Loading data with RAYON_NUM_THREADS=1 does nothing

[teh-cmc]

This is true for both our latest release (0.12.1) and our latest main: RAYON_NUM_THREADS=1 rerun plots.rrd just starts a viewer and then does nothing.

There's no deadlock, the viewer is alive and well: it just doesn't load data.

Either this is a bug and it needs to be fixed, or there is some logical reason for the loading path to require at least two independent compute threads, in which case we need to either document that reason and prevent NUM_THREADS=1 or make sure to spawn a dedicated native thread where required.

Putting on high priority as this might hide something very very wrong.

[teh-cmc]

This is caused by external loaders, removing them all from my PATH fixes the issue -- something is blocking and/or waiting for another task on the compute threadpool?