Hook execution terminates prematurely

(Observed when running CiviHR – a setup in which multiple modules/extensions define hooks)

1. Suppose we have two implementations of hook_civicrm_config (eg a_civicrm_config and b_civicrm_config)
2. Suppose that a_civicrm_config triggers (either directly or indirectly) hook_civicrm_optionValues (e.g. a_civicrm_optionValues)
3. Problem: a_civicrm_config and a_civicrm_optionValues execute (as expected) but b_civicrm_config never executes.

This is because CRM_Utils_Hook::runHooks is non-reentrant – it accepts a reference to the $civiModules array, so all concurrent invocations of runHooks use the same copy of $civiModues, but PHP only allows one implicit iterator for each array.