go/consensus/cometbft/beacon: Fix GetEpochBlock implementation

go/consensus/cometbft/beacon/beacon.go

@@ -114,32 +114,35 @@ func (sc *serviceClient) GetEpochBlock(ctx context.Context, epoch beaconAPI.Epoc
 	}
 
 	// Find historic epoch.
-	//
-	// TODO: This is really really inefficient, and should be optimized,
-	// maybe a cache of the last few epochs, or a binary search.
-	height := consensus.HeightLatest
+	startHeight, err := sc.backend.GetLastRetainedVersion(ctx)
+	if err != nil {
+		return -1, fmt.Errorf("failed to query last retained version: %w", err)
+	}
+	endHeight := currentBlk
 	for {
-		q, err := sc.querier.QueryAt(ctx, height)
+		if startHeight == endHeight {
+			return -1, fmt.Errorf("failed to find historic epoch")
+		}
+
+		midHeight := (endHeight - startHeight) / 2
+		q, err := sc.querier.QueryAt(ctx, midHeight)
 		if err != nil {
 			return -1, fmt.Errorf("failed to query epoch: %w", err)
 		}
 
 		var pastEpoch beaconAPI.EpochTime
-		pastEpoch, height, err = q.Epoch(ctx)
+		pastEpoch, _, err = q.Epoch(ctx)
 		if err != nil {
 			return -1, fmt.Errorf("failed to query epoch: %w", err)
 		}
 
-		if epoch == pastEpoch {
-			return height, nil
-		}
-
-		height--
-
-		// The initial height can be > 1, but presumably this does not
-		// matter, since we validate that epoch > sc.baseEpoch.
-		if pastEpoch == 0 || height <= 1 {
-			return -1, fmt.Errorf("failed to find historic epoch (minimum: %d requested: %d)", pastEpoch, epoch)
+		switch {
+		case epoch == pastEpoch:
+			return midHeight, nil
+		case epoch < pastEpoch:
+			endHeight = midHeight
+		case epoch > pastEpoch:
+			startHeight = midHeight
 		}
 	}
 }