[tor-bugs] #15642 [- Select a component]: Disable default fallback directories when DirAuthorities, AlternateDirAuthority, or FallbackDir are set

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#15642: Disable default fallback directories when DirAuthorities,
AlternateDirAuthority, or FallbackDir are set
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     Reporter:  teor                  |      Owner:
         Type:  defect                |     Status:  new
     Priority:  minor                 |  Milestone:  Tor: 0.2.7.x-final
    Component:  - Select a component  |    Version:  Tor: 0.2.4.7-alpha
   Resolution:                        |   Keywords:  tor-client tor-relay
Actual Points:                        |  Parent ID:
       Points:                        |
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Old description:

> Currently, when we set `DirAuthorities` or `AlternateDirAuthority`, we
> don't add the default directory authorities. But, (as long as
> `FallbackDir` isn't set) we do add the default fallback directories.
> However, the list of default fallback directories is currently `NULL`, so
> this issue has no effect.
>
> But I can't imagine too many scenarios where it would be useful to set an
> `AlternateDirAuthority` or `DirAuthorities`, and still get the default
> `FallbackDir`.
>
> I can imagine this causing similar issues to #13163, where the default
> authorities were added to a custom set of authorities in some
> circumstances.
>
> I'll create a patch to fix this, but it won't actually change tor's
> observable behaviour until we add directories to the default fallback
> directory list.
>
> Bugfix on 90f6071d8dc0 in 0.2.4.7-alpha.

New description:

 Currently, when we set `DirAuthorities` or `AlternateDirAuthority`, we
 don't add the default directory authorities. But, (as long as
 `FallbackDir` isn't set) we do add the default fallback directories.

 It doesn't make sense to me to add the default fallback directories when
 we have custom `DirAuthorities` or `AlternateDirAuthority`. I think we
 should only add the default fallback directories when other directories
 are also set to their defaults.

 However, the list of default fallback directories is currently `NULL`, so
 this issue currently has no effect.

 I also can't imagine any scenarios where it would be useful to set an
 `AlternateDirAuthority` or `DirAuthorities`, and still get the default
 `FallbackDir`.

 I can imagine this causing similar issues to #13163, where the default
 authorities were added to a custom set of authorities in some
 circumstances.

 I'll create a patch to fix this, but it won't actually change tor's
 observable behaviour until we add directories to the default fallback
 directory list.

 Bugfix on 90f6071d8dc0 in 0.2.4.7-alpha.

--

Comment (by teor):

 Edit: make explanation clearer (I hope)

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/15642#comment:1>
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