# Send `count` commands on a deferring client `rd`, reading replies in batches
# of `batch_size` to avoid a TCP deadlock when deferred replies accumulate.
# `cmd_script` is evaluated in the caller's scope once per iteration with the
# variable `i` (0-based index) available, and must send exactly one command.
proc deferred_batch {rd count cmd_script {batch_size 1000}} {
    set pending 0
    for {set idx 0} {$idx < $count} {incr idx} {
        uplevel 1 [list set i $idx]
        uplevel 1 $cmd_script
        incr pending
        if {$pending == $batch_size} {
            for {set i 0} {$i < $pending} {incr i} { $rd read }
            set pending 0
        }
    }
    for {set i 0} {$i < $pending} {incr i} { $rd read }
}