Is "previousblockhash" always returned from getblock RPC calls?

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Does the getblock(hash) RPC call always return a "previousblockhash" field and if not: under what conditions does it not return this?

Background: many algorithms suggest using this field as a way to step back in the blockchain when an orphan or reorganization occurs but the C++ code would indicate this field isn't always present.

if (blockindex->pprev)
        result.push_back(Pair("previousblockhash", blockindex->pprev->GetBlockHash().GetHex()));

Would that be due to chain reorganizations and orphans? Can anyone explain what dynamics would make this check fail and what the next step would be if it does?

Matthew Roberts

Posted 2015-08-08T10:26:56.713

Reputation: 165

Do you really mean orphan blocks, or rather stale blocks? A block can never become an orphan, Bitcoin Core can not connect a block without its previous existing. – Anonymous – 2015-08-08T10:53:20.667

I guess stale block might be the term I'm after then. Good observation – Matthew Roberts – 2015-08-08T11:05:22.650

It won't return information on previousblockhash for the genesis block, for instance. – George Kimionis – 2015-08-08T14:14:05.567

1@Bitcoin Nice analysis, but it's not correct. getblockheader was indeed added because getblock can fail when a block is pruned, but even then, the index entries remain. See my answer. – Pieter Wuille – 2015-08-10T14:06:30.483

@PieterWuille Right, that makes a lot more sense. – Anonymous – 2015-08-10T14:10:43.803

Answers

2

It will only ever not be present for the genesis block.

A block is not added to the block index until all its ancestors are in there too.

The block index is conceptually a tree: every block can have multiple decendents but only one ancestor. A reorganization is switching from one branch of the tree to another, and does not affect this.

Orphan blocks (in the meaning of blocks without known parent) don't exist anymore since Bitcoin Core 0.10 (we only download blocks after validating their headers, so the parent is never unknown), and even before that, orphan blocks were just stored in a queue in memory and not actually added to the index until their parents were found.

Pieter Wuille

Posted 2015-08-08T10:26:56.713

Reputation: 54 032

Asked: 2015-08-08T10:26:56.713

Viewed: 107 times

Active: 2015-08-10T11:00:23.973