1. 引言

Solana runtime支持合约之间通过cross-program invocation 进行相互调用。跨合约调用是通过一个合约触发另一个合约的指令来完成的，invoking合约将halted直到invoked合约处理完该指令。

假设有2个链上合约token和acme，token合约实现了pay()，acme合约实现了launch_missiles()，同时在acme合约中实现了pay_and_launch_missiles()接口来跨链调用token合约的pay()函数。

mod acme {
    use token_instruction;

    fn launch_missiles(accounts: &[AccountInfo]) -> Result<()> {
        ...
    }

    fn pay_and_launch_missiles(accounts: &[AccountInfo]) -> Result<()> {
        let alice_pubkey = accounts[1].key;
        let instruction = token_instruction::pay(&alice_pubkey);
        invoke(&instruction, accounts)?;

        launch_missiles(accounts)?;
    }

invoke()是默认嵌入在Solana runtime中的，负责routing the given instruction to the token program via the instruction’s program_id field。
注意，invoke要求caller传输触发指令所需要的的所有accounts。即要求包含：

the executable account (the ones that matches the instruction’s program id)
以及 the accounts passed to the instruction processor.

Before invoking pay(), the runtime must ensure that acme didn’t modify any accounts owned by token. It does this by applying the runtime’s policy to the current state of the accounts at the time acme calls invoke vs. the initial state of the accounts at the beginning of the acme's instruction. After pay() completes, the runtime must again ensure that token didn’t modify any accounts owned by acme by again applying the runtime’s policy, but this time with the token program ID. Lastly, after pay_and_launch_missiles() completes, the runtime must apply the runtime policy one more time, where it normally would, but using all updated pre_* variables. If executing pay_and_launch_missiles() up to pay() made no invalid account changes, pay() made no invalid changes, and executing from pay() until pay_and_launch_missiles() returns made no invalid changes, then the runtime can transitively assume pay_and_launch_missiles() as whole made no invalid account changes, and therefore commit all these account modifications.

客户端可创建一笔交易来修改2个accounts，每个account属于不同的链上合约：

let message = Message::new(vec![
    token_instruction::pay(&alice_pubkey),
    acme_instruction::launch_missiles(&bob_pubkey),
]);
client.send_and_confirm_message(&[&alice_keypair, &bob_keypair], &message);

也可以直接在客户端调用acme合约，通过acme合约接口来触发token指令：

let message = Message::new(vec![
    acme_instruction::pay_and_launch_missiles(&alice_pubkey, &bob_pubkey),
]);
client.send_and_confirm_message(&[&alice_keypair, &bob_keypair], &message);