AccessPolicyProcessor::processAccessPolicies() creates unnecessary Fibers when checking permissions for the current user

Problem/Motivation

The fix in #3576074: Current user is changed unexpectedly wraps doProcessAccessPolicies() in a child Fiber whenever processAccessPolicies() is called from inside an existing Fiber. The purpose of that Fiber is to contain the accountSwitcher->switchTo() call, but that switch only happens when the account being checked differs from the current user. When checking permissions for the current user, no switch occurs, so the Fiber serves no purpose.

The current condition:

if (!\Fiber::getCurrent()) {
  return $this->doProcessAccessPolicies($account, $scope);
}

means every hasPermission() call fired from within a Fiber (e.g. during placeholder/lazy-builder rendering) creates a child Fiber, even for current-user checks. On a typical page render the vast majority of permission checks are for the current user, so this adds up fast.

Adding a second short-circuit fixes it:

if (!\Fiber::getCurrent() || $account->id() === $this->currentUser->id()) {
  return $this->doProcessAccessPolicies($account, $scope);
}

Measured with xhprof on two separate Drupal 11 projects: on a typical page around 1,000 calls to processAccessPolicies() run per request, the vast majority for the current user. The fix eliminated those unnecessary Fibers, reducing Fiber::start count by ~90% per page and saving ~15–25ms per cold render. Not a dramatic number, but the Fibers don't seem justified for this code path.

Steps to reproduce

Proposed resolution

Update AccessPolicyProcessor::processAccessPolicies with the above condition.

Remaining tasks

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User interface changes

None

Introduced terminology

None

API changes

None

Data model changes

None

Release notes snippet

None