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merge revision(s) 64062: [Backport #14939]

    cont.c (ec_switch): prevent delayed/missed trap interrupt race

    timer-thread may set trap interrupt with rb_threadptr_check_signal
    at any time independent of GVL.  This means timer-thread may set
    the trap interrupt flag on the previous execution context; causing
    the flag to be unnoticed until a future ec switch (or lost
    completely if the ec is done).

    Note: I avoid relying on th->interrupt_lock here and use
    atomics because we won't be able to rely on it for proposed lazy
    timer-thread [Misc #14937].

    This regression affects Ruby 2.5 as it was introduced by moving
    interrupt_flag to `ec' which is an unstable pointer.  Ruby <= 2.4
    was unaffected because vm->main_thread->interrupt_flag never
    changed.

    [ruby-core:88119] [Bug #14939]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@64999 b2dd03c8-39d4-4d8f-98ff-823fe69b080e