ContinuationPreservedEmbedderData-based Node custom labels#153
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@gnurizen This is ready for review; please take a look when you're not too busy sailing in the Ionian Sea. |
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Sep 25, 2025
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Can we separate this into two PRs/commits one to remove hashmap and one to add the new approach? That will make reviewing and more importantly upstreaming this a lot easier. Claude should be able to make short work of it if the git-fu gets tedious. |
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| "license": "Apache-2.0", | ||
| "dependencies": { | ||
| "@polarsignals/custom-labels": "^0.2.0", | ||
| "@polarsignals/custom-labels": "^0.3.1", |
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How do I review what changes went into this bump again?
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We are replacing the manually propagated hashmap-based approach with an approach based on stuffing the labels in v8's ContinuationPreservedEmbedderData (see upcoming commit).
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@gnurizen I broke it into two commits: one which removes the old approach (except tests, which haven't changed) and one which re-adds the new approach. |
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Looks great, just a couple nits, nice work!
Introduce custom labels stored in Node's AsyncLocalStorage. We require that Node use its async_context_frame strategy for storing and propagating this data (which is the default beginning in Node v24, and requires the --experimental-async-context-frame flag in v22 and v23). This strategy works by storing the stored value in v8's ContinuationPreservedEmbedderData, which causes the runtime itself to handle propagation. We refuse to use the previous default AsyncLocalStorage propagation strategy, which is basically similar to our prior approach, which was rejected due to being too slow. The internal v8 data structures we walk are explained in the comment on maybe_add_node_custom_labels.
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This totally removes the previous hashmap-based labels experiment, which was found to be too slow in practice, and replaces it with storing labels in Node's AsyncLocalStorage. We require that Node use its
async_context_framestrategy for storing and propagating this data (which is the default beginning in Node v24, and requires the--experimental-async-context-frameflag in v22 and v23). This strategy works by storing the stored value in v8'sContinuationPreservedEmbedderData, which causes the runtime itself to handle propagation. We refuse to use the previous defaultAsyncLocalStoragepropagation strategy, which is basically similar to our own (failed) approach.The internal v8 data structures we walk are explained in the comment on
maybe_add_node_custom_labels; everything else is basically similar.