it seems that if you do a background index build, we also build it on the background on the slave.

that might be desirable, but if we keep that, we need to (1) have a test for it.

(2) i think there may be an issue: you cannot do two background index builds at once. the administrator might do them one at a time on the master, but if the slave is slower, theymight overlap; in that case, the 2nd index wouldn't get built.

one solution is to do bg indexing only on the primary, but i think users would prefer to have it do background everywhere.

one fix might be : on the slave, do it in the background if one is not in progress; if one is alerady in progress, sleep and poll until that finishes. (kind of like sleepdelay). another option would be to just build int he foreground.