[nicolas-maman]

The first part of #149: the world snapshot (Box3D e77352c's world_snapshot.c). Recording and replay start from one, and it is useful on its own: save states, rollback, debugging a step.

aephysics.snapshot:

• save_world(world_id) -> (bytes, count) keeps everything a step reads:
  • the settings and the scalars a step carries over;
  • the six id pools;
  • the solver sets with their sims;
  • the bodies;
  • the shapes, with their geometry once each however many shapes share it;
  • the fat boxes;
  • the contacts with their manifolds and mesh caches;
  • the joints, the sensors and the islands;
  • the three trees, as dynamic_tree saves them;
  • the pair set, slots in place;
  • the constraint graph's colours.
• What a step builds fresh (events, scratch, the solver's constraints) is not kept, nor is what the host wires in (user data, callbacks, worker count).
• load_world(def, bytes, count) -> WorldId makes a world from def and fills it.
• Refusal, past the reference's checks:
  • a snapshot of another layout is refused, since its tag folds in the size of every record it copies whole and the geometry's versions;
  • every count is checked against the bytes before anything is copied;
  • each geometry block is checked as bytes from outside are (hull_bytes_valid, mesh_bytes_valid, the compound's sections, and the new height_field_bytes_valid);
  • every pointer a record carried is cleared before the record is used;
  • the loaded world must pass the world's own validation.
  • On any refusal, nothing is left behind.
• The loaded world owns the meshes, height fields and compounds it holds (dynamics.adopt_geometry, freed with the world). Hulls go through the hull database.

tests/test_snapshot.ae (50 checks):

• The scene has:
  • a box pyramid reporting to a sensor;
  • spheres, capsules and rocks falling on a mesh with three materials, a height field with holes, and a baked compound;
  • a chain of hinges with a motor;
  • a ragdoll with ankles and its filter joints;
  • a weld, a prismatic spring, a rope, a wheel, a motor and a parallel joint;
  • a disabled body, a kinematic paddle, a bullet, and names on bodies and shapes.
• It is saved and loaded at steps 0, 1, 45 and 200. The loaded world hashes as the original at once (hash_world_state) and steps 150 steps beside it bit for bit, with the same contact, sensor and hit events every step.
• A loaded world saved again gives the same bytes.
• A world stepped on one worker, loaded with four, steps on alike.
• The loaded world's mesh, field and compound are its own.
• Every truncation, a byte too many, and another magic, version or layout are refused, with nothing allocated left over.

Found on the way: aether#2611. Same-named locals of different pointer or struct types in sibling while blocks or if branches pass the type checker, but the generated C declares one variable of the first type, so gcc stops. The code here gives each local its own name.

Local gate on Aether 0.795.0: all 35 suites pass with no warnings, and the golden trace matches bit for bit (1,256 steps). Pins: Aether 0.795.0 and aeb v0.325, the latest releases.

Refs #149 (recording, replay and the name cache are next).