Save State Reset Investigation

Issue: Reset Game does not reliably restore a clean emulator state when a save state fails to load (observed with PicoDrive 32X, potentially other cores).

Related: #3077, Part of epic #2951

Root Cause Analysis

Reset Flow

RetroMenuView "RESET GAME" button
  → emulatorVC.core.resetEmulation()
    → PVEmulatorCore+RunLoop: bridge.resetEmulation()
      → PVLibRetroCore.m / PVPicoDriveBridge.m: core->retro_reset()

Save State Load Flow

loadSaveState(_ state:)
  → core.loadState(fromFileAtPath:)
    → PVLibRetroCore+Saves.m: core->retro_unserialize(data, size)

The Problem

When retro_unserialize() fails (e.g. because the save state was created with an older PicoDrive version that had a different memory layout), the libretro core may:

• (a) Partially apply the state data, leaving internal buffers in an inconsistent state (partially overwritten registers, memory maps, etc.)
• (b) Leave save RAM patched from the battery-save load that happens at ROM load time, but with the game-state data corrupted
• (c) Have core-specific bugs where retro_reset() does not fully reinitialize all state after a failed retro_unserialize() (PicoDrive-specific)

Calling retro_reset() after case (a) or (c) may not fix the inconsistency because:

• retro_reset() is meant to be called on a valid game state (equivalent to pressing the physical reset button on the console)
• If the core's internal pointers or state machine are corrupted, retro_reset() may not know how to recover from that

Evidence

• PicoDrive: retro_reset() calls PicoDrive's reset handler which resets CPU registers and some state, but does NOT fully re-load the ROM or reinitialize all memory regions
• A hard reinitialize (equivalent to retro_deinit() + retro_init() + retro_load_game()) would be needed for a guaranteed clean state, but this is the equivalent of "quit and restart the game" in the app

What We Fixed (App-Level)

Issue #3077 describes the behavior change after a failed save state load (introduced in this PR):

Before: Error is shown, emulator unpauses (potentially in corrupted state)

After: User is offered:

• Reset Game — calls core.resetEmulation() (best effort; may not fix core-level corruption)
• Continue — existing behavior (unpause and continue)

This is the best we can do without core-level changes.

Known Limitations

1. retro_reset() is not a guaranteed fix for post-retro_unserialize() corruption. It is a "soft reset" (hardware reset button equivalent), not a full reinitialization.

2. Core-specific bugs in PicoDrive's retro_reset() cannot be fixed at the app level without modifying upstream libretro-picodrive.

3. The correct deep fix would be to implement a "Reload ROM" option that:

   • Unloads the core via retro_unload_game() + retro_deinit()
   • Re-initializes via retro_init() + retro_load_game()
   • This is equivalent to quitting and relaunching the game
   • Future work: add a "Restart Game" option to the emulator menu (#TODO)

Recommendations for Future Work

1. Add "Restart Game" to the RetroMenuView that fully reinitializes the core (unload + reload ROM). This would definitively fix the corrupt-state problem.

2. Upstream PicoDrive fix: File an issue in the libretro-picodrive repository requesting that retro_reset() fully reinitializes state after a failed retro_unserialize().

3. Core error reporting: When retro_unserialize() returns false, cores should ideally report whether they left the state valid (so retro_reset() is sufficient) or corrupted (so a full reinitialize is needed). This is a libretro protocol gap.

Related Files

• PVCoreBridgeRetro/Sources/PVLibRetro/PVLibRetroCore+Saves.m — loadStateFromFileAtPath:
• PVCoreBridgeRetro/Sources/PVLibRetro/PVLibRetroCore.m — resetEmulation
• Cores/PicoDrive/Sources/PVPicoDriveBridge/PVPicoDriveBridge.m — PicoDrive resetEmulation
• PVUI/Sources/PVUIBase/PVEmulatorVC/PVEmulatorViewController+Saves.swift — load error handling
• PVUI/Sources/PVUIBase/PVEmulatorVC/RetroMenuView.swift — Reset Game menu button