How to send query and capture progress if dbFetch(n = -1)

[gacolitti]

I want to send a query using dbSendQuery() and poll for the progress of the query so I can show a progress bar of my own. But I can only use dbFetch(n = -1).

[jarodmeng]

dbFetch(res) whereby res is the return value of dbSendQuery() can be used to repeatedly fetching results.

The default implementation that calls dbFetch(res) in a loop is here.