Hello Fan and team,

I think I'm not a big fan of the pending_guard and pending_dir_guard
concept. To me it seems like a quick hack that tries to address fundamental
issues with our algorithm that appeared when we tried to adapt the proposal to
the tor codebase.

I think one of the main issues with the current algorithm structure is that
_one run of the algorithm_ can be asked to _setup multiple circuits_, and each
of those circuits has different requirements for guards. That is, since we do
filtering on START based on the requirements of circuit #1, this means that any
other circuits that appear before END is called, also have to adapt to the
requirements of circuit #1. This is obvious in the code since we use
guard_selection->for_directory throughout the whole algorithm run, even though
for_directory was just the restriction of circuit #1.

Specifically about the pending_guard trick, I feel that it interacts in
unpredictable ways with other features of the algorithm. For example, consider
how it interacts with the primary guards heuristic. It could be time for the
algorithm to reenter the primary guards state and retry the top guards in the
list, but because of the pending_guard thing we actually return the 15th guard
to the circuit.

IMO we should revisit the algorithm so that one run of the algorithm can
accomodate multiple circuits by design and without the need for hacks. Here is
an idea towards that direction:

  I think one very important change that we can do to simplify things is to
  remove the need to filter guards based on whether they are dirguards, fast,
  or stable. My suggestion here would be to *only* consider guards that are
  dirguards _and_ fast _and_ stable. This way, any circuit that appears will be
  happy with all the guards in our list and there is no need to do the
  pending_dir_guard trick. See [0] on why I think that's safe to do.

  This is easy to do in the current codebase. You just need to call
  entry_is_live() with need_uptime, need_capacity and for_directory all
  enabled (instead of flags being 0).

  If you do the above, your sampled guard set will be able to accomodate any
  circuit that comes its way and that should simplify logic considerably.

Let me know if the above does not make sense.

Here are some more comments:

- So the above idea addresses a large part of the filtering logic that happens
  on START. The rest of the filtering logic has to do with ClientUsesIPv6,
  ReachableAddreses, etc. . I think it's fine to conduct that filtering on
  START as well.

- I tried to run the branch as of bb3237d, but it segfaulted. Here is where it crashed:

     #1  0x000055555567eb25 in guards_update_state (next=0x5555559c3f40, next@entry=0x5555559c35e8, guards=guards@entry=0x5555559c4620,
      config_name=config_name@entry=0x55555570c47e "UsedGuard") at src/or/prop259.c:1137
     1137                   !strchr(e->chosen_by_version, ' ')) {

  Let me know if you need more info here.

- There is a memleak on 'extended' in filter_set().

  In general, I feel that logic in that function is a bit weird. The function
  is called filter_set() but it can actually end up adding guards to the list.
  Maybe it can be renamed?

- What's up with the each_remaining_by_bandwidth() function name?

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[0]: I think that's OK to do and here is why:

        All Guards are Fast.
        About 95% of Guards are Stable (this will become 100% with #18624)
        About 80% of Guards are V2Dir/dirguards (this will become 100% with #12538)

     #12538 got merged in 0.2.8, so if prop259 gets merged in 0.2.9, by the
     time prop259 becomes active, almost all guards will be dirguards.

     So I think it's fine to only consider guards that are dirguards && fast &&
     stable now, since by the time prop259 gets deployed that will be the case
     for almost 100% of guards.