Merge pull request #7471 from ethereum-optimism/inphi/update-spec

specs: Honest challenger for new resolution

specs/honest-challenger-fdg.md

@@ -64,7 +64,15 @@ then it must dispute claims positioned at odd depths in the game tree.
 Otherwise, it disputes claims positioned at even depths in the game tree.
 This means an honest challenger only responds to claims made by the opposing team.
 
-The next step is to determine whether the claim has a valid commitment (i.e. `ClaimHash`).
+The next step is to determine if the claim, now known to be for the opposing team,
+disputes another claim the honest challenger _agrees_ with.
+An honest challenger agrees with a claim iff every other claim along its path to the
+root claim commits to a valid `ClaimHash`. Put differently, an honest challenger will
+avoid countering a claim if it disagrees with the path of claims leading to that
+specific claim. But if the honest challenger agrees with the path leading to the claim,
+then the claim is countered.
+
+The last step is to determine whether the claim has a valid commitment (i.e. `ClaimHash`).
 If the `ClaimHash` matches the honest challenger's at the same trace index, then we
 disagree with the claim's stance by moving to [defend](./fault-dispute-game.md#defend).
 Otherwise, the claim is [attacked](./fault-dispute-game.md#attack).
@@ -77,18 +85,28 @@ class Team(Enum):
     CHALLENGER = 1
 
 class Claim:
+    parent: Claim
     position: uint64
     claim_hash: ClaimHash
 
 MAX_TRACE = 2**MAX_GAME_DEPTH
 
+def agree_with(claim: Claim, chal_trace: List[ClaimHash, MAX_TRACE]):
+    if chal_trace[claim.trace_index] != claim.claim_hash:
+        return False
+    grand_parent = claim.parent.parent if claim.parent is not None else None
+    if grand_parent is not None:
+        return agree_with(grand_parent)
+    return True
+
 def respond(claim: Claim, chal: Team, chal_trace: List[ClaimHash, MAX_TRACE]):
     if depth(claim.position) % 2 != chal.value:
-        if chal_trace[trace_index(claim.position)] == claim.claim_hash:
-            defend()
-        else:
-            attack()
-    else: pass # no response
+        if claim.parent is None or agree_with(claim.parent, chal_trace):
+            if chal_trace[trace_index(claim.position)] == claim.claim_hash:
+                defend()
+            else:
+                attack()
+        else: pass # avoid supporting invalid claims on the same team
 ```
 
 In attack or defense, the honest challenger submit a `ClaimHash` corresponding to the
@@ -111,10 +129,14 @@ invalid `ClaimHash` commitments. Otherwise, it issues a _defense step_.
 
 ## Resolution
 
-When the [chess clock](./fault-dispute-game.md#game-clock) of a `FaultDisputeGame` team
-runs out, the game can be resolved.
-The honest challenger does this by calling the `resolve` function on the
-`FaultDisputeGame` contract.
+When the [chess clock](./fault-dispute-game.md#game-clock) of a
+[subgame root](./fault-dispute-game.md#resolution) has run out, the subgame can be resolved.
+The honest challenger should resolve all subgames in bottom-up order, until the subgame
+rooted at the FDG root is resolved.
+
+The honest challenger accomplishes this by calling the `resolveClaim` function on the
+`FaultDisputeGame` contract. Once the root claim's subgame is resolved,
+the challenger then finally calls the `resolve` function to resolve the entire game.
 
 The `FaultDisputeGame` does not put a time cap on resolution - because of the liveness
 assumption on honest challengers and the bonds attached to the claims they’ve countered,