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120 lines
4.5 KiB
Markdown
120 lines
4.5 KiB
Markdown
# XPCOM components in Rust
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XPCOM components can be written in Rust.
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## A tiny example
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The following example shows a new type that implements `nsIObserver`.
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First, create a new empty crate (e.g. with `cargo init --lib`), and add the
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following dependencies in its `Cargo.toml` file.
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```toml
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[dependencies]
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libc = "0.2"
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nserror = { path = "../../../xpcom/rust/nserror" }
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nsstring = { path = "../../../xpcom/rust/nsstring" }
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xpcom = { path = "../../../xpcom/rust/xpcom" }
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```
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(The number of `../` occurrences will depend on the depth of the crate in the
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file hierarchy.)
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Next hook it into the build system according to the [build
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documentation](/build/buildsystem/rust.rst).
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The Rust code will need to import some basic types. `xpcom::interfaces`
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contains all the usual `nsI` interfaces.
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```rust
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use libc::c_char;
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use nserror::nsresult;
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use std::sync::atomic::{AtomicBool, Ordering};
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use xpcom::{interfaces::nsISupports, RefPtr};
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```
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The next part declares the implementation.
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```rust
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#[xpcom(implement(nsIObserver), atomic)]
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struct MyObserver {
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ran: AtomicBool,
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}
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```
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This defines the implementation type, which will be refcounted in the specified
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way and implement the listed xpidl interfaces. It will also declare a second
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initializer struct `InitMyObserver` which can be used to allocate a new
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`MyObserver` using the `MyObserver::allocate` method.
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Next, all interface methods are declared in the `impl` block as `unsafe` methods.
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```rust
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impl MyObserver {
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#[allow(non_snake_case)]
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unsafe fn Observe(
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&self,
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_subject: *const nsISupports,
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_topic: *const c_char,
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_data: *const u16,
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) -> nsresult {
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self.ran.store(true, Ordering::SeqCst);
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nserror::NS_OK
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}
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}
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```
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These methods always take `&self`, not `&mut self`, so we need to use interior
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mutability: `AtomicBool`, `RefCell`, `Cell`, etc. This is because all XPCOM
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objects are reference counted (like `Arc<T>`), so cannot provide exclusive access.
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XPCOM methods are unsafe by default, but the
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[xpcom_method!](https://searchfox.org/mozilla-central/source/xpcom/rust/xpcom/src/method.rs)
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macro can be used to clean this up. It also takes care of null-checking and
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hiding pointers behind references, lets you return a `Result` instead of an
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`nsresult,` and so on.
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To use this type within Rust code, do something like the following.
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```rust
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let observer = MyObserver::allocate(InitMyObserver {
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ran: AtomicBool::new(false),
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});
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let rv = unsafe {
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observer.Observe(x.coerce(),
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cstr!("some-topic").as_ptr(),
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ptr::null())
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};
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assert!(rv.succeeded());
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```
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The implementation has an (auto-generated) `allocate` method that takes in an
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initialization struct, and returns a `RefPtr` to the instance.
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`coerce` casts any XPCOM object to one of its base interfaces; in this case,
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the base interface is `nsISupports`. In C++, this would be handled
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automatically through inheritance, but Rust doesn’t have inheritance, so the
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conversion must be explicit.
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## Bigger examples
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The following XPCOM components are written in Rust.
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- [kvstore](https://searchfox.org/mozilla-central/source/toolkit/components/kvstore),
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which exposes the LMDB key-value store (via the [Rkv
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library](https://docs.rs/rkv)) The API is asynchronous, using `moz_task` to
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schedule all I/O on a background thread, and supports getting, setting, and
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iterating over keys.
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- [cert_storage](https://searchfox.org/mozilla-central/source/security/manager/ssl/cert_storage),
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which stores lists of [revoked intermediate certificates](https://blog.mozilla.org/security/2015/03/03/revoking-intermediate-certificates-introducing-onecrl/).
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- [bookmark_sync](https://searchfox.org/mozilla-central/source/toolkit/components/places/bookmark_sync),
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which [merges](https://mozilla.github.io/dogear) bookmarks from Firefox Sync
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with bookmarks in the Places database.
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[There's also some docs on how Rust interacts with Sync](/services/sync/rust-engines.rst)
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- [webext_storage_bridge](https://searchfox.org/mozilla-central/source/toolkit/components/extensions/storage/webext_storage_bridge),
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which powers the WebExtension storage.sync API. It's a self-contained example
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that pulls in a crate from application-services for the heavy lifting, wraps
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that up in a Rust XPCOM component, and then wraps the component in a JS
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interface. There's also some boilerplate there around adding a
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`components.conf` file, and a dummy C++ header that declares the component
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constructor. [It has some in-depth documentation on how it hangs together](../toolkit/components/extensions/webextensions/webext-storage.rst).
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