RFC 86: fetch_tests_from_prefixed_local_storage

rfcs/fetch_tests_from_prefixed_local_storage.md

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+# RFC 86: Add `fetch_tests_from_prefixed_local_storage` method
+
+## Summary
+
+Add `fetch_tests_from_prefixed_local_storage` method that fetches test results from another page via local storage and `/common/PrefixedLocalStorage.js`.
+This is similar to `fetch_tests_from_window`, but is designed for cases where direct communication channels including `postMessage` are not available, like a `Window` opened by `window.open(url, "noopener")`.
+
+This is primarily for the use in Back-foward cache WPTs (see [wpt/pull/28950](https://github.com/web-platform-tests/wpt/pull/28950) for actual `testharness.js` changes and use cases), but can be also used for other tests involving isolated Windows or top-level navigations.
+
+## Details
+
+Testing things requiring isolated windows is not well supported by the WPT frameworks.
+For example, when `A.html` calls `window.open('B.html', 'noopener')` and tests something on `B.html`, `B.html` sends data to `A.html` and `A.html` calls `testharness.js` APIs like `assert_equals()` on the data, like:
+
+A.html:
+```
+async_test(t => {
+  window.open(prefixedLocalStorage.url('B.html'), '_blank', 'noopener');
+  wait_for_result();
+  assert_equals(localStorage.getItem('result'), 'expected');
+});
+```
+
+B.html:
+```
+localStorage.setItem('result', doSomethingAndGetResult());
+```
+
+The problems here are:
+
+- Test logic often spreads across multiple HTMLs, which can make the logic and its failure mode unclear.
+- we need non-`postMessage` communication mechanisms (which tend to be one-off; see below for existing examples).
+
+Instead, this RFC proposes 
+
+- Writing tests using `testharness.js` APIs on isolated Windows (`B.html` in the example above), and
+- Communicating test results within a new API `fetch_tests_from_prefixed_local_storage` in `testharness.js` from the isolated Windows to the main test Window (`A.html` in the example).
+
+A.html:
+```
+const prefixedLocalStorage = new PrefixedLocalStorageTest();
+fetch_tests_from_prefixed_local_storage(prefixedLocalStorage);
+window.open(prefixedLocalStorage.url('B.html'), '_blank', 'noopener');
+```
+
+B.html:
+```
+async_test(t => {
+  assert_equals(doSomethingAndGetResult(), 'expected');
+});
+```
+
+By doing so,
+
+- More test logic can be centralized to `B.html`, especially when the most of the test logic can be done in `B.html` and `window.open() + noopener` is needed just to create a separate browsing context (this is the case for most of the BFCache tests).
+- Communication implementation is encapsulated within `testharness.js`, instead of within individual tests.
+
+## Risks
+
+We might want to change the implementations within `fetch_tests_from_prefixed_local_storage`, when
+
+* More use cases are to be supported (like cross-origin Windows)
+* `localStorage` spec or implementation is changed
+
+This kind of risks is somehow inherent regardless of communication methods we use, because we anyway want communications between isolated Windows, which might be blocked by security/privacy efforts.
+
+Changes needed will be anyway small, because I expect
+
+* We only have to change the implementation within `fetch_tests_from_prefixed_local_storage` and boilerplate-like code on its caller side, (hopefully) not the test contents, given that the APIs supported in the test contents are stable `testharness.js` APIs.
+* The number of tests using `fetch_tests_from_prefixed_local_storage` will be limited.
+
+## Existing communication helpers
+
+Existing tests (WPT or Chromium, mainly those with `window.open()` + `noopener`) uses several communication mechanisms/helpers for this purpose.
+
+- localStorage
+    - [/wpt/common/PrefixedLocalStorage.js](https://github.com/web-platform-tests/wpt/blob/master/common/PrefixedLocalStorage.js)
+        - e.g. used by [/wpt/html/browsers/windows/auxiliary-browsing-contexts/opener-noopener.html](https://github.com/web-platform-tests/wpt/blob/master/html/browsers/windows/auxiliary-browsing-contexts/opener-noopener.html)
+- sessionStorage
+    - [http/tests/navigation/resources/back-to-get-after-post-helper.html](https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/web_tests/http/tests/navigation/resources/back-to-get-after-post-helper.html)
+- Fetch API + server-side stash
+    - [/wpt/scroll-to-text-fragment/stash.js](https://github.com/web-platform-tests/wpt/blob/master/scroll-to-text-fragment/stash.js)
+    - [/wpt/html/browsers/windows/resources/window-name-stash.py](https://github.com/web-platform-tests/wpt/blob/master/html/browsers/windows/resources/window-name-stash.py)
+    - [/wpt_internal/prerender/resources/key-value-store.py](third_party/blink/web_tests/wpt_internal/prerender/resources/key-value-store.py)
+
+## Alternatives Considered
+
+### Fetch API + server-side stash instead of `localStorage`
+
+- pros: More robust than `localStorage`: Fetch API will be less affected by efforts to block communications between windows, and works even in cross-origin cases.
+- cons: Fetch API is async, and pages with open fetch requests are not BFCache-eligible in Chromium. Therefore, a little more coordination is needed to ensure the fetch requests for accessing stash are closed before navigation.
+
+### BroadcastChannel instead of `localStorage`
+
+- cons: Pages with open BroadcastChannels is not BFCache-eligible in Chromium.
+- cons: BroadcastChannel is not supported in Safari.
+
+### Introduce a new class of tests that drive the browser from the outside
+
+Instead of writing tests and passing data within JavaScript, introduce a new class of tests and its infrastructure controling the browser "from the outside", much more like WebDriver tests.
+
+- cons: Design and implementation costs are much higher.
+- It would be preferable to create a design that covers a broader use cases like navigation, BFCache, prerendering, etc., but right now I don't have clear ideas (both on design/implementation and use cases).