degesch: add backlog/scrollback functionality

Finally!  I went with possibly the simplest solution, which is to
run less, instead of badly reimplementing its functionality.

degesch.c

@@ -1364,6 +1364,8 @@ struct app_context
 	bool awaiting_mirc_escape;          ///< Awaiting a mIRC attribute escape
 	char char_buf[MB_LEN_MAX + 1];      ///< Buffered multibyte char
 	size_t char_buf_len;                ///< How much of an MB char is buffered
+
+	bool running_backlog_helper;        ///< Running a backlog helper
 }
 *g_ctx;
 
@@ -1640,6 +1642,10 @@ static struct config_schema g_config_behaviour[] =
 	  .type      = CONFIG_ITEM_BOOLEAN,
 	  .default_  = "off",
 	  .on_change = on_config_logging_change },
+	{ .name      = "backlog_helper",
+	  .comment   = "Shell command to display a buffer's history",
+	  .type      = CONFIG_ITEM_STRING,
+	  .default_  = "\"LESSSECURE=1 less -M -R +G\"" },
 	{ .name      = "save_on_quit",
 	  .comment   = "Save configuration before quitting",
 	  .type      = CONFIG_ITEM_BOOLEAN,
@@ -2839,11 +2845,18 @@ log_formatter (struct app_context *ctx,
 		&& buffer == ctx->current_buffer->server->buffer))
 		can_leak = true;
 
-	if (buffer == ctx->current_buffer)
+	bool displayed = true;
+	if (ctx->running_backlog_helper)
+		// Another process is using the terminal
+		displayed = false;
+	else if (buffer == ctx->current_buffer)
 		buffer_line_display (ctx, line, false);
 	else if (!ctx->isolate_buffers && can_leak)
 		buffer_line_display (ctx, line, true);
 	else
+		displayed = false;
+
+	if (!displayed)
 	{
 		buffer->unseen_messages_count++;
 		if (flags & BUFFER_LINE_HIGHLIGHT)
@@ -2957,6 +2970,8 @@ buffer_open_log_file (struct app_context *ctx, struct buffer *buffer)
 	if (!(buffer->log_file = fopen (path.str, "ab")))
 		log_global_error (ctx, "Couldn't open log file `#s': #s",
 			path.str, strerror (errno));
+	else
+		set_cloexec (fileno (buffer->log_file));
 	str_free (&path);
 }
 
@@ -8807,6 +8822,38 @@ make_completions (struct app_context *ctx, char *line, int start, int end)
 
 // --- Common code for user actions --------------------------------------------
 
+static void
+suspend_terminal (struct app_context *ctx)
+{
+#ifdef HAVE_READLINE
+	rl_deprep_terminal ();
+#elif defined (HAVE_EDITLINE)
+	el_set (ctx->input.editline, EL_PREP_TERM, 0);
+#endif
+
+	input_hide (&ctx->input);
+	poller_fd_reset (&ctx->tty_event);
+	// TODO: also disable the date change timer
+}
+
+static void
+resume_terminal (struct app_context *ctx)
+{
+#ifdef HAVE_READLINE
+	rl_prep_terminal (true);
+#elif defined (HAVE_EDITLINE)
+	el_set (ctx->input.editline, EL_PREP_TERM, 1);
+#endif
+
+	// In theory we could just print all unseen messages but this is safer
+	buffer_print_backlog (ctx, ctx->current_buffer);
+	// Now it's safe to process any user input
+	poller_fd_set (&ctx->tty_event, POLLIN);
+	input_show (&ctx->input);
+}
+
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+
 static bool
 redraw_screen (struct app_context *ctx)
 {
@@ -8849,6 +8896,59 @@ await_mirc_escape (struct app_context *ctx)
 	ctx->char_buf_len = 0;
 }
 
