Nannou: Examples exit with panic when pressing "mouse back-button"

[karroffel]

Report

I have a mouse with dedicated back and forward buttons and I noticed that all the examples panic when I press the "back" button (which seems like it's received as "mouse button 9").

This is a backtrace I got from the simple_polyline example:

thread 'main' panicked at 'index out of bounds: the len is 9 but the index is 9', src/state.rs:224:13
stack backtrace:
   0: std::sys::unix::backtrace::tracing::imp::unwind_backtrace
             at src/libstd/sys/unix/backtrace/tracing/gcc_s.rs:39
   1: std::sys_common::backtrace::_print
             at src/libstd/sys_common/backtrace.rs:71
   2: std::panicking::default_hook::{{closure}}
             at src/libstd/sys_common/backtrace.rs:59
             at src/libstd/panicking.rs:197
   3: std::panicking::default_hook
             at src/libstd/panicking.rs:211
   4: std::panicking::rust_panic_with_hook
             at src/libstd/panicking.rs:474
   5: std::panicking::continue_panic_fmt
             at src/libstd/panicking.rs:381
   6: rust_begin_unwind
             at src/libstd/panicking.rs:308
   7: core::panicking::panic_fmt
             at src/libcore/panicking.rs:85
   8: core::panicking::panic_bounds_check
             at src/libcore/panicking.rs:61
   9: nannou::state::mouse::ButtonMap<S>::press
             at src/state.rs:224
  10: nannou::app::process_and_emit_winit_event
             at src/app.rs:1831
  11: nannou::app::poll_and_process_events
             at src/app.rs:1681
  12: nannou::app::run_loop_mode_refresh_sync
             at src/app.rs:1490
  13: nannou::app::run_loop
             at src/app.rs:1251
  14: nannou::app::Builder<M,E>::run
             at src/app.rs:636
  15: nannou::app::Builder::sketch
             at src/app.rs:666
  16: nannou::sketch
             at src/lib.rs:76
  17: simple_polyline::main
             at examples/simple_polyline.rs:4
  18: std::rt::lang_start::{{closure}}
             at /rustc/0ff088c9ee6a132daf256383e56004bee34fbaae/src/libstd/rt.rs:64
  19: std::panicking::try::do_call
             at src/libstd/rt.rs:49
             at src/libstd/panicking.rs:293
  20: __rust_maybe_catch_panic
             at src/libpanic_unwind/lib.rs:85
  21: std::rt::lang_start_internal
             at src/libstd/panicking.rs:272
             at src/libstd/panic.rs:388
             at src/libstd/rt.rs:48
  22: std::rt::lang_start
             at /rustc/0ff088c9ee6a132daf256383e56004bee34fbaae/src/libstd/rt.rs:64
  23: main
  24: __libc_start_main
  25: _start

Investigation

In src/state.rs the maximum number of buttons on a mouse is assumed to be 9:

https://github.com/nannou-org/nannou/blob/a5486e12b425df14cf2bb2c04496b5d3c38e1854/src/state.rs#L80-L81

With this constant, a fixed-size array is created to hold all the button states:

https://github.com/nannou-org/nannou/blob/a5486e12b425df14cf2bb2c04496b5d3c38e1854/src/state.rs#L108-L111

This function generates the array index from a mouse button event:

https://github.com/nannou-org/nannou/blob/a5486e12b425df14cf2bb2c04496b5d3c38e1854/src/state.rs#L273-L280

However, in multiple places this function is used to directly access the fixed-size array:

https://github.com/nannou-org/nannou/blob/a5486e12b425df14cf2bb2c04496b5d3c38e1854/src/state.rs#L222-L246

Since the button-ID reported seems to be 9, the index 9 is out of bounds.

Potential solutions

Either the fixed-size array could be replaced by a Vec<ButtonPosition<S>> and initialized to N button states. Then when a new button-index is encountered this array could be resized lazily and filled with default states.

Maybe another solution is to have button_to_idx return an Option<usize> instead, any unexpected button input could then be discarded.

[JoshuaBatty]

Hi @karroffel thanks heaps for bringing this to our attention and for the detailed report.

I actually have a new fancy mouse with heaps of buttons arriving from DHL any minute now so i'll look into addressing this and submit a PR hopefully within the next few days.