The following test is throwing an AssertionError:

let controller;
const stream = new ReadableStream({ start(c) { controller = c; } });
const [branch1, branch2] = stream.tee();

controller.error("error");

branch1.cancel().catch(_=>_);
branch2.cancel().catch(_=>_);

This can be seen in our CI logs, e.g. the latest run on master:

AssertionError: false == true
    at resolvePromise (eval at setup (/home/runner/work/streams/streams/reference-implementation/run-web-platform-tests.js:56:14), <anonymous>:6894:3)
    at eval (eval at setup (/home/runner/work/streams/streams/reference-implementation/run-web-platform-tests.js:56:14), <anonymous>:4831:5)
    at processTicksAndRejections (internal/process/task_queues.js:93:5)
  √ ReadableStream teeing: erroring a teed stream should error both branches

which refers to this line in the reference implementation:

assert(stateIsPending(p) === true);

This indicates that we're attempting to resolve an already resolved promise. Although implementers are required to ignore such attempts, we try to avoid doing this in the specification, which is why we added this check in #1102. Unfortunately, it looks like we missed a case where this assertion fails...

This PR fixes it by checking if both branches have already been canceled in "upon rejection of reader.[[closedPromise]]". If that's the case, then we will have already resolved cancelPromise and we don't need to resolve it again.

• At least two implementers are interested (and none opposed):
  • …
  • …
• Tests are written and can be reviewed and commented upon at:
  • Streams: add missing asserts in tee test web-platform-tests/wpt#28153
• Implementation bugs are filed:
  • Chrome: N/A (already passes updated tests)
  • Firefox: N/A (already passes updated tests)
  • Safari: https://bugs.webkit.org/show_bug.cgi?id=223558

(See WHATWG Working Mode: Changes for more details.)

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