Subject: Re: [bitcoindev] Unbreaking testnet4

From: Sjors Provoost <[email-masked]>
Date: Mon, 28 Apr 2025 14:47:13 +0200

Jameson Lopp wrote:

> Encoding an "end of life date" into testnets is actually an interesting idea worth discussing. As far as I'm aware it's never been done before on any network. 

Keep in mind that testnet-specific code has to live right next to, even inside of, mainnet consensus code. We want the change to be as simple as possible, so as to not accidentally break mainnet.

Unless and until coin expiration is something we're seriously considering for mainnet, we'd rather not implement it for testnet.

This particular idea probably requires a lot of changes all over the place (consensus, mempool, wallet) because it breaks the assumption that coins don't expire.

Something I've proposed in person a few times, is to double the coins every halving. In terms of code, it boils down to changing GetBlockSubsidy:

CAmount nSubsidy = 50 * COIN;
// Subsidy is cut in half every 210,000 blocks which will occur approximately every 4 years.
If (consensusParams.inflation) {
    // Except on testnet5
    nSubsidy <<= halvings;
} else {
  nSubsidy >>= halvings;
}

This will eventually overflow, but that seems fine for a testnet. Along with the timewarp fix, the network might even grind to a halt in 2106, long before we overflow 64 bit numbers.

Rust Bitcoin [0] currently refuses amounts above 21 million BTC, but they would have many years to fix that.

Strong inflation has been battle tested by governments around the world for millennia as a way to discourage saving.

- Sjors

[0] https://github.com/rust-bitcoin/rust-bitcoin/issues/4273

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