被回滚区块是否还保存在bitcoin全节点

之前有同事询问，过去被回滚的区块是否还能在全节点找到？ 当时没有具体了解bitcoin在forked block上的处理，给他的结论是全节点不会保存被forked block信息，如果保留也是短暂时间内的。这个结论的主要依据是bitcoin源码中定义了一个BlockIndex的链表数据，而这个结构只保留主链上的block，所以想当然的认为全节点是不会保留forked block信息的。

另外一种情况是: 历史上发生了区块回滚的事件，一个新建的bitcoin全节点，通过区块同步的方式下载所有区块链数据，那么可以肯定的是，在下载到最高度的区间内的所有forked block信息，将会全部丢失。因为区块同步下载的过程是不会涉及到forked block的。

获取forked block数据

bitcon给出的API getchaintips中能够获得相关信息，具体使用方法如下:

> bitcoin-cli getchaintips
[
  {
    "height": 580618,
    "hash": "00000000000000000021873a6237b6bf299c334925062609b5e91d39d46ca044",
    "branchlen": 0,
    "status": "active"
  },
  {
    "height": 578141,
    "hash": "0000000000000000001253a5f37d3763dbe928d21f7d72a708f05268c044179c",
    "branchlen": 1,
    "status": "valid-fork"
  },
  {
    "height": 573313,
    "hash": "0000000000000000000883f9f6eb9a459426f715ee3895a4dbb6e51e9237eceb",
    "branchlen": 1,
    "status": "valid-fork"
  },
  {
    "height": 570069,
    "hash": "000000000000000000214b7d19ea458da8f8e80bc437587a02eb418b5fd8b3aa",
    "branchlen": 1,
    "status": "valid-headers"
  },
  {
    "height": 478576,
    "hash": "000000000000000001416af072f8989829f4c60a1a9658e1cec08411798e4ffa",
    "branchlen": 18,
    "status": "headers-only"
  }
]

该API返回的是一个json数组，各个字段的含义如下:

• height: 发生fork事件的高度
• hash: forked block hash值
• branchlen: 发生fork的深度
• status: forked block的验证状态

对于status字段，其可能的值，以及具体含义可以查看如下代码:

std::string status;
if (chainActive.Contains(block)) {
    // This block is part of the currently active chain.
    status = "active";
} else if (block->nStatus & BLOCK_FAILED_MASK) {
    // This block or one of its ancestors is invalid.
    status = "invalid";
} else if (!block->HaveTxsDownloaded()) {
    // This block cannot be connected because full block data for it or one of its parents is   missing.
    status = "headers-only";
} else if (block->IsValid(BLOCK_VALID_SCRIPTS)) {
    // This block is fully validated, but no longer part of the active chain. It was probably   the active block once, but was reorganized.
    status = "valid-fork";
} else if (block->IsValid(BLOCK_VALID_TREE)) {
    // The headers for this block are valid, but it has not been validated. It was probably     never part of the most-work chain.
    status = "valid-headers";
} else {
    // No clue.
    status = "unknown";
}
obj.pushKV("status", status);

分析

之所以bitcoin全节点保存了这些数据，是因为有一个全局的block容器mapBlockIndex，来处理该节点遇到的所有block，以及header数据。上面提到只保留主链的容器是:

/** The currently-connected chain of blocks (protected by cs_main). */
extern CChain& chainActive;