+static void
+exec_backlog_helper (const char *command, FILE *backlog)
+{
+	dup2 (fileno (backlog), STDIN_FILENO);
+
+	// Put the child into a new foreground process group
+	hard_assert (setpgid (0, 0)!= -1);
+	hard_assert (tcsetpgrp (STDOUT_FILENO, getpgid (0)) != -1);
+
+	execl ("/bin/sh", "/bin/sh", "-c", command, NULL);
+	print_error ("%s: %s", "Failed to launch backlog helper", strerror (errno));
+	_exit (EXIT_FAILURE);
+}
+
+static void
+display_backlog (struct app_context *ctx)
+{
+	hard_assert (!ctx->running_backlog_helper);
+
+	FILE *backlog = tmpfile ();
+	set_cloexec (fileno (backlog));
+
+	// TODO: retrieve the lines with formatting
+	for (struct buffer_line *line = ctx->current_buffer->lines;
+		line; line = line->next)
+		buffer_line_write_to_log (ctx, line, backlog);
+
+	rewind (backlog);
+	suspend_terminal (ctx);
+
+	pid_t child = fork ();
+	if (child == 0)
+		exec_backlog_helper (get_config_string
+			(ctx->config.root, "behaviour.backlog_helper"), backlog);
+
+	if (child == -1)
+	{
+		int saved_errno = errno;
+		resume_terminal (ctx);
+		log_global_error (ctx, "#s: #s",
+			"Failed to launch backlog helper", strerror (saved_errno));
+	}
+	else
+	{
+		// Make sure the child has its own process group
+		(void) setpgid (child, child);
+
+		ctx->running_backlog_helper = true;
+	}
+
+	fclose (backlog);
+}
+
 static void
 bind_common_keys (struct app_context *ctx)
 {
@@ -8866,6 +8966,8 @@ bind_common_keys (struct app_context *ctx)
 		input_bind (self, key_f5, "previous-buffer");
 	if (key_f6)
 		input_bind (self, key_f6, "next-buffer");
+	if (key_ppage)
+		input_bind (self, key_ppage, "display-backlog");
 
 	if (clear_screen)
 		input_bind_control (self, 'l', "redraw-screen");
@@ -8906,6 +9008,17 @@ on_readline_next_buffer (int count, int key)
 	return 0;
 }
 
+static int
+on_readline_display_backlog (int count, int key)
+{
+	(void) count;
+	(void) key;
+
+	struct app_context *ctx = g_ctx;
+	display_backlog (ctx);
+	return 0;
+}
+
 static int
 on_readline_redraw_screen (int count, int key)
 {
@@ -8998,6 +9111,7 @@ app_readline_init (void)
 	rl_add_defun ("previous-buffer",  on_readline_previous_buffer,  -1);
 	rl_add_defun ("next-buffer",      on_readline_next_buffer,      -1);
 	rl_add_defun ("goto-buffer",      on_readline_goto_buffer,      -1);
+	rl_add_defun ("display-backlog",  on_readline_display_backlog,  -1);
 	rl_add_defun ("redraw-screen",    on_readline_redraw_screen,    -1);
 	rl_add_defun ("insert-attribute", on_readline_insert_attribute, -1);
 	rl_add_defun ("send-line",        on_readline_return,           -1);
@@ -9057,6 +9171,15 @@ on_editline_next_buffer (EditLine *editline, int key)
 	return CC_NORM;
 }
 
+static unsigned char
+on_editline_display_backlog (EditLine *editline, int key)
+{
+	(void) editline;
+	(void) key;
+
+	display_backlog (g_ctx);
+}
+
 static unsigned char
 on_editline_redraw_screen (EditLine *editline, int key)
 {
@@ -9173,6 +9296,7 @@ app_editline_init (struct input *self)
 		{ "goto-buffer",      "Go to buffer",    on_editline_goto_buffer      },
 		{ "previous-buffer",  "Previous buffer", on_editline_previous_buffer  },
 		{ "next-buffer",      "Next buffer",     on_editline_next_buffer      },
+		{ "display-backlog",  "Display backlog", on_editline_display_backlog  },
 		{ "redraw-screen",    "Redraw screen",   on_editline_redraw_screen    },
 		{ "insert-attribute", "mIRC formatting", on_editline_insert_attribute },
 		{ "send-line",        "Send line",       on_editline_return           },
@@ -9376,29 +9500,34 @@ static volatile sig_atomic_t g_termination_requested;
 static volatile sig_atomic_t g_winch_received;
 
