LibCore: Allow adding/removing signal handlers while handling signals

This allows adding and removing of asynchronous signal handlers while
executing signal handlers, even if it is for the same signal that is
being handled right now.

Libraries/LibCore/EventLoop.cpp

@@ -82,8 +82,7 @@ static NeverDestroyed<IDAllocator> s_id_allocator;
 static HashMap<int, NonnullOwnPtr<EventLoopTimer>>* s_timers;
 static HashTable<Notifier*>* s_notifiers;
 int EventLoop::s_wake_pipe_fds[2];
-HashMap<int, EventLoop::SignalHandlers> EventLoop::s_signal_handlers;
-int EventLoop::s_handling_signal = 0;
+HashMap<int, NonnullRefPtr<EventLoop::SignalHandlers>> EventLoop::s_signal_handlers;
 int EventLoop::s_next_signal_id = 0;
 pid_t EventLoop::s_pid;
 static RefPtr<LocalServer> s_rpc_server;
@@ -428,43 +427,75 @@ EventLoop::SignalHandlers::SignalHandlers(int signo)
 
 EventLoop::SignalHandlers::~SignalHandlers()
 {
-    if (m_valid) {
 #ifdef EVENTLOOP_DEBUG
-        dbgln("Core::EventLoop: Unregistering handler for signal {}", m_signo);
+    dbgln("Core::EventLoop: Unregistering handler for signal {}", m_signo);
 #endif
-        signal(m_signo, m_original_handler);
-    }
+    signal(m_signo, m_original_handler);
 }
 
 void EventLoop::SignalHandlers::dispatch()
 {
+    TemporaryChange change(m_calling_handlers, true);
     for (auto& handler : m_handlers)
         handler.value(m_signo);
+    if (!m_handlers_pending.is_empty()) {
+        // Apply pending adds/removes
+        for (auto& handler : m_handlers_pending) {
+            if (handler.value) {
+                auto result = m_handlers.set(handler.key, move(handler.value));
+                ASSERT(result == AK::HashSetResult::InsertedNewEntry);
+            } else {
+                m_handlers.remove(handler.key);
+            }
+        }
+        m_handlers_pending.clear();
+    }
 }
 
 int EventLoop::SignalHandlers::add(Function<void(int)>&& handler)
 {
     int id = ++EventLoop::s_next_signal_id; // TODO: worry about wrapping and duplicates?
-    m_handlers.set(id, move(handler));
+    if (m_calling_handlers)
+        m_handlers_pending.set(id, move(handler));
+    else
+        m_handlers.set(id, move(handler));
     return id;
 }
 
 bool EventLoop::SignalHandlers::remove(int handler_id)
 {
     ASSERT(handler_id != 0);
+    if (m_calling_handlers) {
+        auto it = m_handlers.find(handler_id);
+        if (it != m_handlers.end()) {
+            // Mark pending remove
+            m_handlers_pending.set(handler_id, nullptr);
+            return true;
+        }
+        it = m_handlers_pending.find(handler_id);
+        if (it != m_handlers_pending.end()) {
+            if (!it->value)
+                return false; // already was marked as deleted
+            it->value = nullptr;
+            return true;
+        }
+        return false;
+    }
     return m_handlers.remove(handler_id);
 }
 
 void EventLoop::dispatch_signal(int signo)
 {
-    // We need to protect the handler from being removed while handling it
-    TemporaryChange change(s_handling_signal, signo);
     auto handlers = s_signal_handlers.find(signo);
     if (handlers != s_signal_handlers.end()) {
+        // Make sure we bump the ref count while dispatching the handlers!
+        // This allows a handler to unregister/register while the handlers
+        // are being called!
+        auto handler = handlers->value;
 #ifdef EVENTLOOP_DEBUG
         dbgln("Core::EventLoop: dispatching signal {}", signo);
 #endif
-        handlers->value.dispatch();
+        handler->dispatch();
     }
 }
 
@@ -489,15 +520,14 @@ void EventLoop::handle_signal(int signo)
 int EventLoop::register_signal(int signo, Function<void(int)> handler)
 {
     ASSERT(signo != 0);
-    ASSERT(s_handling_signal != signo); // can't register the same signal while handling it
     auto handlers = s_signal_handlers.find(signo);
     if (handlers == s_signal_handlers.end()) {
-        SignalHandlers signal_handlers(signo);
-        auto handler_id = signal_handlers.add(move(handler));
+        auto signal_handlers = adopt(*new SignalHandlers(signo));
+        auto handler_id = signal_handlers->add(move(handler));
         s_signal_handlers.set(signo, move(signal_handlers));
         return handler_id;
     } else {
-        return handlers->value.add(move(handler));
+        return handlers->value->add(move(handler));
     }
 }
 
@@ -506,11 +536,8 @@ void EventLoop::unregister_signal(int handler_id)
     ASSERT(handler_id != 0);
     int remove_signo = 0;
     for (auto& h : s_signal_handlers) {
-        auto& handlers = h.value;
-        if (handlers.m_signo == s_handling_signal) {
-            // can't remove the same signal while handling it
-            ASSERT(!handlers.have(handler_id));
-        } else if (handlers.remove(handler_id)) {
+        auto& handlers = *h.value;
+        if (handlers.remove(handler_id)) {
             if (handlers.is_empty())
                 remove_signo = handlers.m_signo;
             break;
@@ -529,7 +556,6 @@ void EventLoop::notify_forked(ForkEvent event)
         s_timers->clear();
         s_notifiers->clear();
         s_signal_handlers.clear();
-        s_handling_signal = 0;
         s_next_signal_id = 0;
         s_pid = 0;
         s_rpc_server = nullptr;