feat(function): resolve page methods when the snippet asks for them - #950
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`extendPage` can only carry values decided before the snippet starts, so
anything it might read had to be produced up front, on every run, whether
or not it reached that method.
`hostPage` attaches methods the host resolves at the moment they are
called:
createFunction(({ page }) => page.content(), {
hostPage: { content: () => fetchThePage(url) }
})
Nothing is serialized into the isolate. The call travels over the
`isolated-function` channel when the snippet makes it, so a snippet that
returns without touching `page` resolves nothing, and neither does a
branch it does not take:
async ({ page }) => (false ? await page.content() : 'skipped')
That last case is the one static inspection cannot answer, because
whether the call happens is only known at runtime.
A `hostPage` key satisfies its method exactly as an `extendPage` key
does, so `needsBrowser` sees both and a snippet reading only these still
skips Chromium. Both kinds can sit on the same page; host methods are
applied first so an eager value with the same name wins, matching the
existing precedence where extendPage shadows a real page method.
Requires isolated-function 0.2.10 for the channel.
8 tests, including the untaken branch, the coexistence of both kinds, and
that a host method never reaches `pageValues`.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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Included review availability: This review used your included allowance. Your plan provides up to 4 included reviews per hour; 3 remain after this review. 📝 WalkthroughWalkthroughThe function APIs accept ChangesHost page methods
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~20 minutes Change: Feature Sequence Diagram(s)sequenceDiagram
participant UserFunction
participant GeneratedPageMethod
participant IsolatedHost
participant HostPage
UserFunction->>GeneratedPageMethod: call method with arguments
GeneratedPageMethod->>IsolatedHost: invoke __isolated_host with method and arguments
IsolatedHost->>HostPage: resolve requested method
HostPage-->>IsolatedHost: return method result
IsolatedHost-->>UserFunction: return method result
Merge Risk: ⚪ Minimal · up to The change adds an opt-in hostPage option that runs host-provided page methods only when a snippet calls them. No actionable merge-blocking risk is evident from the supplied review context. Security Architecture ReviewSecurity architecture risk: 🟡 Moderate · up to Callers can now give isolated code access to host-side methods. That access is limited to methods the caller supplies, but side-effecting methods need particular care when a call times out or is retried. Retained concerns
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🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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- 🪄 Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @packages/function/README.md:
- Around line 159-162: Update both hostPage examples using createFunction in the
README to retain and invoke the returned callable, awaiting its result so the
examples demonstrate their documented outcomes and whether fetchThePage runs.
- Line 165: Update both README statements that say nothing is serialized for
host methods: clarify that the methods remain on the host, while arguments and
results cross the channel and must use values supported by its serializer. Keep
the existing contrast with extendPage accurate.
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…annel Both examples built a function and never called it, so neither produced the result its comment claimed. They now await the call, and the output was checked by running them: the first resolves the html with one host call, the second returns "skipped" with none. "Nothing is serialized into the isolate" was wrong about the part that matters. The method stays on the host, but its arguments and its result do cross the channel and have to be values the channel can carry, which is why a BigInt rejects the call rather than resolving it. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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- 🪄 Fix CodeRabbit comments on this PR
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only still-valid issues, skip the rest with a brief reason, keep changes
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Inline comments:
Review comments at @packages/function/README.md:
- Line 172: Update the method-precedence explanation in the README to state that
when hostPage and extendPage define the same method name, the extendPage value
overwrites the hostPage assignment. Keep the existing guidance about Chromium
startup intact.
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A non-function hostPage value counted as a stub and then failed inside the snippet. Reject it at setup, and leave a name that extendPage also defines off the channel so only the eager value is reachable. Co-authored-by: Cursor <cursoragent@cursor.com>
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Inline comments:
Review comments at @packages/function/src/template.js:
- Line 239: Update the host method assignment guarded by `covered.has(name)` so
an own `__proto__` method from `hostPage` cannot mutate the host object’s
prototype and disappear from `exposedHost`; reject that method explicitly or
store it in a way that preserves it as an own property.
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Assigning that name sets the host object's prototype instead of storing a method, so the key disappears. Reject it before the assignment. Co-authored-by: Cursor <cursoragent@cursor.com>
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Why
extendPagecan only carry values decided before the snippet starts, so anything it might read had to be produced up front, on every run, whether or not it reached that method.hostPageattaches methods the host resolves at the moment they are called:fetchThePageruns because the snippet awaitedcontent.What it buys
Measured against a host that counts its calls, with
getBrowserlessthrowing so a started browser would fail the test:() => 420[]async ({ page }) => (await page.content()).length["content"]async ({ page }) => (await page.metadata()).title["metadata"]async ({ page }) => (false ? await page.content() : "skipped")[]The last row is the point. Whether that call happens is only knowable at runtime, so static inspection of the source has to assume it does and resolve up front. The channel resolves nothing.
How it fits the existing page
Nothing is serialized into the isolate.
extendPageJSON values still travel aspageValuesand becomeasync () => value; ahostPagemethod becomes a call over theisolated-functionchannel instead:A
hostPagekey satisfies its method exactly as anextendPagekey does, soneedsBrowsersees both and a snippet reading only these still skips Chromium. Both kinds can sit on the same page; host methods are applied first, so an eager value with the same name wins — matching the existing precedence whereextendPageshadows a real page method.Additive throughout: without
hostPage, every generated program is byte-identical to before and no channel is opened.Dependency
isolated-function~0.2.8to~0.2.10, which is where the channel lands (#83). Worth knowing the symptom if anyone runs an older one locally: the lazy calls fail with an empty error object rather than anything descriptive, because the option is simply ignored.Tests
8 new, 96 passing in the package,
standardclean. Beyond the table above: ahostPagekey satisfiesneedsBrowserwhile an unprovided method still requires a browser, a host method never reachespageValues, and eager and host-backed methods coexist on one page.Scope
Only
packages/function. No overlap with #914 (packages/capture) or #885 (packages/goto).🤖 Generated with Claude Code
Note
Medium Risk
Exposes a new host RPC surface to sandboxed snippets (untrusted args, call limits); behavior is additive when
hostPageis omitted.Overview
Adds
hostPage, acreateFunctionoption for lazypagemethods the host runs only when untrusted snippet code actually calls them, via theisolated-functionhost channel (globalThis.__isolated_host) instead of pre-serializing likeextendPage.needsBrowsertreatshostPagekeys likeextendPagestubs so snippets that only use those methods can still skip Chromium.extendPagewins on name clashes—shadowed host methods are not exposed on the channel. Validation rejects non-function values and__proto__keys.Wires
hostPagethrough template generation, the VMhostoption, and browser/no-browser run paths; bumpsisolated-functionto~0.2.10. README andhost-page.jstests cover lazy resolution, branching, caching per method/args, and coexistence withextendPage.Reviewed by Cursor Bugbot for commit 9453ef7. Bugbot is set up for automated code reviews on this repo. Configure here.
Summary by CodeRabbit
BigIntare rejected.