 static void
-sigterm_handler (int signum)
+postpone_signal_handling (char id)
 {
-	(void) signum;
-
-	g_termination_requested = true;
-
 	int original_errno = errno;
-	if (write (g_signal_pipe[1], "t", 1) == -1)
+	if (write (g_signal_pipe[1], &id, 1) == -1)
 		soft_assert (errno == EAGAIN);
 	errno = original_errno;
 }
 
 static void
-sigwinch_handler (int signum)
+signal_superhandler (int signum)
 {
-	(void) signum;
+	switch (signum)
-
+	{
-	g_winch_received = true;
+	case SIGWINCH:
-
+		g_winch_received = true;
-	int original_errno = errno;
+		postpone_signal_handling ('w');
-	if (write (g_signal_pipe[1], "w", 1) == -1)
+		break;
-		soft_assert (errno == EAGAIN);
+	case SIGINT:
-	errno = original_errno;
+	case SIGTERM:
+		g_termination_requested = true;
+		postpone_signal_handling ('t');
+		break;
+	case SIGCHLD:
+		postpone_signal_handling ('c');
+		break;
+	default:
+		hard_assert (!"unhandled signal");
+	}
 }
 
 static void
@@ -9418,28 +9547,76 @@ setup_signal_handlers (void)
 
 	signal (SIGPIPE, SIG_IGN);
 
+	// So that we can write to the terminal while we're running a backlog
+	// helper.  This is also inherited by the child so that it doesn't stop
+	// when it calls tcsetpgrp().
+	signal (SIGTTOU, SIG_IGN);
+
 	struct sigaction sa;
 	sa.sa_flags = SA_RESTART;
-	sa.sa_handler = sigwinch_handler;
+	sa.sa_handler = signal_superhandler;
 	sigemptyset (&sa.sa_mask);
 
-	if (sigaction (SIGWINCH, &sa, NULL) == -1)
+	if (sigaction (SIGWINCH, &sa, NULL) == -1
-		exit_fatal ("sigaction: %s", strerror (errno));
+	 || sigaction (SIGINT,   &sa, NULL) == -1
-
+	 || sigaction (SIGTERM,  &sa, NULL) == -1
-	sa.sa_handler = sigterm_handler;
+	 || sigaction (SIGCHLD,  &sa, NULL) == -1)
-	if (sigaction (SIGINT, &sa, NULL) == -1
-	 || sigaction (SIGTERM, &sa, NULL) == -1)
 		exit_fatal ("sigaction: %s", strerror (errno));
 }
 
 // --- I/O event handlers ------------------------------------------------------
 
+static bool
+try_reap_child (struct app_context *ctx)
+{
+	int status;
+	pid_t zombie = waitpid (-1, &status, WNOHANG | WUNTRACED);
+
+	if (zombie == -1)
+	{
+		if (errno == ECHILD)  return false;
+		if (errno == EINTR)   return true;
+		exit_fatal ("%s: %s", "waitpid", strerror (errno));
+	}
+	if (!zombie)
+		return false;
+
+	if (!ctx->running_backlog_helper)
+	{
+		print_debug ("an unknown child has died");
+		return true;
+	}
+	if (WIFSTOPPED (status))
+	{
+		// We could also send SIGCONT but what's the point
+		kill (-zombie, SIGKILL);
+		return true;
+	}
+
+	ctx->running_backlog_helper = false;
+	hard_assert (tcsetpgrp (STDOUT_FILENO, getpgid (0)) != -1);
+	resume_terminal (ctx);
+
+	if (WIFSIGNALED (status))
+		log_global_error (ctx,
+			"Child died from signal #d", WTERMSIG (status));
+	else if (WIFEXITED (status) && WEXITSTATUS (status) != 0)
+		log_global_error (ctx,
+			"Child returned status #d", WEXITSTATUS (status));
+	return true;
+}
+
 static void
 on_signal_pipe_readable (const struct pollfd *fd, struct app_context *ctx)
 {
 	char dummy;
 	(void) read (fd->fd, &dummy, 1);
 
+	// Reap all dead children (since the signal pipe may overflow etc. we run
+	// waitpid() in a loop to return all the zombies it knows about).
+	while (try_reap_child (ctx))
+		;
+
 	if (g_termination_requested && !ctx->quitting)
 		// TODO: this way we don't send a QUIT message but just close the
 		//   connection from our side and wait for a full close.