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Parent -> child 1, cdrom1 still reading disc1
       -> child 2, cdrom2 *finished* reading disc2 -> exit0
       -> child 3, cdrom3 still reading disc3

Parent -> child 1, cdrom1 still reading disc1
       -> child 2 (done)
       -> child 3, cdrom3 still reading disc3
       -> child 4, cdrom2 read disc4