[4.0.7/4.0.8] Deterministic boot panic at addition.rs:425 — duplicate path within one add_files_to_directory batch dereferences its own future FileInfoIdx

[safnob78]

Summary

Deterministic panic on every boot at src/replacement/addition.rs:425:31 — index out of bounds — reproduced on both v4.0.8 and v4.0.7, Smash 13.0.4, Skyline beta, all support plugins current. This appears to be the same panic as closed issue #607 (which was on real hardware, FW 19.0.1 / Atmosphere), so the mechanism is not emulator-specific; analysis below is platform-independent.

Trigger mod: "MicroMoveset Little Mac c100 ~ Broly DBS" by Mastaklo (reported to the author separately). With the mod installed the panic fires 100% of boots at the splash screen; removing it boots clean.

Root cause (from reading add_files_to_directory, v4.0.8)

1. For each incoming file processed in the second loop, the function rewrites its lookup entry to a future table position:
   ctx.file_info_indices[...].file_info_index = FileInfoIdx((ctx.file_infos.len() + file_infos.len()) as u32) — a position that only becomes valid when ctx.file_infos.extend_from_slice(&file_infos) runs at the end of the call.
2. contained_files only guards against files already in the target directory (populated in the first loop). The incoming files slice is never checked for internal duplicates.
3. Therefore, if the same path occurs twice in one batch: the first occurrence is pushed and its file_info_index now points past the current end of ctx.file_infos; the second occurrence passes the contained_files check, get_path_idx succeeds, and line 425 (ctx.file_infos[usize::from(...file_info_index)]) reads the not-yet-materialized index → panic. The overshoot equals the number of pushes between the two occurrences.

Supporting measurements

• len 633302 / index 633652 (overshoot 350). Removing exactly 2 files the mod shipped in the batch region (Windows build junk) → len 633300 / index 633648 (overshoot 348).
• Adding 35 unrelated files to the mod → len 633335 / index 633683: length grew by exactly 35, overshoot unchanged at 348 — consistent with a fixed duplicate-pair distance in the plugin-built batch, independent of surrounding files.
• The batch appears to be built by the mod's plugin.nro, whose strings show it iterates all eight colors (c00_index…c07_index, characall_label_c00…c07) while the mod ships a single added slot — a plausible duplicate factory.

Suggested hardening

Insert each processed file's hash into contained_files inside the second loop (or dedupe the incoming slice up front), so a duplicated path is skipped like an already-present file instead of dereferencing a future FileInfoIdx. That one change turns this class of malformed plugin batch from a boot-killing panic into a logged skip.

Happy to provide full logs or run test builds.