how to check if a blockhash has expired using @solana/web3.js?

Question

I'm using getLatestBlockhash() to pass into a transaction.

const { blockhash } = await connection.getLatestBlockhash("finalized");

However, I want to invalidate the transaction on my backend if a block has passed and the user has not approved the transaction. How I would check if a block has expired using @solana/web3.js?

Answer 1

There's no way to "invalidate" a transaction that has been fully signed and broadcast, it can only be allowed to expire.

The reply of getLatestBlockhash also has a lastValidBlockheight field. getEpochInfo can be polled until the value in its blockHeight field exceeds the blockhash's last valid height. The cookbook has a pretty good explainer on the lifetime of transactions

Answer 2

You can use the web3 method simulateTransaction. If the blockhash has expired then this method returns an error indicating that the blockhash has expired.

The blockhash is valid if it is within the last 150 blocks.

Do note though that sendTransaction will also check that the blockhash is valid for you, so long as the you do not skip the preflight checks, skipPreflight: false, which is one of the sendOptions in the sendTransaction method. More on that from the cookbook here

The solana cookbook also goes into more detail here on keeping the transaction fresh. It references an RPC method that isn't yet available on the web3 library specifically for the purpose of checking a blockhash is still valid, called isBlockhashValid.