PyBullet inverse-dynamics cache is stale after mass changes and body reuse

Environment

• OS: Ubuntu 24.04.5 LTS, Linux 7.0.0-28-generic x86_64
• Compiler: GCC 11.5.0
• Python: 3.12.3
• Source: bulletphysics/bullet3 master at 63c4d67e337017f9d8b298c900e9aabdb69296e7
• PyBullet: built locally from that exact checkout

Description

calculateInverseDynamics, calculateMassMatrix, and Jacobian calculation share an inverse-dynamics model cached by raw btMultiBody*. changeDynamics can change mass or inertia without invalidating the model, and removeBody deletes a multibody without erasing its cache entry. A replacement can reuse both body ID and allocation, so calculations for the replacement use the deleted body's parameters.

Expected behavior: derived dynamics calculations reflect the current body's mass and inertia after a dynamics mutation or body replacement.

Actual behavior: for the reproducer's one-DOF fixed-base model, a fresh 10 kg link returns inverse torque -90 and mass matrix [[10]]. If a cached 1 kg body is changed to 10 kg, or removed and replaced by a 10 kg body at reused ID 0, both operations still return the 1 kg results -9 and [[1]].

Sources:

• https://github.com/bulletphysics/bullet3/blob/63c4d67e337017f9d8b298c900e9aabdb69296e7/examples/SharedMemory/PhysicsServerCommandProcessor.cpp#L1996-L2029
• https://github.com/bulletphysics/bullet3/blob/63c4d67e337017f9d8b298c900e9aabdb69296e7/examples/SharedMemory/PhysicsServerCommandProcessor.cpp#L10656-L10981
• https://github.com/bulletphysics/bullet3/blob/63c4d67e337017f9d8b298c900e9aabdb69296e7/examples/SharedMemory/PhysicsServerCommandProcessor.cpp#L12739-L12832

Reproduction package:

inverse-dynamics-cache-invalidation-poc.zip

Steps to reproduce

1. Build PyBullet from the commit above using poc/README.md.
2. Run python poc/reproducer.py fresh.
3. Run python poc/reproducer.py reuse.
4. Run python poc/reproducer.py mutate.

The exact baseline and fixed outputs are in poc/observed-output.txt.