[nooga]

The crash

Pressing New Project, or opening another project, while the current project's master effects were still sounding could segfault on the audio thread:

Segmentation fault in std.atomic load
  handoff.zig:24 adopt ← fy_raw_machine.zig:1046 adoptAssets ← adoptImpl
  ← engine.zig:594 adoptTrack (master's audioEffects) ← adoptMachines ← render

It reproduced after playing a song with a long verb2 cathedral tail and a master limiter2, then pressing New Project.

Cause

document.apply replaced the master's effect chain by calling deinit on the old effects in place. Every other path (removeEffect, replaceEffect, replaceMachine) hands old machines to retired_machines, which frees them only after the audio thread acknowledges the new chain. Regular tracks don't hit this because they're replaced whole while the device is stopped. The master is restored in place.

When the engine is still ringing at restart, Track.adoptMachines(fade=true) keeps the old chain for one more fade-out chunk. So adoptTrack called adopt_updates on the freed effect states.

Fix

• document.apply now removes the master's effects with Track.removeEffect, so they retire like any other removed effect.
• The UI loop now collects the master's retired machines every frame, as it does for the tracks through publishSnapshot. The old chain's memory is freed as soon as audio moves past it, not at the next master edit.

Verification

• A new test, realtime: a project load retires the master's effects until audio adopts the new chain, fails before the fix (the effect is freed while the audio side still holds it) and passes after. It covers the fading adoption, the switch, and the free afterwards.
• zig build test passes.
• zig build bench -- --all --check --no-sheets is bit-exact on all 164 golden cases.
• The GUI repro itself (play, then New Project) has not been re-run by hand.

🤖 Generated with Claude Code