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Mention pitfall in ObjectTracerContext::with_tracer doc comment #1259

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Mention pitfall in ObjectTracerContext::with_tracer doc comment #1259

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14 changes: 10 additions & 4 deletions src/vm/scanning.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -61,10 +61,16 @@ pub trait ObjectTracerContext<VM: VMBinding>: Clone + Send + 'static {

/// Create a temporary `ObjectTracer` and provide access in the scope of `func`.
///
/// When the `ObjectTracer::trace_object` is called, if the traced object is first visited
/// in this transitive closure, it will be enqueued. After `func` returns, the implememtation
/// will create work packets to continue computing the transitive closure from the newly
/// enqueued objects.
/// A typical use-case of this method is to call `ObjectTracer::trace_object` in a loop for
/// finalization candidates to create `ProcessEdgesWork` packets.
//// **Note**: Do not use this API in such a way that only one object is enqueued at a time.
/// This can result in creating a work packet for a single object, exhausting (physical) memory
/// if there are enough finalization candidates.
Comment on lines +64 to +68
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I think the description is too specific to the bug you fixed recently. It's not specific to finalization (it's also applicable to SoftReference), and it is better to provide an example of the right way to use it, and then describe the consequence of flipping the order of the loop and the closure. You can leave this to me. I plan to write a dedicated porting guide section for weak references and finalizers, and update this doc comment at the same time.

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Okay. Thank you.

///
/// When the `ObjectTracer::trace_object` is called, if the traced object is visited in this
/// transitive closure for the first time, it will be enqueued. After `func` returns, the
/// implementation will create work packets to continue computing the transitive closure from
/// the newly enqueued objects.
///
/// API functions that provide `QueuingTracerFactory` should document
/// 1. on which fields the user is supposed to call `ObjectTracer::trace_object`, and
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