Chain Reorganization Guide

// Monitor for reorgs
const monitorReorgs = async (callback, checkInterval = 5000) => {
	let lastBlock = await web3.eth.getBlockNumber();
	let lastBlockHash = (await web3.eth.getBlock(lastBlock)).hash;

	return setInterval(async () => {
		const currentBlock = await web3.eth.getBlockNumber();
		const currentBlockHash = (await web3.eth.getBlock(lastBlock)).hash;

		if (currentBlockHash !== lastBlockHash) {
			// Reorg detected
			callback({
				oldBlock: lastBlock,
				oldHash: lastBlockHash,
				newBlock: currentBlock,
				newHash: currentBlockHash,
			});
		}

		lastBlock = currentBlock;
		lastBlockHash = currentBlockHash;
	}, checkInterval);
};

// Get common ancestor block
const findCommonAncestor = async (hash1, hash2) => {
	let block1 = await web3.eth.getBlock(hash1);
	let block2 = await web3.eth.getBlock(hash2);

	while (block1.number > block2.number) {
		block1 = await web3.eth.getBlock(block1.parentHash);
	}
	while (block2.number > block1.number) {
		block2 = await web3.eth.getBlock(block2.parentHash);
	}
	while (block1.hash !== block2.hash) {
		block1 = await web3.eth.getBlock(block1.parentHash);
		block2 = await web3.eth.getBlock(block2.parentHash);
	}
	return block1;
};