[tor-bugs] #29209 [Core Tor/Tor]: Reduce visibility of more data type internals

[Tor Bug Tracker & Wiki]

#29209: Reduce visibility of more data type internals
----------------------------------------+----------------------------------
 Reporter:  nickm                       |          Owner:  (none)
     Type:  task                        |         Status:  needs_review
 Priority:  Medium                      |      Milestone:  Tor:
                                        |  0.4.1.x-final
Component:  Core Tor/Tor                |        Version:
 Severity:  Normal                      |     Resolution:
 Keywords:  technical-debt refactoring  |  Actual Points:  3.5
Parent ID:                              |         Points:  15
 Reviewer:                              |        Sponsor:  Sponsor31-can
----------------------------------------+----------------------------------
Changes (by asn):

 * status:  new => needs_review
 * actualpoints:   => 3.5


Comment:

 OK, I finally have something to show here:
 https://github.com/torproject/tor/pull/929

 So basically this branch does the following:
 - Moves `crypt_path_t` related functions to its own file
 `src/core/or/crypt_path.c`. Previously they were all over the place, even
 tho they were cpath-specific.
 - Starts splitting `crypt_path_t` into a public struct and a private
 struct (using an opaque pointer).
 - Moves `crypt_path_t.crypto` to the private part of the struct by
 defining appropriate functions for doing circuit crypto.

 This took much longer than anticipated because I first experimented with
 encapsulating others parts of `crypt_path_t` before deciding to go with
 `crypto`. In particular, I first started hiding `deliver_window` and
 `package_window` but I quickly realized that this would cause lots of
 conflicts with #26288. I think after #26288 gets merged, these two fields
 might be the next candidates for hiding.

 Now in terms of general notes, this project took me more than 3 days of
 careful refactoring work, and I've only hided like 10% of the cpath
 structure. Hiding the whole structure is far far away since there are
 quite complicated structures involved like `extend_info_t`.

 In terms of future goals here, I think hiding `extend_info_t` into its own
 interface would be quite convenient since that structure is used and poked
 in weird ways all over the codebase. Furthermore, that would mean we can
 eventually encapsulate `crypt_path_t` even better. Other plausible medium-
 term targets would be structures like `tor_cert_t` which are mostly
 hidden, but not completely.

 I also added some TODO notes in the top of `crypt_path.c` with more cpath-
 specific things we can move without too much trouble.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/29209#comment:3>
Tor Bug Tracker & Wiki <https://trac.torproject.org/>
The Tor Project: anonymity online