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+# `RemoteContext`: API for script execution in another context
+
+`RemoteContext` in `/common/dispatcher/dispatcher.js` provides an interface to
+execute JavaScript in another global object (page or worker, the "executor"),
+based on:
+
+- [WPT RFC 88: context IDs from uuid searchParams in URL](https://github.com/web-platform-tests/rfcs/pull/88),
+- [WPT RFC 89: execute_script](https://github.com/web-platform-tests/rfcs/pull/89) and
+- [WPT RFC 91: RemoteContext](https://github.com/web-platform-tests/rfcs/pull/91).
+
+Tests can send arbitrary javascript to executors to evaluate in its global
+object, like:
+
+```
+// injector.html
+const argOnLocalContext = ...;
+
+async function execute() {
+  window.open('executor.html?uuid=' + uuid);
+  const ctx = new RemoteContext(uuid);
+  await ctx.execute_script(
+      (arg) => functionOnRemoteContext(arg),
+      [argOnLocalContext]);
+};
+```
+
+and on executor:
+
+```
+// executor.html
+function functionOnRemoteContext(arg) { ... }
+
+const uuid = new URLSearchParams(window.location.search).get('uuid');
+const executor = new Executor(uuid);
+```
+
+For concrete examples, see
+[events.html](../../html/browsers/browsing-the-web/back-forward-cache/events.html)
+and
+[executor.html](../../html/browsers/browsing-the-web/back-forward-cache/resources/executor.html)
+in back-forward cache tests.
+
+Note that `executor*` files under `/common/dispatcher/` are NOT for
+`RemoteContext.execute_script()`. Use `remote-executor.html` instead.
+
+This is universal and avoids introducing many specific `XXX-helper.html`
+resources.
+Moreover, tests are easier to read, because the whole logic of the test can be
+defined in a single file.
+
+## `new RemoteContext(uuid)`
+
+- `uuid` is a UUID string that identifies the remote context and should match
+  with the `uuid` parameter of the URL of the remote context.
+- Callers should create the remote context outside this constructor (e.g.
+  `window.open('executor.html?uuid=' + uuid)`).
+
+## `RemoteContext.execute_script(fn, args)`
+
+- `fn` is a JavaScript function to execute on the remote context, which is
+  converted to a string using `toString()` and sent to the remote context.
+- `args` is null or an array of arguments to pass to the function on the
+  remote context. Arguments are passed as JSON.
+- If the return value of `fn` when executed in the remote context is a promise,
+  the promise returned by `execute_script` resolves to the resolved value of
+  that promise. Otherwise the `execute_script` promise resolves to the return
+  value of `fn`.
+
+Note that `fn` is evaluated on the remote context (`executor.html` in the
+example above), while `args` are evaluated on the caller context
+(`injector.html`) and then passed to the remote context.
+
+## Return value of injected functions and `execute_script()`
+
+If the return value of the injected function when executed in the remote
+context is a promise, the promise returned by `execute_script` resolves to the
+resolved value of that promise. Otherwise the `execute_script` promise resolves
+to the return value of the function.
+
+When the return value of an injected script is a Promise, it should be resolved
+before any navigation starts on the remote context. For example, it shouldn't
+be resolved after navigating out and navigating back to the page again.
+It's fine to create a Promise to be resolved after navigations, if it's not the
+return value of the injected function.
+
+## Calling timing of `execute_script()`
+
+When `RemoteContext.execute_script()` is called when the remote context is not
+active (for example before it is created, before navigation to the page, or
+during the page is in back-forward cache), the injected script is evaluated
+after the remote context becomes active.
+
+Multiple calls to `RemoteContext.execute_script()` will result in multiple scripts
+being executed in remote context and ordering will be maintained.
+
+## Errors from `execute_script()`
+
+Errors from `execute_script()` will result in promise rejections, so it is
+important to await the result.  This can be `await ctx.execute_script(...)` for
+every call but if there are multiple scripts to executed, it may be preferable
+to wait on them in parallel to avoid incurring full round-trip time for each,
+e.g.
+
+```js
+await Promise.all(
+  ctx1.execute_script(...),
+  ctx1.execute_script(...),
+  ctx2.execute_script(...),
+  ctx2.execute_script(...),
+  ...
