Fix crash after multiple terminal resizes

When Kakoune's terminal is shown on my laptop monitor and I plug
in my external monitor, the terminal's workspace will move to that
external monitor. When this happens, Kakoune may segfault.
There are multiple resize events (SIGWINCH) in quick succession;
it crashes because we handle SIGWINCH during rendering.

The problem happens during execution of "TerminalUI::Screen::output"
(frame #18). When we receive SIGWINCH while writing to stdout, write(2)
fails with EAGAIN, prompting us to handle pending events (See ae001a1f9
(Run EventManager whenever writing to a file descriptor would block,
2022-05-10)).  We update the screen size in check_resize() here:

	#4  Kakoune::TerminalUI::check_resize (force=<optimized out>) at terminal_ui.cc:683
	#5  Kakoune::TerminalUI::get_next_key () at terminal_ui.cc:719
	#6  operator() (__closure=0x555555984198) at terminal_ui.cc:484
	#7  std::__invoke_impl<void, Kakoune::TerminalUI::TerminalUI()::<lambda(Kakoune::FDWatcher&, Kakoune::FdEvents, Kakoune::EventMode)>&, Kakoune::FDWatcher&, Kakoune::FdEvents, Kakoune::EventMode> (__f=...) at /usr/include/c++/12.2.1/bits/invoke.h:61
	#8  std::__invoke_r<void, Kakoune::TerminalUI::TerminalUI()::<lambda(Kakoune::FDWatcher&, Kakoune::FdEvents, Kakoune::EventMode)>&, Kakoune::FDWatcher&, Kakoune::FdEvents, Kakoune::EventMode> (__fn=...) at /usr/include/c++/12.2.1/bits/invoke.h:111
	#9  std::_Function_handler<void(Kakoune::FDWatcher&, Kakoune::FdEvents, Kakoune::EventMode), Kakoune::TerminalUI::TerminalUI()::<lambda(Kakoune::FDWatcher&, Kakoune::FdEvents, Kakoune::EventMode)> >::_M_invoke(const std::_Any_data &, Kakoune::FDWatcher &, Kakoune::FdEvents &&, Kakoune::EventMode &&) (__functor=..., __args#0=..., __args#1=<optimized out>, __args#2=<optimized out>) at /usr/include/c++/12.2.1/bits/std_function.h:290
	#10 std::function<void (Kakoune::FDWatcher&, Kakoune::FdEvents, Kakoune::EventMode)>::operator()(Kakoune::FDWatcher&, Kakoune::FdEvents, Kakoune::EventMode) const (__args#2=<optimized out>, __args#1=<optimized out>,  __args#0=...) at /usr/include/c++/12.2.1/bits/std_function.h:591
	#11 Kakoune::FDWatcher::run (mode=Kakoune::EventMode::Urgent, events=<optimized out>) at event_manager.cc:28
	#12 Kakoune::EventManager::handle_next_events (mode=mode@entry=Kakoune::EventMode::Urgent, sigmask=sigmask@entry=0x0, block=<optimized out>, block@entry=false) at event_manager.cc:143
	#13 Kakoune::write (fd=1, data=...) at file.cc:273
	#14 Kakoune::BufferedWriter<4096>::flush () at string.hh:236
	#15 Kakoune::BufferedWriter<4096>::write (data="t file.hh:145
	#16 Kakoune::TerminalUI::Screen::set_face (face=..., writer=...) at terminal_ui.cc:255
	#17 operator() (line=..., __closure=<synthetic pointer>) at terminal_ui.cc:326
	#18 Kakoune::TerminalUI::Screen::output (force=force@entry=true, synchronized=<optimized out>, writer=...) at terminal_ui.cc:402
	#19 Kakoune::TerminalUI::redraw (force=force@entry=true) at terminal_ui.cc:571
	#20 Kakoune::TerminalUI::refresh (force=<optimized out>) at terminal_ui.cc:592
	#21 Kakoune::Client::redraw_ifn () at client.cc:282
	#22 Kakoune::ClientManager::redraw_clients () at client_manager.cc:232
	#23 Kakoune::run_server (session=..., server_init=..., client_init=..., init_buffer="fish-rust/src/ast.rs", init_coord=..., flags=Kakoune::ServerFlags::None, ui_type=Kakoune::UIType::Terminal,
	    debug_flags=<optimized out>, files=ArrayView<Kakoune::StringView> = {...}) at main.cc:893
	#24 main (argc=<optimized out>, argv=<optimized out>) at main.cc:1243

Thereafter, "TerminalUI::Screen::output" resumes and crashes due to
a buffer overflow in "lines" which has been resized.

src/file.cc

@@ -252,13 +252,14 @@ bool regular_file_exists(StringView filename)
            (st.st_mode & S_IFMT) == S_IFREG;
 }
 
+template<bool atomic>
 void write(int fd, StringView data)
 {
     const char* ptr = data.data();
     ssize_t count   = (int)data.length();
 
     int flags = fcntl(fd, F_GETFL, 0);
-    if (EventManager::has_instance())
+    if (not atomic and EventManager::has_instance())
         fcntl(fd, F_SETFL, flags | O_NONBLOCK);
     auto restore_flags = on_scope_end([&] { fcntl(fd, F_SETFL, flags); });
 
@@ -269,12 +270,15 @@ void write(int fd, StringView data)
             ptr += written;
             count -= written;
         }
-        else if (errno == EAGAIN and EventManager::has_instance())
+        else if (errno == EAGAIN and not atomic and EventManager::has_instance())
             EventManager::instance().handle_next_events(EventMode::Urgent, nullptr, false);
         else
             throw file_access_error(format("fd: {}", fd), strerror(errno));
     }
 }
+template void write<true>(int fd, StringView data);
+template void write<false>(int fd, StringView data);
+
 
 void write_to_file(StringView filename, StringView data)
 {
@@ -295,7 +299,7 @@ void write_buffer_to_fd(Buffer& buffer, int fd)
         eoldata = "\n";
 
 
-    BufferedWriter writer{fd};
+    BufferedWriter<false> writer{fd};
     if (buffer.options()["BOM"].get<ByteOrderMark>() == ByteOrderMark::Utf8)
         writer.write("\xEF\xBB\xBF");
 

src/file.hh

@@ -39,6 +39,7 @@ bool fd_readable(int fd);
 bool fd_writable(int fd);
 String read_fd(int fd, bool text = false);
 String read_file(StringView filename, bool text = false);
+template<bool force_blocking = false>
 void write(int fd, StringView data);
 void write_to_file(StringView filename, StringView data);
 
@@ -121,7 +122,7 @@ CandidateList complete_filename(StringView prefix, const Regex& ignore_regex,
 
 CandidateList complete_command(StringView prefix, ByteCount cursor_pos = -1);
 
-template<int buffer_size = 4096>
+template<bool atomic, int buffer_size = 4096>
 struct BufferedWriter
 {
     BufferedWriter(int fd)
@@ -149,7 +150,7 @@ struct BufferedWriter
 
     void flush()
     {
-        Kakoune::write(m_fd, {m_buffer, m_pos});
+        Kakoune::write<atomic>(m_fd, {m_buffer, m_pos});
         m_pos = 0;
     }
 

src/terminal_ui.cc

@@ -208,7 +208,7 @@ void TerminalUI::Window::draw(DisplayCoord pos,
         lines[(int)pos.line].append({}, size.column - pos.column, default_face);
 }
 
-struct Writer : BufferedWriter<>
+struct Writer : BufferedWriter<true>
 {
     using Writer::BufferedWriter::BufferedWriter;
     ~Writer() noexcept(false) = default;