soft-body simulation hangs with ConfigFlag.VF_SS

[stephengold]

This issue was reported by Tharg, using the HelloSoftBody tutorial app with VF_SS set.

Here is a native stack (from GDB):

(gdb) bt
#0  0x00007f75f30c1d35 in buildTreeBottomUp (leafNodes=..., adj=...)
    at /home/travis/build/stephengold/Libbulletjme/src/main/native/bullet3/BulletSoftBody/btSoftBody.cpp:85
#1  0x00007f75f30d206e in btSoftBody::rebuildNodeTree (this=0x7f75c0b680b0)
    at /home/travis/build/stephengold/Libbulletjme/src/main/native/bullet3/BulletSoftBody/btSoftBody.cpp:2729
#2  0x00007f75f30d1b7a in btSoftBody::initializeFaceTree (this=0x7f75c0b680b0)
    at /home/travis/build/stephengold/Libbulletjme/src/main/native/bullet3/BulletSoftBody/btSoftBody.cpp:2693
#3  0x00007f75f30ce6cb in btSoftBody::predictMotion (this=0x7f75c0b680b0, 
    dt=0.0166666675)
    at /home/travis/build/stephengold/Libbulletjme/src/main/native/bullet3/BulletSoftBody/btSoftBody.cpp:2111
#4  0x00007f75f3336ee5 in btDefaultSoftBodySolver::predictMotion (
    this=0x7f75c0b51f60, timeStep=0.0166666675)
    at /home/travis/build/stephengold/Libbulletjme/src/main/native/bullet3/BulletSoftBody/btDefaultSoftBodySolver.cpp:143
#5  0x00007f75f32e1e5f in btSoftRigidDynamicsWorld::predictUnconstraintMotion (
    this=0x7f75c0b10160, timeStep=0.0166666675)
    at /home/travis/build/stephengold/Libbulletjme/src/main/native/bullet3/BulletSoftBody/btSoftRigidDynamicsWorld.cpp:74
#6  0x00007f75f32fe8f5 in btDiscreteDynamicsWorld::internalSingleStepSimulation
    (this=0x7f75c0b10160, timeStep=0.0166666675)
    at /home/travis/build/stephengold/Libbulletjme/src/main/native/bullet3/BulletDynamics/Dynamics/btDiscreteDynamicsWorld.cpp:463
#7  0x00007f75f32e1f2d in btSoftRigidDynamicsWorld::internalSingleStepSimulation (this=0x7f75c0b10160, timeStep=0.0166666675)
    at /home/travis/build/stephengold/Libbulletjme/src/main/native/bullet3/BulletSoftBody/btSoftRigidDynamicsWorld.cpp:88

[stephengold]

TODO:

1. Verify this is the first simulation step.
2. Verify N = 2459

[stephengold]

Actually, N = 4914.

Then newLeafNodes.size() = 2597, then 1398, then 773, then 455, then 290, then 203, then 156, ...

It eventually gets to 1, but then buildTreeBottomUp() gets invoked again (on the same simulation step), and that's when the hang occurs.

Note that buildTreeBottomUp() is invoked from initializeFaceTree() the 1st time and then from rebuildNodeTree() the 2nd time, both times on the same soft body.

The 2nd time, N seems to be stuck at 2459.

[stephengold]

The 2nd time, adj is based on the soft body's links, but the body has zero links!

[stephengold]

Adding links to the body resolves this issue. I'll add an option to the SoftBodyControl constructor.

[stephengold]

Added at b2cd333