[tor-bugs] #13790 [Tor]: Refactor and add comments to new_route_len()

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#13790: Refactor and add comments to new_route_len()
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 Reporter:  dgoulet      |          Owner:
     Type:  enhancement  |         Status:  new
 Priority:  normal       |      Milestone:
Component:  Tor          |        Version:
 Keywords:               |  Actual Points:
Parent ID:               |         Points:
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 (function in src/or/circuitbuild.c)

 At the moment, this function is kind of scary and it should be refactored
 and heavily documented to avoid future confusion. (This ticket exists
 because two different developers at different time span over a week
 expressed concerns about the behavior of HS and this function).

 Bit of info on {{{new_route_len()}}}, this function is called to possibly
 extend a circuit with an extra hop that matches specific purpose(s). Right
 now it's testing the given purpose if it is *not* ESTABLISH_INTRO and
 TESTING, it will extend it to 4 hops (with exit information).

 However, there are other purposes that do NOT need 4 hops. Fortunately, it
 seems that there are no code paths that end up calling this function with
 exit info and a purpose that should not be extended (investigated by
 special, sysrqb, asn and me). But, we all agree that this is very fragile
 thus the purpose condition in this function should be applied on only
 purposes that need 4 hops.

 Furthermore, add more comments to make sure no more confusion happens with
 that fairly important function.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/13790>
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