+)
+```
+
+## Evaluation timing of injected functions
+
+The script injected by `RemoteContext.execute_script()` can be evaluated any
+time during the remote context is active.
+For example, even before DOMContentLoaded events or even during navigation.
+It's the responsibility of test-specific code/helpers to ensure evaluation
+timing constraints (which can be also test-specific), if any needed.
+
+### Ensuring evaluation timing around page load
+
+For example, to ensure that injected functions (`mainFunction` below) are
+evaluated after the first `pageshow` event, we can use pure JavaScript code
+like below:
+
+```
+// executor.html
+window.pageShowPromise = new Promise(resolve =>
+    window.addEventListener('pageshow', resolve, {once: true}));
+
+
+// injector.html
+const waitForPageShow = async () => {
+  while (!window.pageShowPromise) {
+    await new Promise(resolve => setTimeout(resolve, 100));
+  }
+  await window.pageShowPromise;
+};
+
+await ctx.execute(waitForPageShow);
+await ctx.execute(mainFunction);
+```
+
+### Ensuring evaluation timing around navigation out/unloading
+
+It can be important to ensure there are no injected functions nor code behind
+`RemoteContext` (such as Fetch APIs accessing server-side stash) running after
+navigation is initiated, for example in the case of back-forward cache testing.
+
+To ensure this,
+
+- Do not call the next `RemoteContext.execute()` for the remote context after
+  triggering the navigation, until we are sure that the remote context is not
+  active (e.g. after we confirm that the new page is loaded).
+- Call `Executor.suspend(callback)` synchronously within the injected script.
+  This suspends executor-related code, and calls `callback` when it is ready
+  to start navigation.
+
+The code on the injector side would be like:
+
+```
+// injector.html
+await ctx.execute_script(() => {
+  executor.suspend(() => {
+    location.href = 'new-url.html';
+  });
+});
+```
+
+## Future Work: Possible integration with `test_driver`
+
+Currently `RemoteContext` is implemented by JavaScript and WPT-server-side
+stash, and not integrated with `test_driver` nor `testharness`.
+There is a proposal of `test_driver`-integrated version (see the RFCs listed
+above).
+
+The API semantics and guidelines in this document are designed to be applicable
+to both the current stash-based `RemoteContext` and `test_driver`-based
+version, and thus the tests using `RemoteContext` will be migrated with minimum
+modifications (mostly in `/common/dispatcher/dispatcher.js` and executors), for
+example in a
+[draft CL](https://chromium-review.googlesource.com/c/chromium/src/+/3082215/).
+
+
+# `send()`/`receive()` Message passing APIs
+
+`dispatcher.js` (and its server-side backend `dispatcher.py`) provides a
+universal queue-based message passing API.
+Each queue is identified by a UUID, and accessed via the following APIs:
+
+-   `send(uuid, message)` pushes a string `message` to the queue `uuid`.
+-   `receive(uuid)` pops the first item from the queue `uuid`.
+-   `showRequestHeaders(origin, uuid)` and
+    `cacheableShowRequestHeaders(origin, uuid)` return URLs, that push request
+    headers to the queue `uuid` upon fetching.
+
+It works cross-origin, and even access different browser context groups.
+
+Messages are queued, this means one doesn't need to wait for the receiver to
+listen, before sending the first message
+(but still need to wait for the resolution of the promise returned by `send()`
+to ensure the order between `send()`s).
+
+## Executors
+
+Similar to `RemoteContext.execute_script()`, `send()`/`receive()` can be used
+for sending arbitrary javascript to be evaluated in another page or worker.
+
+- `executor.html` (as a Document),
+- `executor-worker.js` (as a Web Worker), and
+- `executor-service-worker.js` (as a Service Worker)
+
+are examples of executors.
+Note that these executors are NOT compatible with
+`RemoteContext.execute_script()`.
+
+## Future Work
+
+`send()`, `receive()` and the executors below are kept for COEP/COOP tests.
+
+For remote script execution, new tests should use
+`RemoteContext.execute_script()` instead.
+
+For message passing,
+[WPT RFC 90](https://github.com/web-platform-tests/rfcs/pull/90) is still under
+discussion.