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author | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-19 00:47:55 +0000 |
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committer | Daniel Baumann <daniel.baumann@progress-linux.org> | 2024-04-19 00:47:55 +0000 |
commit | 26a029d407be480d791972afb5975cf62c9360a6 (patch) | |
tree | f435a8308119effd964b339f76abb83a57c29483 /third_party/rust/uniffi_bindgen/src/bindings/kotlin/templates/ObjectRuntime.kt | |
parent | Initial commit. (diff) | |
download | firefox-26a029d407be480d791972afb5975cf62c9360a6.tar.xz firefox-26a029d407be480d791972afb5975cf62c9360a6.zip |
Adding upstream version 124.0.1.upstream/124.0.1
Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
Diffstat (limited to 'third_party/rust/uniffi_bindgen/src/bindings/kotlin/templates/ObjectRuntime.kt')
-rw-r--r-- | third_party/rust/uniffi_bindgen/src/bindings/kotlin/templates/ObjectRuntime.kt | 161 |
1 files changed, 161 insertions, 0 deletions
diff --git a/third_party/rust/uniffi_bindgen/src/bindings/kotlin/templates/ObjectRuntime.kt b/third_party/rust/uniffi_bindgen/src/bindings/kotlin/templates/ObjectRuntime.kt new file mode 100644 index 0000000000..b9352c690f --- /dev/null +++ b/third_party/rust/uniffi_bindgen/src/bindings/kotlin/templates/ObjectRuntime.kt @@ -0,0 +1,161 @@ +// Interface implemented by anything that can contain an object reference. +// +// Such types expose a `destroy()` method that must be called to cleanly +// dispose of the contained objects. Failure to call this method may result +// in memory leaks. +// +// The easiest way to ensure this method is called is to use the `.use` +// helper method to execute a block and destroy the object at the end. +interface Disposable { + fun destroy() + companion object { + fun destroy(vararg args: Any?) { + args.filterIsInstance<Disposable>() + .forEach(Disposable::destroy) + } + } +} + +inline fun <T : Disposable?, R> T.use(block: (T) -> R) = + try { + block(this) + } finally { + try { + // N.B. our implementation is on the nullable type `Disposable?`. + this?.destroy() + } catch (e: Throwable) { + // swallow + } + } + +// The base class for all UniFFI Object types. +// +// This class provides core operations for working with the Rust `Arc<T>` pointer to +// the live Rust struct on the other side of the FFI. +// +// There's some subtlety here, because we have to be careful not to operate on a Rust +// struct after it has been dropped, and because we must expose a public API for freeing +// the Kotlin wrapper object in lieu of reliable finalizers. The core requirements are: +// +// * Each `FFIObject` instance holds an opaque pointer to the underlying Rust struct. +// Method calls need to read this pointer from the object's state and pass it in to +// the Rust FFI. +// +// * When an `FFIObject` is no longer needed, its pointer should be passed to a +// special destructor function provided by the Rust FFI, which will drop the +// underlying Rust struct. +// +// * Given an `FFIObject` instance, calling code is expected to call the special +// `destroy` method in order to free it after use, either by calling it explicitly +// or by using a higher-level helper like the `use` method. Failing to do so will +// leak the underlying Rust struct. +// +// * We can't assume that calling code will do the right thing, and must be prepared +// to handle Kotlin method calls executing concurrently with or even after a call to +// `destroy`, and to handle multiple (possibly concurrent!) calls to `destroy`. +// +// * We must never allow Rust code to operate on the underlying Rust struct after +// the destructor has been called, and must never call the destructor more than once. +// Doing so may trigger memory unsafety. +// +// If we try to implement this with mutual exclusion on access to the pointer, there is the +// possibility of a race between a method call and a concurrent call to `destroy`: +// +// * Thread A starts a method call, reads the value of the pointer, but is interrupted +// before it can pass the pointer over the FFI to Rust. +// * Thread B calls `destroy` and frees the underlying Rust struct. +// * Thread A resumes, passing the already-read pointer value to Rust and triggering +// a use-after-free. +// +// One possible solution would be to use a `ReadWriteLock`, with each method call taking +// a read lock (and thus allowed to run concurrently) and the special `destroy` method +// taking a write lock (and thus blocking on live method calls). However, we aim not to +// generate methods with any hidden blocking semantics, and a `destroy` method that might +// block if called incorrectly seems to meet that bar. +// +// So, we achieve our goals by giving each `FFIObject` an associated `AtomicLong` counter to track +// the number of in-flight method calls, and an `AtomicBoolean` flag to indicate whether `destroy` +// has been called. These are updated according to the following rules: +// +// * The initial value of the counter is 1, indicating a live object with no in-flight calls. +// The initial value for the flag is false. +// +// * At the start of each method call, we atomically check the counter. +// If it is 0 then the underlying Rust struct has already been destroyed and the call is aborted. +// If it is nonzero them we atomically increment it by 1 and proceed with the method call. +// +// * At the end of each method call, we atomically decrement and check the counter. +// If it has reached zero then we destroy the underlying Rust struct. +// +// * When `destroy` is called, we atomically flip the flag from false to true. +// If the flag was already true we silently fail. +// Otherwise we atomically decrement and check the counter. +// If it has reached zero then we destroy the underlying Rust struct. +// +// Astute readers may observe that this all sounds very similar to the way that Rust's `Arc<T>` works, +// and indeed it is, with the addition of a flag to guard against multiple calls to `destroy`. +// +// The overall effect is that the underlying Rust struct is destroyed only when `destroy` has been +// called *and* all in-flight method calls have completed, avoiding violating any of the expectations +// of the underlying Rust code. +// +// In the future we may be able to replace some of this with automatic finalization logic, such as using +// the new "Cleaner" functionaility in Java 9. The above scheme has been designed to work even if `destroy` is +// invoked by garbage-collection machinery rather than by calling code (which by the way, it's apparently also +// possible for the JVM to finalize an object while there is an in-flight call to one of its methods [1], +// so there would still be some complexity here). +// +// Sigh...all of this for want of a robust finalization mechanism. +// +// [1] https://stackoverflow.com/questions/24376768/can-java-finalize-an-object-when-it-is-still-in-scope/24380219 +// +abstract class FFIObject( + protected val pointer: Pointer +): Disposable, AutoCloseable { + + private val wasDestroyed = AtomicBoolean(false) + private val callCounter = AtomicLong(1) + + open protected fun freeRustArcPtr() { + // To be overridden in subclasses. + } + + override fun destroy() { + // Only allow a single call to this method. + // TODO: maybe we should log a warning if called more than once? + if (this.wasDestroyed.compareAndSet(false, true)) { + // This decrement always matches the initial count of 1 given at creation time. + if (this.callCounter.decrementAndGet() == 0L) { + this.freeRustArcPtr() + } + } + } + + @Synchronized + override fun close() { + this.destroy() + } + + internal inline fun <R> callWithPointer(block: (ptr: Pointer) -> R): R { + // Check and increment the call counter, to keep the object alive. + // This needs a compare-and-set retry loop in case of concurrent updates. + do { + val c = this.callCounter.get() + if (c == 0L) { + throw IllegalStateException("${this.javaClass.simpleName} object has already been destroyed") + } + if (c == Long.MAX_VALUE) { + throw IllegalStateException("${this.javaClass.simpleName} call counter would overflow") + } + } while (! this.callCounter.compareAndSet(c, c + 1L)) + // Now we can safely do the method call without the pointer being freed concurrently. + try { + return block(this.pointer) + } finally { + // This decrement always matches the increment we performed above. + if (this.callCounter.decrementAndGet() == 0L) { + this.freeRustArcPtr() + } + } + } +} |