Simplify and refine async context tracking #282
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harrishancock
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Jan 23, 2023
kentonv
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Jan 24, 2023
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Ok, now that it's clear this is definitely the direction we want to go, I've merged the two PRs and cleaned up more of the bits that are made obsolete by relying on the v8 API. Thenables are still a challenge here but there's a possible workaround using |
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@harrishancock ... given the substantial changes, I've cleared your ok. Can you give it another look over? |
harrishancock
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Jan 25, 2023
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It looks reasonable to me. I think @kentonv's reviews here are a bit more valuable, though.
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I can't figure out how to get a diff from what I've already reviewed to the current state of the PR. It seems like several additional changes have been rebased together with stuff I've reviewed already, and the whole thing was also rebased on master in the meantime too. |
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There are enough additional cleanups and a number of new commits here that it's worth looking at it as a fresh review. |
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kentonv
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Jan 30, 2023
kentonv
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Jan 30, 2023
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looks good but needs squash |
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Squashed and rebased. Will land once updated internal CI comes up green |
kentonv
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It occurred to me that we really do not need a root AsyncContextFrame instance at all since there's nothing actually being stored there ever. Just have AsyncContextFrame::current() return an empty kj::Maybe in that case. Also fixes a couple of minor bugs.
It turns out that v8 has a (pretty much undocumented) API for setting continuation context for Promises: `v8::Context::SetContinuationPreservedEmbedderData(...)` Using is allows us to simplify (but not quite eliminate yet) the promise hook and eliminate the use of the frame stack in favor of the simpler kj::Maybe approach Kenton suggested previously.
Removing obsolete bits, updating docs/comments
Capture the current AsyncContextFrame when awaitIoImpl is called and ensure that the promise continuation enters that context when resolving the JS promise.
By default, capture the current AsyncContextFrame when the
transform() method is called such that all of the handler
functions will enter that context. This approach gives us
the most flexibility. If a user really prefers to capture
the context when `on()` or `onDocument()` is called, the
Element and Document handlers can be easily wrapped.
// Default:
```js
class ElementHandler {
element(element) {
if (als.getStore() !== 321) throw new Error("Incorrect context");
}
text(text) {
if (als.getStore() !== 321) throw new Error("Incorrect context");
}
}
const res = new Response(new ReadableStream({
start(c) {
const enc = new TextEncoder();
c.enqueue(enc.encode('<div>text</div>'));
c.close();
}
}));
// In this variation, the HTMLWriter captures the async context at the
// moment transform is called.
const rewriter = new HTMLRewriter();
als.run(123, () => rewriter.on('div', new ElementHandler()));
return als.run(321, () => rewriter.transform(res));
```
// Override:
```js
class ElementHandler extends AsyncResource {
constructor() { super('') }
element(element) {
this.runInAsyncScope(() => {
if (als.getStore() !== 123) throw new Error("Incorrect context");
});
}
text(text) {
this.runInAsyncScope(() => {
if (als.getStore() !== 123) throw new Error("Incorrect context");
});
}
}
const res = new Response(new ReadableStream({
start(c) {
const enc = new TextEncoder();
c.enqueue(enc.encode('<div>text</div>'));
c.close();
}
}));
// In this variation, the ElementHandler captures the async context.
const rewriter = new HTMLRewriter();
als.run(123, () => rewriter.on('div', new ElementHandler()));
return als.run(321, () => rewriter.transform(res));
```
Alternative override:
```js
class ElementHandler {
element = AsyncResource.bind(() => {
if (als.getStore() !== 123) throw new Error("Incorrect context");
});
text = AsyncResource.bind(() => {
if (als.getStore() !== 123) throw new Error("Incorrect context");
});
}
const res = new Response(new ReadableStream({
start(c) {
const enc = new TextEncoder();
c.enqueue(enc.encode('<div>text</div>'));
c.close();
}
}));
// In this variation, the ElementHandler captures the async context.
const rewriter = new HTMLRewriter();
als.run(123, () => rewriter.on('div', new ElementHandler()));
return als.run(321, () => rewriter.transform(res));
```
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There are two key changes here:
Also fixes a couple of minor bugs.