Cleanup SIGHUP handling and forking server to background

Ensure we ignore SIGHUP once the TerminalUI is gone as it will be
sent again on fork, fix the parent process terminating due to trying
to write to stdout after it was closed.

Fixes #4960

src/terminal_ui.cc

@@ -429,7 +429,7 @@ static constexpr StringView assistant_dilbert[] =
 template<typename T> T sq(T x) { return x * x; }
 
 static sig_atomic_t resize_pending = 0;
-static sig_atomic_t stdin_closed = 0;
+static sig_atomic_t terrminal_hangedup = 0;
 
 template<sig_atomic_t* signal_flag>
 static void signal_handler(int)
@@ -465,7 +465,7 @@ TerminalUI::TerminalUI()
     enable_mouse(true);
 
     set_signal_handler(SIGWINCH, &signal_handler<&resize_pending>);
-    set_signal_handler(SIGHUP, &signal_handler<&stdin_closed>);
+    set_signal_handler(SIGHUP, &signal_handler<&terrminal_hangedup>);
     set_signal_handler(SIGTSTP, [](int){ TerminalUI::instance().suspend(); });
 
     check_resize(true);
@@ -474,11 +474,13 @@ TerminalUI::TerminalUI()
 
 TerminalUI::~TerminalUI()
 {
-    enable_mouse(false);
-    restore_terminal();
-    tcsetattr(STDIN_FILENO, TCSAFLUSH, &m_original_termios);
+    if (not terrminal_hangedup) {
+        enable_mouse(false);
+        restore_terminal();
+        tcsetattr(STDIN_FILENO, TCSAFLUSH, &m_original_termios);
+    }
     set_signal_handler(SIGWINCH, SIG_DFL);
-    set_signal_handler(SIGHUP, SIG_DFL);
+    set_signal_handler(SIGHUP, SIG_IGN);
     set_signal_handler(SIGTSTP, SIG_DFL);
 }
 
@@ -666,10 +668,10 @@ void TerminalUI::check_resize(bool force)
 
 Optional<Key> TerminalUI::get_next_key()
 {
-    if (stdin_closed)
+    if (terrminal_hangedup)
     {
         set_signal_handler(SIGWINCH, SIG_DFL);
-        set_signal_handler(SIGHUP, SIG_DFL);
+        set_signal_handler(SIGHUP, SIG_IGN);
         if (m_window)
             m_window.destroy();
         m_stdin_watcher.disable();
@@ -691,7 +693,7 @@ Optional<Key> TerminalUI::get_next_key()
         if (unsigned char c = 0; read(STDIN_FILENO, &c, 1) == 1)
             return c;
 
-        stdin_closed = 1;
+        terrminal_hangedup = 1;
         return {};
     };