Await a promise SHARED between callers while letting each caller give up on
its own signal.
A single-flight fetch (one in-flight promise many callers join) must not run
under any one caller's signal: a superseded first caller would abort the work
every joiner is waiting on. The shared work runs under its owner's lifetime
signal instead, and each caller waits through this helper, which rejects with
the caller's abort reason the moment that caller's signal fires while leaving
the shared promise untouched.
Rejects synchronously-ish (on the next microtask) for an already-aborted
signal, and detaches its listener once the shared promise settles, so a
long-lived signal does not accumulate listeners across waits.
Await a promise SHARED between callers while letting each caller give up on its own signal.
A single-flight fetch (one in-flight promise many callers join) must not run under any one caller's signal: a superseded first caller would abort the work every joiner is waiting on. The shared work runs under its owner's lifetime signal instead, and each caller waits through this helper, which rejects with the caller's abort reason the moment that caller's signal fires while leaving the shared promise untouched.
Rejects synchronously-ish (on the next microtask) for an already-aborted signal, and detaches its listener once the shared promise settles, so a long-lived signal does not accumulate listeners across waits.