在checkConnectBlock(...)方法中，找到了如下的判断规则: 如果当前处于某个checkpoint范围内并且当前需要检测的区块高度比这个chekpoint的高度低，则认为这是一个安全的区块，不进行script的校验。
// Don't run scripts if this node is before the latest known good
// checkpoint since the validity is verified via the checkpoints (all
// transactions are included in the merkle root hash and any changes
// will therefore be detected by the next checkpoint). This is a huge
// optimization because running the scripts is the most time consuming
// portion of block handling.
checkpoint := b.LatestCheckpoint()
runScripts := true
if checkpoint != nil && node.height <= checkpoint.Height {
runScripts = false
}
....
// 下面提到ECDSA signature 检查是非常耗费CPU资源的。
// Now that the inexpensive checks are done and have passed, verify the
// transactions are actually allowed to spend the coins by running the
// expensive ECDSA signature check scripts. Doing this last helps
// prevent CPU exhaustion attacks.
if runScripts {
err := checkBlockScripts(block, view, scriptFlags, b.sigCache,
b.hashCache)
if err != nil {
return err
}
}
