Lightmass Build Hanging at Specific Percentages

Lighting will build indefinitely in certain circumstances and can be verified in Swarm Agent that it will continue to process the maps even though the log shows it has completed. This is happening in 4.9, but works correctly in 4.8.

Users Video demonstration: [Link Removed]

1. Create a blank project.
2. In the Default Scene with the Template Floor, delete the floor mesh.
3. Build Lighting
4. Open Swarm Agent from the task bar to check status

Results: Light Build in UE4 will only display completion of 75%. Looking at Swarm Agent the process is still running indefinitely. Looking at the Logs in Swarm Agent the builder shows that it completes at 100%, but it continues to process.

Expected: Lighting will build.

Additional Repro notes:

Base test: Default scene completes at 100%

• Open default level with floor mesh and build lighting

Instance 1: Lighting will hang at 75%

• In a default scene remove the template floor mesh and build lighting.

Instance 2: Lighting will hang at 80%

• In a default scene remove the floor mesh and add the cube static mesh from the modes panel and build lighting.

Instance 3: Lighting will complete at 100%

• In a default scene remove the floor mesh and add a BSP from the modes panel and build lighting.