend/resume threads for conservative GC stack scanning.
+    // Once JSC installs its own SIGUSR1 handler it owns the signal (Go no
+    // longer handles it), and forcing SA_ONSTACK makes that handler run on
+    // Go's alternate signal stack, breaking GC thread synchronisation and
+    // freezing WebKit during idle collection. See issue #5527.
 #if defined(SIGXCPU)
     fix_signal(SIGXCPU);
 #endif

v3/pkg/application/linux_cgo_gtk3.go

@@ -222,9 +222,12 @@ static void install_signal_handlers() {
 	#if defined(SIGSEGV)
 		fix_signal(SIGSEGV);
 	#endif
-	#if defined(SIGUSR1)
-		fix_signal(SIGUSR1);
-	#endif
+	// NOTE: Do NOT add SA_ONSTACK to SIGUSR1. WebKit's JavaScriptCore uses
+	// SIGUSR1 to suspend/resume threads for conservative GC stack scanning.
+	// Once JSC installs its own SIGUSR1 handler it owns the signal (Go no
+	// longer handles it), and forcing SA_ONSTACK makes that handler run on
+	// Go's alternate signal stack, breaking GC thread synchronisation and
+	// freezing WebKit during idle collection. See issue #5527.
 	#if defined(SIGXCPU)
 		fix_signal(SIGXCPU);
 	#endif