How to tell if a Task is blocked?

I have a Task executed with @async. This task writes to a Channel multiple times (it is not known exactly how often), then reads from another Channel once. This should repeat until some condition is met. The main thread should wait for all output of the task, then write to it once. This should repeat until the task is finished. What is sent to the task depends on what the task sent before.

Here is a contrived example:

function f(input, output)
  while true
    put!(output, rand(-1:1))
    if rand(1:10) == 1
      take!(input) == 0 && return

input = Channel{Int}(Inf)
output = Channel{Int}(Inf)

task = @async f(input, output)

while !istaskdone(task)
  s = 0
  sleep(0.01) # FIXME: wait until task is trying to take! from input
  while isready(output)
    s += take!(output)
  put!(input, s)

In this situation, how can I reliably tell if task is blocked or is still sending input? I’d be happy about any hints or suggestions.

(Note: I asked this question a few days ago on on reddit and didn’t get a solution. Reddit user u/wherrera10 suggested using an unbuffered Channel and isready, but this only works to tell if a task is blocked trying to put! to an unbuffered Channel)