[wp-trac] [WordPress Trac] #65167: WP_Hook: Self-removing callback causes the next priority to be skipped

[WordPress Trac]

#65167: WP_Hook: Self-removing callback causes the next priority to be skipped
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 Reporter:  obenland      |      Owner:  (none)
     Type:  defect (bug)  |     Status:  new
 Priority:  normal        |  Milestone:  Awaiting Review
Component:  Plugins       |    Version:  4.7
 Severity:  normal        |   Keywords:
  Focuses:                |
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 When a callback running inside `apply_filters()` is the only one at its
 priority and it removes itself (or another callback empties that bucket),
 `WP_Hook` silently skips the callbacks at the next remaining priority, but
 only when at least one earlier-priority callback has already run.

 ==== Conditions to reproduce

 - A filter has callbacks at three priorities, e.g. 5, 10, 20.
 - The callback at priority 10 is the only one at that priority and calls
 `remove_filter()` on itself.
 - During `apply_filters()`, the callback at priority 20 is silently
 skipped.

 If priority 5 is removed from the example, all callbacks fire correctly.
 The bug only manifests when at least one earlier-priority callback has
 already run.

 ==== Root cause

 The bug predates `WP_Hook`. The pre-WP_Hook array-based `apply_filters()`
 had the same misbehavior: when a callback unset the priority bucket the
 internal array pointer was on, PHP silently auto-advanced the pointer to
 the next valid key, and the trailing `next()` then overshot it. `WP_Hook`
 (introduced in [38571] / 4.7.0) replaced PHP's implicit auto-advance with
 an explicit forward-walk in `resort_active_iterations()`, but preserved
 the same overshoot. The add-side of `resort_active_iterations()` already
 has symmetric handling for callbacks added at the current priority during
 iteration; the remove side is missing the equivalent fix.

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Ticket URL: <https://core.trac.wordpress.org/ticket/65167>
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