[tor-bugs] #13802 [Core Tor/Tor]: Add general trace-event logging instrumentation to Tor

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#13802: Add general trace-event logging instrumentation to Tor
-------------------------------------------------+-------------------------
 Reporter:  dgoulet                              |          Owner:  pastly
     Type:  enhancement                          |         Status:
                                                 |  merge_ready
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  0.3.0.x-final
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  tor-03-unspecified-201612, review-   |  Actual Points:
  group-15                                       |
Parent ID:  #13792                               |         Points:  3
 Reviewer:  dgoulet                              |        Sponsor:
                                                 |  SponsorR-can
-------------------------------------------------+-------------------------
Changes (by dgoulet):

 * status:  needs_review => merge_ready


Comment:

 Ok so I've cleaned up a bit the branch that is squashed all the commit
 into one and rebased it on master. I've also improved the comments and
 minor fixes.

 I think this is ready for merge, it is working fine. Note that this is
 just the "tracing infrastructure". It is not used in the code base
 anywhere and you need to specifically enable it at compile time for the
 "tracepoint to log_debug()" framework (`--enable-tracing-debug`).

 The point of having this skeleton is that once it's in, we can start
 adding tracepoint to subsystems that we want to trace using a specific
 framework (here we only have the log_debug() framework and even there it's
 very basic). For instance, KIST work at NRL are using this infrastructure
 to route their tracepoint to log_debug() but with some extra code around
 it. See doc/HACKING/Tracing.md for more information.

 @nickm, if you prefer having it in use in the codebase before merging, I'm
 fine with it that means we differ this until we actually have tracepoint
 upstream. I think our friend at NRL can survive without it upstream yet I
 think because most of their stuff is a "new framework" plugged in this
 infrastructure.

 Branch `ticket13802_030_01`.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/13802#comment:42>
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