To remove a to-be-closed variable from the stack in the C API a
function must use 'lua_settop' or 'lua_pop'. Previous implementation of
'luaL_pushresult' was not closing the box. (This commit also added
tests to check that box is being closed "as soon as possible".)
The variable to be closed in a generic 'for' loop now is the
4th value produced in the loop initialization, instead of being
the loop state (the 2nd value produced). That allows a loop to
use a state with a '__toclose' metamethod but do not close it.
(As an example, 'f:lines()' might use the file 'f' as a state
for the loop, but it should not close the file when the loop ends.)
The new syntax is <local *toclose x = f()>. The mark '*' allows other
attributes to be added later without the need of new keywords; it
also allows better error messages. The API function was also renamed
('lua_tobeclosed' -> 'lua_toclose').
The implicit variable 'state' in a generic 'for' is marked as a
to-be-closed variable, so that the state will be closed as soon
as the loop ends, no matter how.
Taking advantage of this new facility, the call 'io.lines(filename)'
now returns the open file as a second result. Therefore,
an iteraction like 'for l in io.lines(name)...' will close the
file even when the loop ends with a break or an error.
A closing method cannot be called in its own stack slot, as there may
be returning values in the stack after that slot, and the call would
corrupt those values. Instead, the closing method must be copied to the
top of the stack to be called.
Moreover, even when a function returns no value, its return istruction
still has to have its position (which will set the stack top) after
the local variables, otherwise a closing method might corrupt another
not-yet-called closing method.
Start of the implementation of "scoped variables" or "to be closed"
variables, local variables whose '__close' (or themselves) are called
when they go out of scope. This commit implements the syntax, the
opcode, and the creation of the corresponding upvalue, but it still
does not call the finalizations when the variable goes out of scope
(the most important part).
Currently, the syntax is 'local scoped name = exp', but that will
probably change.
From the point of view of 'git', all names are relative to the root
directory of the project. So, file names in '$Id:' also should be
relative to that directory: the proper name for test file 'all.lua'
is 'testes/all.lua'.
Roberto Ierusalimschy