Based on conversation with milkie - not 100% sure if this is correct behavior or not (or possibly just a normal consequence of yielding/timing) so i'm opening this to track efforts to probe further.

• on a standalone mongod, create a collection and fill it with test data (1M docs)
• run createIndex with {background:true}
• immediately after the createIndex command, run a convertToCapped command on the same collection.

expected:

• the background index build should abort and convertToCapped should run

actual:

• the background index command runs to completion, and the convertToCapped runs only after the index build is complete.