sciond: Add method to filter paths with a policy

[lukedirtwalker]

Filters paths with the given policy, using the policy's Act method.

Fixes #2821

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This change is 

[oncilla]

Reviewed 5 of 5 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @lukedirtwalker)

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go/lib/pathpol/acl.go, line 35 at r1 (raw file):

	}
	lastRule := entries[len(entries)-1].Rule
	if lastRule != nil && (lastRule.IfIDs[0] != 0 || lastRule.ISD != 0 || lastRule.AS != 0) {

I suggest you make this a method on HopPredicate:

func (hp *HopPredicate) matchesAll() bool {
  if hp == nil {
    return true
  }
  // hp.AS == 0 implies that there is exactly one 0 interfaces.
  return hp.ISD == 0 && hp.AS == 0
}

[lukedirtwalker]

Reviewable status: 4 of 6 files reviewed, 1 unresolved discussion (waiting on @oncilla)

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go/lib/pathpol/acl.go, line 35 at r1 (raw file):

Previously, Oncilla wrote…

I suggest you make this a method on HopPredicate:

func (hp *HopPredicate) matchesAll() bool {
  if hp == nil {
    return true
  }
  // hp.AS == 0 implies that there is exactly one 0 interfaces.
  return hp.ISD == 0 && hp.AS == 0
}

Done.

[oncilla]

Reviewed 2 of 2 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved

[oncilla]

Reviewable status: complete! all files reviewed, all discussions resolved

[oncilla]

Reviewed 1 of 1 files at r3.
Reviewable status: complete! all files reviewed, all discussions resolved