main: Do not update the display on backspace when there is no input to remove

On machines with a slow CPU (Atom) and a highres screen drawing the
diskcrypt dialog may take longer then the keyrepeat speed, this leads to
a long delay before showing keypresses when doing the following:

1) Type long password
2) Realize it is wrong, press + hold backspace
   the key-repeat will now generate backspace key presses faster then we
   process them as main.c does an update_display for each press
3) Users releases backspace when we've processed input-length backspace
   key-presses, but since we were drawing slower then key-presses were
   coming in many more backspace keypresses are in the keyboard buffer
4) User types first character of the right password, this shows up up to
   a couple of seconds later because first we are still processing all
   the queued up backspace presses and doing a redraw for each.

This commit fixes this by skipping the redraws in on_backspace when there
is no more input left in the input buffer.

https://bugs.freedesktop.org/show_bug.cgi?id=104714

1 files changed, 2 insertions, 0 deletions

diff --git a/src/main.c b/src/main.c
index 08c7fe1a..f1e0fa7c 100644
--- a/src/main.c
+++ b/src/main.c
@@ -1570,6 +1570,8 @@ on_backspace (state_t *state)
 
         bytes = ply_buffer_get_bytes (state->entry_buffer);
         size = ply_buffer_get_size (state->entry_buffer);
+        if (size == 0)
+                return;
 
         bytes_to_remove = MIN (size, PLY_UTF8_CHARACTER_SIZE_MAX);
         while ((previous_character_size = ply_utf8_character_get_size (bytes + size - bytes_to_remove, bytes_to_remove)) < bytes_to_remove) {