Now, simply remove the userdata table immediately before dropping the userdata.
This does two things, it prevents __gc from double dropping the userdata, and
after the first call to __gc, it prevents the userdata from being identified as
any particular userdata type, so it cannot be misused after being finalized.
This change thus removes the userdata invalidation error, and simplifies a lot
of userdata handling code.
It also fixes a panic bug. Because there is no predictable order for
finalizers, it is possible to run a userdata finalizer that does not resurrect
itself before a lua table finalizer that accesses that userdata, and this means
that there were several asserts that were possible to trigger in normal Lua code
in util.rs related to `WrappedError`.
Now, finalized userdata is simply a userdata with no methods, so any use of
finalized userdata becomes a normal script runtime error (though, with a
potentially confusing error message). As a future improvement, we could set
a metatable on finalized userdata that provides a better error message.
Also make sure that panic messages clearly state that they are internal errors,
so people report them as a bug. Since the only panics left are all internal
errors, just move the internal error message into the panic / assert macros.
- Update readme, changelog, cargo version number in preparation for release
- Remove panicking behavior on recursive callback calls, add additional error
variant for recursive callback errors.
CallbackError now, instead of displaying the cause description, instead prints
"callback error: <traceback>". Since the cause is already in the cause chain of
the error, this avoids repeatedly printing the cause of callback errors along
the chain, and also actually prints the callback when using Display on each
error in the chain.
Since this is `rlua::Error`, it should be clear that it refers to
Lua-related errors. Downstream crates want to define their own error
enums, which can add a prefix like "lua error:" to disambiguate.
Previously, the traceback would be printed, but not the actual error.
I've removed traceback printing completely, not sure if that's a good
idea. A `Display` impl that outputs multiple lines feels weird.
I didn't yet document *everything* there is to say (in particular, how
exactly custom Rust errors can be passed through Lua), but I've some
changes to this type in mind that I'll do next.
Rename the following:
LuaNil => Nil
LuaExternalError => ExternalError
LuaExternalResult => ExternalResult
LuaCallback => Callback (internal only)
Use qualified re-exports at the top of the module.
Add a new public 'prelude' module which re-exports everything with a
non-conflicting name (Adds back the Lua prefix), and is meant to be imported
unqualified.
It, ahem "should not" be possible to exhaust lua stack space in normal usage,
and causing stack errors to be Err is slightly obnoxious. I have been wanting
to make this change for a while, and removing the callback API from tables makes
this sensible *I think*.
I can think of a couple of ways that this is not technically true, but I think
that they are acceptable, or should be handled differently.
One, you can make arbitrarily sized LuaVariadic values. I think this is maybe a
bug already, because there is an argument limit in Lua which is lower than the
stack limit. I'm not sure what happens there, but if it is a stack based panic,
(or any panic?) it is a bug.
Two, I believe that if you recurse over and over between lua -> rust -> lua ->
rust etc, and call rlua API functions, you might get a stack panic. I think for
trusted lua code, this is morally equivalent to a regular stack overflow in
plain rust, which is already.. well it's not a panic but it's some kind of safe
crash I'm not sure, so I think this is acceptable. For *untrusted* lua code,
this could theoretically be a problem if the API provided a callback that would
call back into lua, then some lua script could force a stack based panic. There
are so many concerns with untrusted lua code, and this library is NOT safe
enough yet for untrusted code (it doesn't even provide an option to limit lua to
the safe API subset yet!), so this is not currently an issue. When the library
provides support for "safe lua", it should come with big warnings anyway, and
being able to force a stack panic is pretty minor in comparison.
I think if there are other ways to cause unbounded stack usage, that it is a
bug, or there can be an error just for that situation, like argument count
limits.
This commit also fixes several stupid bugs with tests, stack checking, and
panics.
The current situation with error_chain is less than ideal, and there are lots of
conflicting interests that are impossible to meet at once. Here is an
unorganized brain dump of the current situation, stay awhile and listen!
This change was triggered ultimately by the desire to make LuaError implement
Clone, and this is currently impossible with error_chain. LuaError must
implement Clone to be a proper lua citizen that can live as userdata within a
lua runtime, because there is no way to limit what the lua runtime can do with a
received error. Currently, this is solved by there being a rule that the error
will "expire" if the error is passed back into rust, and this is very
sub-optimal. In fact, one could easily imagine a scenario where lua is for
example memoizing some function, and if the function has ever errored in the
past the function should continue returning the same error, and this situation
immediately fails with this restriciton in place.
Additionally, there are other more minor problems with error_chain which make
the API less good than it could be, or limit how we can use error_chain. This
change has already solved a small bug in a Chucklefish project, where the
conversion from an external error type (Borrow[Mut]Error) was allowed but not
intended for user code, and was accidentally used. Additionally, pattern
matching on error_chain errors, which should be common when dealing with Lua, is
less convenient than a hand rolled error type.
So, if we decide not to use error_chain, we now have a new set of problems if we
decide interoperability with error_chain is important. The first problem we run
into is that there are two natural bounds for wrapped errors that we would
pick, (Error + Send + Sync), or just Error, and neither of them will
interoperate well with error_chain. (Error + Send + Sync) means we can't wrap
error chain errors into LuaError::ExternalError (they're missing the Sync
bound), and having the bounds be just Error means the opposite, that we can't
hold a LuaError inside an error_chain error.
We could just decide that interoperability with error_chain is the most
important qualification, and pick (Error + Send), but this causes a DIFFERENT
set of problems. The rust ecosystem has the two primary error bounds as Error
or (Error + Send + Sync), and there are Into impls from &str / String to
Box<Error + Send + Sync> for example, but NOT (Error + Send). This means that
we are forced to manually recreate the conversions from &str / String to
LuaError rather than relying on a single Into<Box<Error + Send + Sync>> bound,
but this means that string conversions have a different set of methods than
other error types for external error conversion. I have not been able to figure
out an API that I am happy with that uses the (Error + Send) bound. Box<Error>
is obnoxious because not having errors implement Send causes needless problems
in a multithreaded context, so that leaves (Error + Send + Sync). This is
actually a completely reasonable bound for external errors, and has the nice
String Into impls that we would want, the ONLY problem is that it is a pain to
interoperate with the current version of error_chain.
It would be nice to be able to specify the traits that an error generated by the
error_chain macro would implement, and this is apparently in progress in the
error_chain library. This would solve both the problem with not being able to
implement Clone and the problems with (Error + Send) bounds. I am not convinced
that this library should go back to using error_chain when that functionality is
in stable error_chain though, because of the other minor usability problems with
using error_chain.
In that theoretical situation, the downside of NOT using error_chain is simply
that there would not be automatic stacktraces of LuaError. This is not a huge
problem, because stack traces of lua errors are not extremely useful, and for
external errors it is not too hard to create a different version of the
LuaExternalResult / LuaExternalError traits and do conversion from an
error_chain type into a type that will print the stacktrace on display, or
use downcasting in the error causes.
So in summary, this library is no longer using error_chain, and probably will
not use it again in the future. Currently this means that to interoperate with
error_chain, you should use error_chain 0.8.1, which derives Sync on errors, or
wait for a version that supports user defined trait derives. In the future
when error_chain supports user defined trait derives, users may have to take an
extra step to make wrapped external errors print the stacktrace that they
capture.
This change works, but is not entirely complete. There is no error
documentation yet, and the change brought to a head an ugly module organization
problem. There will be more commits for documentation and reorganization, then
a new stable version of rlua.
Alex Orlenko