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Automatic Close of Unreachable Context and Engine Objects in Polyglot API

As of Polyglot version 24.2.0, Context and Engine objects are automatically closed when they are no longer strongly referenced. This feature ensures that unreachable Context and Engine instances are correctly cleaned up to minimize memory leaks. However, it remains highly recommended to manage the lifecycle of Context and Engine explicitly, ideally using a try-with-resources statement, rather than relying on garbage collection for this purpose.

Context Lifecycle Management

A Context instance is eligible for automatic closure when no strong references to it remain. These references can include the following:

• Direct strong references to the Context object.
• Strong references to objects bound to the Context, such as Value and PolyglotException instances.
• Active polyglot threads or any host thread that has explicitly entered the Context. A Context that is explicitly entered will not be garbage-collected until it is explicitly left.
• Inner Context objects, as inner contexts prevent their parent contexts from being closed if they remain reachable.

When any of these conditions hold, the Context instance remains active and will not be garbage-collected and closed. The presence of active system threads does not prevent a Context from being closed.

Engine Lifecycle Management

The Engine object follows a similar approach to lifecycle management. An Engine will only be closed when it no longer has any strong references to it, such as:

• Direct references to the Engine.
• Strong references to Language, Instrument, or ExecutionListener instances that were obtained from the Engine.

Furthermore, an Engine cannot be closed if there is an active Context within it or if any Context objects that uses this Engine remain reachable. Similar to Context objects, the presence of active system threads does not interfere with the automatic closure of an Engine.

TruffleContext and Inner Contexts

In scenarios where a language implementation creates inner contexts using TruffleLanguage.Env.newInnerContextBuilder(), both the Polyglot API Context representing this inner context and the TruffleContext must be unreachable to enable automatic closure. This situation is illustrated in the diagram below, showing an engine with one top-level context and one inner context. Strong references are represented by solid lines, and weak references by dashed lines.

When an inner Context is no longer referenced by the embedder, and the corresponding TruffleContext instance is no longer referenced by its language implementation, weak references from PolyglotContext to Context and TruffleContext are added to a reference queue. During the next lifecycle event (such as when a new context is created or closed), this queue is processed, and any unreachable context is closed automatically.