validation: fix verification progress during IBD. #22804

Description

Motivation: Currently verificationprogress attribute of getblockchaininfo returns incorrect value(1.92338932716318e-06) during the headers synchronization phase of IBD. This PR fixes this by returning 0 instead of ...e-0x.

<details> <summary>getblockchaininfo on master</summary>

$ ./src/bitcoin-cli -signet getblockchaininfo                                                                                                                                                               
{
  "chain": "signet",
  "blocks": 0,
  "headers": 32410,
  "bestblockhash": "00000008819873e925422c1ff0f99f7cc9bbb232af63a077a480a3633bee1ef6",
  "difficulty": 0.001126515290698186,
  "time": 1598918400,
  "mediantime": 1598918400,
  "verificationprogress": 1.921578597095376e-06,
  "initialblockdownload": true,
  "chainwork": "000000000000000000000000000000000000000000000000000000000049d414",
  "size_on_disk": 17095548,
  "pruned": false,
  "softforks": {
    "bip34": {
      "type": "buried",
      "active": true,
      "height": 1
    },
    "bip66": {
      "type": "buried",
      "active": true,
      "height": 1
    },
    "bip65": {
      "type": "buried",
      "active": true,
      "height": 1
    },
    "csv": {
      "type": "buried",
      "active": true,
      "height": 1
    },
    "segwit": {
      "type": "buried",
      "active": true,
      "height": 1
    },
    "taproot": {
      "type": "bip9",
      "bip9": {
        "status": "active",
        "start_time": -1,
        "timeout": 9223372036854775807,
        "since": 0,
        "min_activation_height": 0
      },
      "height": 0,
      "active": true
    }
  },
  "warnings": "This is a pre-release test build - use at your own risk - do not use for mining or merchant applications"
}

</details>

<details> <summary>getblockchaininfo in this PR</summary>

$ ./src/bitcoin-cli -signet getblockchaininfo                                                                                                                                                               
{
  "chain": "signet",
  "blocks": 0,
  "headers": 21940,
  "bestblockhash": "00000008819873e925422c1ff0f99f7cc9bbb232af63a077a480a3633bee1ef6",
  "difficulty": 0.001126515290698186,
  "time": 1598918400,
  "mediantime": 1598918400,
  "verificationprogress": 0,
  "initialblockdownload": true,
  "chainwork": "000000000000000000000000000000000000000000000000000000000049d414",
  "size_on_disk": 8546460,
  "pruned": false,
  "softforks": {
    "bip34": {
      "type": "buried",
      "active": true,
      "height": 1
    },
    "bip66": {
      "type": "buried",
      "active": true,
      "height": 1
    },
    "bip65": {
      "type": "buried",
      "active": true,
      "height": 1
    },
    "csv": {
      "type": "buried",
      "active": true,
      "height": 1
    },
    "segwit": {
      "type": "buried",
      "active": true,
      "height": 1
    },
    "taproot": {
      "type": "bip9",
      "bip9": {
        "status": "active",
        "start_time": -1,
        "timeout": 9223372036854775807,
        "since": 0,
        "min_activation_height": 0
      },
      "height": 0,
      "active": true
    }
  },
  "warnings": "This is a pre-release test build - use at your own risk - do not use for mining or merchant applications"
}

</details>

Conext:

• During IBD all block headers are downloaded before the transactions of each block.
• verificationprogress is calculated using the GuessVerificationProgress method in src/validation.cpp https://github.com/bitcoin/bitcoin/blob/0492b56e38c24e599a221b5bb276957c674cfafe/src/validation.cpp#L4650
• Currently nChainTx is artificially set to 1 when the are no predecessor blocks. https://github.com/bitcoin/bitcoin/blob/0492b56e38c24e599a221b5bb276957c674cfafe/src/validation.cpp#L4960
• This results in the value of verificationprogress to be 1/fTxTotal(...e-0x) during block headers download in IBD.

Changes:

• This PR fixes the issue by returning 0.0 in GuessVerificationProgress option when nHeight == 0

Reviewing

Testing:

You can test this by running bitconid with -reindex option and immediately running getblockchainfo afterward.

$ ./src/bitcoind --daemon -signet -reindex
$ ./src/bitcoin-cli -signet getblockchaininfo

Concept ACK

• Currently nChainTx is artificially set to 1 when the are no predecessor blocks. https://github.com/bitcoin/bitcoin/blob/0492b56e38c24e599a221b5bb276957c674cfafe/src/validation.cpp#L4960

Note that the code line you mentioned is currently never executed in the codebase. It resides in ChainstateManager::PopulateAndValidateSnapshot, which is in turn called by ChainstateManager::ActivateSnapshot, which is not called anywhere (besides of fuzzing and unit tests). The value 1 for nChainTx simply results from the coinbase transaction of the genesis block that is embedded in the code (and hence already available before IBD starts). So in that sense one could argue that the calculation is actually not entirely wrong; the verification value is (hopefully) only used for statistical purposes anyways and if the progress value before IBD is zero or almost-zero doesn't matter that much.

So with that in mind, I'm updating Concept ACK ~0 (neutral) on this, would love to hear others opinion on this.

Agree with @theStack

If too much precision is an issue, it could make sense to round to the closest 1/10000 in the display.

Thanks @theStack and @MarcoFalke for the review and insights!

Will be closing this PR as GuessVerificationProgress is working as intended and will create another one that uses @MarcoFalke's suggestion instead.