Bug 1987362 [wpt PR 54725] - WPT: Snapshot internal observers before Subscriber iterates over them, a=testonly

Automatic update from web-platform-tests
WPT: Snapshot internal observers before Subscriber iterates over them

This behavior was already implemented in
https://crrev.com/c/6311209, which was a follow-up bug fix to
https://crrev.com/c/6221901. It was only recently spec'ed in
WICG/observable#214, and this CL adds a WPT for
it.

This behavior was obliquely tested by:
https://source.chromium.org/chromium/chromium/src/+/main:third_party/blink/web_tests/external/wpt/dom/observable/tentative/observable-from.any.js;l=675-740;drc=5f8643cf150e97b694f2214ee092c34cc0ccf5fe,
as discovered in
keithamus/observable-polyfill#38, however this
CL adds a simpler, more direct test for this behavior.

R=jarhar

Bug: 40282760
Change-Id: I43748a09a2f5a235c25ffdbd807e630c187ef2b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6915657
Reviewed-by: Joey Arhar <jarhar@chromium.org>
Commit-Queue: Dominic Farolino <dom@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1511774}

--

wpt-commits: 826a9b6061c2541d55b9f04d45b8deb102283b4c
wpt-pr: 54725

Author: domfarolino

testing/web-platform/tests/dom/observable/tentative/observable-constructor.any.js

@@ -1199,3 +1199,33 @@ test(() => {
     'after final abort'
   ]);
 }, "Teardown runs after last unsubscribe regardless of unsubscription order");
+
+test(() => {
+  const results = [];
+  const source = new Observable(subscriber => {
+    subscriber.next(1);
+    subscriber.next(2);
+    subscriber.complete();
+  });
+
+  source.subscribe(v => {
+    results.push(`${v}-first-sub`);
+    if (v === 1) {
+      // This new subscription adds a new internal observer to the subscriber's
+      // internal observer list, but it does not get the value `1` that the
+      // subscriber is currently pushing. This is because the Subscriber
+      // iterates over a snapshot of its internal observers to push values to.
+      // The first value that this new subscription will see is `2`.
+      //
+      // See https://github.com/WICG/observable/pull/214.
+      source.subscribe(v => results.push(`${v}-second-sub`));
+    }
+  });
+
+  assert_array_equals(results, [
+    "1-first-sub",
+
+    "2-first-sub",
+    "2-second-sub",
+  ]);
+}, "Subscriber iterates over a snapshot of its internal observers");