127 lines
7.0 KiB
Markdown
127 lines
7.0 KiB
Markdown
# rlua -- High level bindings between Rust and Lua
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[](https://travis-ci.org/chucklefish/rlua)
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[API Documentation](https://docs.rs/rlua)
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[Examples](examples/examples.rs)
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This library is a high level interface between Rust and Lua. Its major goal is
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to expose as easy to use, practical, and flexible of an API between Rust and Lua
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as possible, while also being completely safe.
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There are other high level Lua bindings systems for rust, and this crate is an
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exploration of a different part of the design space. The other high level
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interface to Lua that I am aware of right now
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is [hlua](https://github.com/tomaka/hlua/) which you should definitely check out
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and use if it suits your needs. This crate has the following differences with
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hlua:
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* Handles to Lua values use the Lua registry, not the stack
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* Handles to Lua values are all internally mutable
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* Handles to Lua values have non-mutable borrows to the main Lua object, so
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there can be multiple handles or long lived handles
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* Targets Lua 5.3
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The key difference here is that rlua handles rust-side references to Lua values
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in a fundamentally different way than hlua, more similar to other Lua bindings
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systems like [Selene](https://github.com/jeremyong/Selene) for C++. Values like
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LuaTable and LuaFunction that hold onto Lua values in the Rust stack, instead of
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pointing at values in the Lua stack, are placed into the registry with luaL_ref.
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In this way, it is possible to have an arbitrary number of handles to internal
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Lua values at any time, created and destroyed in arbitrary order. This approach
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IS slightly slower than the approach that hlua takes of only manipulating the
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Lua stack, but this, combined with internal mutability, allows for a much more
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flexible API.
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There are currently a few notable missing pieces of this API:
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* Security limits on Lua code such as total instruction limits and recursion
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limits to prevent DOS from malicious Lua code, as well as control over which
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libraries are available to scripts.
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* Lua profiling support
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* "Context" or "Sandboxing" support. There should be the ability to set the
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`_ENV` upvalue of a loaded chunk to a table other than `_G`, so that you can
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have different environments for different loaded chunks.
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* More fleshed out Lua API, there is some missing nice to have functionality
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not exposed like storing values in the registry, and manipulating `LuaTable`
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metatables.
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* Benchmarks, and quantifying performance differences with what you would
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might write in C.
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Additionally, there are ways I would like to change this API, once support lands
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in rustc. For example:
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* Currently, variadics are handled entirely with tuples and traits implemented
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by macro for tuples up to size 12, it would be great if this was replaced
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with real variadic generics when this is available in rust.
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It is also worth it to list some non-goals for the project:
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* Be a perfect zero cost wrapper over the Lua C API
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* Allow the user to do absolutely everything that the Lua C API might allow
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## API stability or lack thereof
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This library is very much Work In Progress, so there is a lot of API churn. I
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believe the library should be stable and usable enough to realistically use in a
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real project, but the API has probably not settled down yet. I currently follow
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"pre-1.0 semver" (if such a thing exists), but there have been a large number of
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API version bumps, and there will may continue to be. If you have a dependency
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on rlua, you might want to consider adding a 0.x version bound.
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## Safety and panics
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The goal of this library is complete safety, it should not be possible to cause
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undefined behavior whatsoever with the API, even in edge cases. There is,
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however, QUITE a lot of unsafe code in this crate, and I would call the current
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safety level of the crate "Work In Progress". If you find the ability to cause
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UB with this API *at all*, please file a bug report.
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There are, however, a few ways to cause *panics* and even *aborts* with this
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API. Usually these panics or aborts are alternatives to what would otherwise be
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unsafety.
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Panic / abort considerations when using this API:
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* The API should be panic safe currently, whenever a panic is generated the
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Lua stack is cleared and the `Lua` instance should continue to be usable.
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* Panic unwinds in Rust callbacks should currently be handled correctly, the
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unwind is caught and carried across the Lua API boundary, and Lua code
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cannot catch rust panics. This is done by overriding the normal Lua 'pcall'
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and 'xpcall' with custom versions that cannot catch rust panics being piped
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through the normal Lua error system.
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* There are a few panics marked "internal error" that should be impossible to
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trigger. If you encounter one of these this is a bug.
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* When the internal version of Lua is built using the `gcc` crate (the
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default), `LUA_USE_APICHECK` is enabled. Any abort caused by this internal
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Lua API checking should be considered a bug.
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* The library internally calls lua_checkstack to ensure that there is
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sufficient stack space, and if the stack cannot be sufficiently grown this
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is a panic. There should not be a way to cause this using the API, if you
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encounter this, it is a bug.
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* This API attempts only to handle errors in Lua C API functions that can
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cause an error either directly or by running arbitrary Lua code directly,
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not functions that can cause memory errors (marked as 'm' in the Lua C API
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docs). This means that we must take care to ensure that gc or memory errors
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cannot occur, because this would unsafely longjmp potentially across rust
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frames. The allocator provided to lua is libc::malloc with an extra guard
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to ensure that OOM errors are immediate aborts, because otherwise this would
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be unsafe. Similarly, 'setmetatable' is wrapped so that any `__gc`
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metamethod specified in lua scripts will *abort* if the metamethod causes an
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error rather than longjmp like a normal error would. Lua objects can also
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be resurrected with user provided `__gc` metamethods
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(See [here](https://www.lua.org/manual/5.3/manual.html#2.5.1) for details),
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and this includes userdata, so it is possible to trigger a panic from lua by
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resurrecting a userdata and re-using it after it has been garbage collected.
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It is an eventual goal of the library to ensure that lua scripts cannot
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cause panics or aborts, but currently this is not true and this is a known
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limitation. Lua scripts should NOT be able to cause unsafety, though, this
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is always considered a bug.
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* There are currently no recursion limits on callbacks. This could cause one
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of two problems, either the API will run out of stack space and cause a
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panic in Rust, or more likely it will cause an internal `LUA_USE_APICHECK`
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abort, from exceeding LUAI_MAXCCALLS.
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* There are currently no checks on argument sizes, and I think you may be able
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to cause an abort by providing a large enough `LuaVariadic`.
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