Kikobeats / isolated-function / 36459718807

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feat: let a snippet call back into the host (#83)

* feat: let a snippet call back into the host

A snippet could only ever be handed values decided before it started. Work
it might need had to be done up front, for every run, whether or not the
snippet reached it.

`host` exposes methods it can call as `globalThis.__isolated_host(name, args)`,
resolved when asked for and never before:

  isolatedFunction(async () => (await __isolated_host('content')).length, {
    host: { content: () => expensiveFetch(url) }
  })

`expensiveFetch` runs because the snippet awaited it. A snippet returning
`42` without asking never triggers it.

The channel is an `ipc` stdio slot, opened only when `host` is passed, so a
run without it spawns exactly as before and has no `process.send` at all.
Verified the slot survives the three things it had to: `node -` reading the
program from stdin, `--permission` with no `--allow-*` flags (the permission
model does not gate IPC), and the `sh -c 'ulimit -t N && exec node'` wrapper.
The channel is closed before the result is written, or the child would never
exit and would be killed at its timeout.

The channel cannot be hidden from snippet code. Deleting `process.send` is
not a boundary: `process.channel.fd` stays readable and `fs.writeSync(3, …)`
reaches the host even under `--permission`, which I confirmed before
designing around it. So the host treats every message as untrusted input. A
method missing from `host`, or inherited rather than own, is refused without
being reached; a malformed message is ignored; and `maxHostCalls` bounds
distinct calls per run so a snippet cannot amplify cost. Repeats of a
resolved call are free, keyed on method and arguments.

`hostMethods` joins the shell cache key. Without it a build cached for a run
with no channel would be reused for one that needs it.

14 tests, including the two that matter for the boundary: a snippet writing
straight to fd 3 cannot invoke an unexposed method, and forged or ma... (continued)

305 of 327 branches covered (93.27%)

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198 of 198 new or added lines in 6 files covered. (100.0%)

1243 of 1272 relevant lines covered (97.72%)

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