[Vanilla Bug] Homing projectiles disappear once their target object is unavailable

[askhati]

Description:

Homing projectiles (weapon projectile ROT>0) - noticeably missiles, and the CyCommando plasma cannon - will disappear mid-flight if their target becomes unavailable (is destroyed, or cloaks).

Conditions:

Weapon projectile must have ROT>0

Steps To Reproduce:

1. Use a unit with homing projectiles (e.g. Hover MLRS) to fire at a target unit (e.g. Titan)
2. Use a second unit to deal massive damage to the Titan while the HMLRS rockets are in mid-flight (enough to destroy the Titan)

Expected Behaviour:

The homing projectiles should keep flying towards the last known position of the now-destroyed (or now-cloaked) unit.

Actual Behaviour:

Homing projectiles just disappear mid-air.

[Crimsonum]

This bug also happens when the target switches between mobile/deployed state, e.g. Tick Tank. It can thus be easily reproduced when playing as Nod: Make a Rocket Infantry fire at a Tick Tank and (un)deploy the tank before the missile hits.

[Rampastring]

To fix this, we need to figure out the algorithm of BulletClass::AI for both ROT > 0 projectiles as well as ROT <= 0 projectiles. It's a rather complex function to reverse-engineer.

I'm attaching slightly cleaned up pseudocode for the function. @tomsons26 provided me with the original pseudo, but I cleaned it up a bit further. bulletclass__ai_ts.txt

I made a test where I made the game skip entirely calling the bullet explosion / disappearing code regardless of the projectile's state. It resulted in behaviour where ROT > 0 projectiles circle around their target infinitely, until the target dies. When the target dies, the ROT > 0 projectiles stop instantly. ROT = 0 projectiles, however, continue moving on their own for some time even after their target dies. Video of ROT > 0 projectile behaviour

This kinda points towards the game having no code for properly advancing the movement of ROT > 0 projectiles whose target is dead. This might not be the ultimate truth though.

[Rampastring]

Part of BulletClass::AI, specifically the code that deals with ROT > 0 projectiles (from 0x00444814 on), re-written in C++ for Vinifera to some degree: https://cdn.discordapp.com/attachments/741394749063430144/950888946953494538/bulletclass__ai_rot_over_zero.cpp

It's far from complete, but is likely a good base for further research. I took the BulletClass::AI pseudocode posted above in my earlier comment, cleaned it up further and did more research until I was able to write this.