This procedure is defined in the server but not documented via ad_proc or proc_doc and may be intended as a private interface.
The procedure is defined as:
proc tdav::read_lock {uri} {
set f [open [tdav::get_lock_file $uri] {CREAT RDONLY}]
set s [read $f]
set e "list ${s}"
set l [eval $e]
close $f
return $l
}