const maxWeight = ...
sema := semaphore.NewWeighted(maxWeight)
...
for ... {
    size := object.Size()
    if size > maxWeight {
        // Object is too large to fit in memory, switch to a streaming mode.
        object.StreamingMode(true)
        size = maxWeight // use the maximum weight to have exclusive lock over semaphore
    }
    sema.Acquire(size)
    go func() {
        defer sema.Release(size)
        ...
    }
}

// Capacity returns the maximum combined weight this semaphore is constructed with.
func (s *Weighted) Capacity() int64 {
    return s.size
}