debug.getinfo doesn't check for '>'

[Xmilia Hermit <xmilia.hermit@...>]

Hi,

debug.getinfo does not guard against '>' as the first character in 
what/options. This can result in lua_getinfo expecting a function on the 
top of the stack but db_getinfo not pushing it, when a call stack index 
is supplied instead of a function.
When a different thread is inspected, this can result in stack top being 
reduced by one every time debug.getinfo is called.
After reducing stack top of a different thread, calling debug.getinfo 
with a function object instead of a call stack index can result in the 
function object being written somewhere in the middle of the stack of 
the other thread as can be seen in the following test.


local c = coroutine.create(function(changed)
   coroutine.yield() -- Here debug.getinfo will write the print 
function into 'changed'.
   coroutine.yield(changed)
end)

local value = "Random"

coroutine.resume(c, value)

debug.getinfo(c, 1, ">") -- This should 'error bad argument #3 to 
'getinfo' (invalid option)' as debug.getinfo(c, 1, "X") does.
assert(debug.getinfo(c, 1, ">f").func == value) -- Reading the local 
variable 'changed' from the coroutine.
debug.getinfo(c, print, "")

local _, val = coroutine.resume(c)
-- assert(val == value, "Value changed") -- This should be true,
assert(val == print) -- but this is


I am aware that using the debug library wrong can cause unexpected 
behaviours, but debug.getinfo already errors when an unexpected option 
is supplied, therefore '>' shouldn't be allowed too.

Regards,
Xmilia