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Promoted `with`, `take`, `finalize` to be methods of Managed_Resource rather than static methods always taking `resource`, for consistency reasons. This required function dispatch boilerplate, similarly to `Ref`. In future iterations we will address this boilerplate code. Related to https://www.pivotaltracker.com/story/show/182212217
108 lines
5.0 KiB
Markdown
108 lines
5.0 KiB
Markdown
---
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layout: developer-doc
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title: Managed Resources
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category: semantics
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tags: [resources, finalization, cleanup]
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order: 9
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---
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# Managed Resources
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Enso is a language targeting an audience with possibly low programming skills
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and aims to be as user-friendly as possible. Therefore, it is crucial to provide
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some mechanisms to automatically clean up unclosed resources (such as file
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handles, sockets, machine pointers, etc.). The Managed Resources system solves
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this problem, by allowing library authors to attach garbage collection hooks to
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certain objects, such that a clean up action can be performed as soon as the
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runtime discovers the resource will never be used again. This document outlines
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the behavior of this system, as well as important notes regarding its use.
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<!-- MarkdownTOC levels="2,3" autolink="true" -->
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- [Basic Usage](#basic-usage)
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- [Semantics and Guarantees](#semantics-and-guarantees)
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- [Execution Guarantess](#execution-guarantees)
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- [Multiple Managed Resources Wrapping The Same Underlying Resource](#multiple-managed-resources-wrapping-the-same-underlying-resource)
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- [Thread Safety](#thread-safety)
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<!-- /MarkdownTOC -->
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## Basic Usage
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A new managed resource is created by calling the
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`Managed_Resource.register resource finalizer` method. The `resource` is the
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object being finalized and `finalizer` is a one-argument function that will be
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passed the `resource` upon finalization. this call returns an object wrapping
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the original resource. The underlying resource will be finalized as soon as the
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_returned wrapper_ is garbage collected. It is therefore crucial to stop using
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`resource` right after the call, as it may be finalized at any point after this
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call.
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> #### Important
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>
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> Due to the limitations of current implementation of Enso, the `finalizer`
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> passed to `Managed_Resource.register` must not be a lambda. This is because
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> lambdas implicitly capture the whole lexical scope they are defined in, so in
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> `res = Managed_Resource.register object (o -> o.close)`, the `finalizer`
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> closes over the value of `res`, preventing it from being garbage collected.
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> The same limitation concerns the underscore-lambda syntax, as `_.close` is
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> equivalent to `o -> o.close`. The finalizer should be a (possibly curried)
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> call to a function defined outside of the lexical scope of the
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> `Managed_Resource.register` call.
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To perform operations on the underlying resource, use the
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`<managed-resource>.with action` method, where `<managed-resource>` is the
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object returned from the call to `Managed_Resource.register`, and `action` is a
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function taking the underlying object as its only argument. It is important that
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the object passed to `action` is not stored and is not used past the return of
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`action`. This means in particular that it is unsafe to give another thread a
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reference to that object, if the thread remains alive past the return of
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`action`. If such an operation is necessary, the other thread should call `with`
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itself, using a reference to the original manged resource.
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A managed resource can be closed manually, using `<managed-resource>.close`. The
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underlying object is then finalized immediately.
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The finalization of a resource can be aborted using
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`Managed_Resource.take resource`. This call will abort any automatic
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finalization actions and return the original underlying object. The return value
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is no longer managed by the runtime and must either be finalized manually or
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wrapped in a new managed resource using a call to `Managed_Resource.register`.
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## Semantics and Guarantees
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This section outlines the runtime semantics and guarantees provided by the
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managed resources system.
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### Execution Guarantees
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The finalizer attached to a managed resource is guaranteed to be executed
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at-most-once.
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There are no guarantees that the finalizer will ever _be_ executed. It is
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executed as soon as the runtime garbage-collects the managed resource, but this
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is not to say "as soon as the managed resource becomes unreachable". The runtime
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is free to run garbage collection at any point, including to not run it at all
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over the course of program execution. A call to `Runtime.gc` serves as hint to
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the runtime system to perform garbage collection, but does not guarantee that
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garbage collection will actually run.
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The finalizer may be run from any application thread, with no guarantees as to
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which thread will perform the finalization.
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### Multiple Managed Resources Wrapping The Same Underlying Resource
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In case the same underlying resource is used in multiple managed resources, it
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will be finalized as soon as the first managed resource is garbage collected.
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Moreover, the finalizer will be called for each garbage collected managed
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resource, possibly leading to multiple-finalization of the underlying object.
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Therefore, using the same underlying resource with multiple managed resource
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instances should be considered an error.
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### Thread Safety
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Operations on managed resources are thread safe. Therefore, the safety
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guarantees of the underlying resources are the limitation – if the underlying
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resource is not thread-safe, calls to `Managed_Resource.with` will also not be
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thread-safe.
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