When a let binding gets used across a blocking operation, the binding leaks out of the let.
The stuff binding gets stuck into the the state-machine state. This persists across the recur, until the binding is next encountered.
(In our case, a few GBs had accumuled by that time.)
Ideally, the fields in the state that represent local variables would be nulled when the variables go out of scope.
related : ASYNC-219