Creates a new physics shape instance when the mesh is replaced, so that all other linked objects remain unaffected.
The bug in question is this one:
You can find some example files there.
Detailed description of the bug from the link.
"The use of replace mesh , do something with mesh , reinstance physics mesh doesn't work because replace mesh switches out the graphics mesh without regard whether it was shared or inherited or not. But the physics mesh gets inherited from whatever mesh the shape was inherited from when the object was created. Which is also why this new demofile https://developer.blender.org/F110777 demonstrates another bug, this time with the replace mesh actuator:
Creating an object and then a linked duplicate, giving the original but not the linked duplicate a replace mesh actuator and using it with a cylinder will switch out shapes for the original object but also because of it's linked nature the physics shape for the linked duplicate - but not the duplcates graphics mesh.
So really when anything gets replaced, things break. Either the graphics mesh gets replaced and the physics mesh doesn't or both do and then the physics meshes on all relative objects break."
me in that linked bug report
You can use the linked demofile in a fairly straightforward way, there are 3 objects with the described setup and an always sensor executes the mesh replacement. You can then see how the physics mesh gets switched out when it shouldn't be. Or the graphics mesh when it shouldn't be etc. Since the behavior of all objects spawned with scene.addObject are linked copies, this behavior applies to all objects that are spawned while the game runs. Changing one changes all of them.
This patch "fixes" that by introducing a flag which if enabled creates a new instance for the physics mesh. This physics mesh can then be replaced or written to without affecting the original mesh or it's linked copies.
This is good because this way I can create mesh shapes while the engine runs, instead of just spawning previously created mesh shapes.