[tor-bugs] #27244 [Core Tor/Tor]: Use mmap to hold our cached consensus, when we even need it stored in RAM at all.

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#27244: Use mmap to hold our cached consensus, when we even need it stored in RAM
at all.
-------------------------------------------------+-------------------------
 Reporter:  nickm                                |          Owner:  (none)
     Type:  defect                               |         Status:  new
 Priority:  Medium                               |      Milestone:  Tor:
                                                 |  0.3.5.x-final
Component:  Core Tor/Tor                         |        Version:
 Severity:  Normal                               |     Resolution:
 Keywords:  035-roadmap-master, 035-triaged-     |  Actual Points:
  in-20180711                                    |
Parent ID:  #27243                               |         Points:
 Reviewer:                                       |        Sponsor:
                                                 |  Sponsor8
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Comment (by nickm):

 Turns out not to be redundant with #27247.  Clients no longer need to keep
 a cached_dir_t, and that ticket stops them -- but when they load it, they
 still load it with read_file_to_string().

 There's a slight difficulty in moving to mmap, however.  When we mmap() a
 file, it is not a NUL-terminated string, but rather a non-terminated pile
 of bytes with an associated size_t.

 There are three cases when a client needs a consensus text:
   1. It's parsing it from disk
   2. It's applying a consensus diff.
   3. It's writing it to the controller

 Case 3 is easy enough, since the client doesn't need to examine the bytes
 at all.

 But for case 1 and case 2, our networkstatus parsing code and our
 consensus diff code both assume that the mapped object is NUL-terminated.

 So before we can do this, we'll need to carefully revise a bunch of
 functions.

--
Ticket URL: <https://trac.torproject.org/projects/tor/ticket/27244#comment:3>
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