Logic for Checking if Essential Nodes are Confirmable

[rauljordan]

Background

As part of reimbursing challenge bonds in-protocol, our researchers at Offchain Labs have determined that honest parties may need to keep defending honest edges within subchallenges for a period of time that may extend beyond the confirmation of a challenge. Defending honest edges within a subchallenge can stop once the closest "essential edge" of an edge within the subchallenge is confirmable.

We can think of an essential edge, for purposes of this PR, as the honest root of a subchallenge, or of a top-level challenge. To check if an essential node is confirmable, all essential paths down from it to terminal edges must have a path weight >= CHALLENGE_PERIOD. This path weight is the sum of the local timers of edges along the path. To achieve this, we do a depth-first search of the graph to extract all essential paths. For each path, we store its weight in a min-heap. Once the min element of this heap has a weight >= CHALLENGE_PERIOD, we know the essential edge is confirmable.

This PR includes the DFS computation and a function called IsConfirmableEssentialNode which performs the operations above. It is tested across challenge levels and uses algorithmic utilities from the standard library. We perform DFS using a visitor stack to avoid recursion in our algorithm.

Note

See the test Test_findEssentialPaths to understand how the algorithm works and what it is meant to output for a sample challenge tree under paths_test.go and also setupEssentialPathsTest

[codecov]

Codecov Report

Attention: Patch coverage is 87.11340% with 25 lines in your changes are missing coverage. Please review.

Project coverage is 60.00%. Comparing base (c3d517f) to head (43f4cee).

Additional details and impacted files

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