race conditions in service startup
There have always been a variety of possibilities for race conditions in service startup because of the nature of how multiple different things can call the functions that do the startup. That's a larger architectural issue which we're discussing options for properly addressing in the future.
The more immediate issue is after removing the "exit if booting" check from rc.newwanip(v6) in 2.3, which fixed a variety of edge case bugs with interfaces that are slow to come online during boot, some systems end up running certain things twice at almost exactly the same time. For instance, #6160, and probably #6132.
Adding locks in vpn_ipsec_configure was fine for strongswan in #6160. Might be fine in other areas, though adding locking like that can be risky in potentially breaking things that are fine now, if some of those functions end up recursing.