} "ready line1 line2 {done\n}"
test io-53.4 {CopyData: background write overflow} {stdio unix openpipe fileevent fcopy} {
    set big bbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbbb\n
    variable x
    for {set x 0} {$x < 12} {incr x} {
	append big $big
    }
#    file delete $path(test1)
    file delete $path(pipe)
    set f1 [open $path(pipe) w]
    puts $f1 {
	puts ready
	fcopy stdin stdout -command { set x }
	vwait x
#	set f [open $path(test1) w]
#	fconfigure $f -translation lf
#	puts $f "done"
#	close $f
    }
    close $f1
    set f1 [open "|[list [interpreter] $path(pipe)]" r+]
    set result [gets $f1]
    fconfigure $f1 -blocking 0
    puts $f1 $big
    flush $f1
    after 500
    set result ""
    fileevent $f1 read [namespace code {
	append result [read $f1 1024]
	if {[string length $result] >= [string length $big]} {
	    set x done
	}
    }]