Prevent segfault when all todo items are hidden

To prevent from illegal memory access, we checked whether the list of
todo items is empty before performing an interactive todo item action
such as flagging or removal. However, since 1a4bf2b (Add a "hide
completed" view to the todo panel, 2016-01-17), it is possible to hide
completed items from the todo panel. Thus, it may occur that the todo
list box is empty while the list of todo items is not.

To detect such situations as well, teach todo_get_item() to return NULL
if the requested item does not exist. Then, instead of checking whether
the todo item list is non-empty, make sure that the item returned by
todo_get_item() is non-NULL when performing an action.

Reported-by: Vlad Glagolev <scm@vaygr.net>
Signed-off-by: Lukas Fleischer <lfleischer@calcurse.org>

2 files changed, 28 insertions, 21 deletions

diff --git a/src/todo.c b/src/todo.c
index 9a6f72f..8027803 100644
--- a/src/todo.c
+++ b/src/todo.c
@@ -59,6 +59,9 @@ struct todo *todo_get_item(int item_number, int skip_completed)
 	else
 		i = LLIST_NTH(&todolist, item_number);
 
+	if (!i)
+		return NULL;
+
 	return LLIST_GET_DATA(i);
 }
 
diff --git a/src/ui-todo.c b/src/ui-todo.c
index 8eaef08..50bb30d 100644
--- a/src/ui-todo.c
+++ b/src/ui-todo.c
@@ -87,15 +87,14 @@ void ui_todo_delete(void)
 	const int nb_erase_choice = 2;
 	int answer;
 
-	if (!LLIST_FIRST(&todolist) ||
-	    (conf.confirm_delete
-	     && (status_ask_bool(del_todo_str) != 1))) {
+	struct todo *item = ui_todo_selitem();
+
+	if (!item || (conf.confirm_delete &&
+		      (status_ask_bool(del_todo_str) != 1))) {
 		wins_erase_status_bar();
 		return;
 	}
 
-	struct todo *item = ui_todo_selitem();
-
 	if (item->note)
 		answer = status_ask_choice(erase_warning, erase_choice,
 					   nb_erase_choice);
@@ -121,12 +120,12 @@ void ui_todo_delete(void)
 /* Edit the description of an already existing todo item. */
 void ui_todo_edit(void)
 {
-	if (!LLIST_FIRST(&todolist))
-		return;
-
 	struct todo *item = ui_todo_selitem();
 	const char *mesg = _("Enter the new TODO description:");
 
+	if (!item)
+		return;
+
 	status_mesg(mesg, "");
 	updatestring(win[STA].p, &item->mesg, 0, 1);
 	todo_resort(item);
@@ -144,11 +143,11 @@ void ui_todo_pipe(void)
 	int pid;
 	FILE *fpout;
 
-	if (!LLIST_FIRST(&todolist))
-		return;
-
 	struct todo *item = ui_todo_selitem();
 
+	if (!item)
+		return;
+
 	status_mesg(_("Pipe item to external command:"), "");
 	if (getstring(win[STA].p, cmd, BUFSIZ, 0, 1) != GETSTRING_VALID)
 		return;
@@ -284,10 +283,11 @@ void ui_todo_update_panel(int which_pan)
 /* Change an item priority by pressing '+' or '-' inside TODO panel. */
 void ui_todo_chg_priority(int diff)
 {
-	if (!LLIST_FIRST(&todolist))
+	struct todo *item = ui_todo_selitem();
+
+	if (!item)
 		return;
 
-	struct todo *item = ui_todo_selitem();
 	int id = item->id + diff;
 	struct todo *item_new;
 
@@ -304,19 +304,21 @@ void ui_todo_chg_priority(int diff)
 
 void ui_todo_popup_item(void)
 {
-	if (!LLIST_FIRST(&todolist))
+	struct todo *item = ui_todo_selitem();
+
+	if (!item)
 		return;
 
-	struct todo *item = ui_todo_selitem();
 	item_in_popup(NULL, NULL, item->mesg, _("TODO:"));
 }
 
 void ui_todo_flag(void)
 {
-	if (!LLIST_FIRST(&todolist))
+	struct todo *item = ui_todo_selitem();
+
+	if (!item)
 		return;
 
-	struct todo *item = ui_todo_selitem();
 	todo_flag(item);
 	ui_todo_load_items();
 	io_set_modified();
@@ -325,19 +327,21 @@ void ui_todo_flag(void)
 
 void ui_todo_view_note(void)
 {
-	if (!LLIST_FIRST(&todolist))
+	struct todo *item = ui_todo_selitem();
+
+	if (!item)
 		return;
 
-	struct todo *item = ui_todo_selitem();
 	todo_view_note(item, conf.pager);
 }
 
 void ui_todo_edit_note(void)
 {
-	if (!LLIST_FIRST(&todolist))
+	struct todo *item = ui_todo_selitem();
+
+	if (!item)
 		return;
 
-	struct todo *item = ui_todo_selitem();
 	todo_edit_note(item, conf.editor);
 	io_set_modified();
 }