[luatex] Can't open pipe in .tex doc

[Michael Talbot-Wilson]

$ cat textst7.tex && luatex textst7
\directlua {a = io.popen("ls /")
if a == nil then
    print "Woe is me!"
else
    for c in a:lines(1) do print(c) end
    io.close(a)
end}
\bye
This is LuaTeX, Version beta-0.70.2-2012062812 (TeX Live 2012)
  restricted \write18 enabled.
(./textst7.texWoe is me!
  )
No pages of output.
Transcript written on textst7.log.


Why?  Works as standalone lua.

What I want is for my TeX document to read BLOBs (PDF images) and
other stuff from a PostgreSQL database.  So "ls /" would become a psql
(SQL) command.