Resources

People often ask me "How did you learn how to hack?" The answer: by reading. This page is a collection of the blog posts and other articles that I have accumulated over the years of my journey. Enjoy!

When making a Cross Program Invocation (CPI) in Solana via invoke or invoke_signed, you provide a set of accounts to be used. In raw Solana, you pass in AccountInfo directly, which is a handle to the in-memory runtime state. In Anchor, you pass in Account<'info, T>., which is a deserialized version of T and acts as a cached value.

Native Solana programs do not operate on the ledger directly. Instead, accounts are loaded into the runtime as a working set. Instructions mutate this in-memory state. Many things, like lamports, are read directly from the runtime state every time. If you reborrow the data, then the underlying bytes will also be updated.

In Anchor, the type T on the AccountInfo is a deserialized snapshot of the account data bytes. At the start of the instruction, Anchor constructs the accounts by deserializing them in a generated handler from the info.data on the account. This means that the data is copied onto the stack/heap as a Rust value and is NOT a live reference to the runtime bytes. At the end of the instruction, Anchor will serialize the data structure and write it back to the runtime.

In practice, this has a strange consequence: if a CPI modifies an account, the cached version will have stale data. For instance, for balance on a token account, a token transfer would show the same balance before and after the CPI, regardless of whether the account balance changed.

To solve this problem, Anchor accounts have reload(). This will reload the data from storage via re-reading and deserializing the data within AccountInfo.data. The account data is now no longer stale.

The author gives some tips on when to call reload(). It's required when A) a CPI can be used to mutate account data, B) the account needs to be read/validated later and C) you are reading a cached struct. If lamports or native runtime fields are being read, then reloading isn't necessary.

Overall, a great post on Solana CPI reloading and why it must be done. I had always wondered why lamports didn't need to be reloaded but the data did; now I know!