- It is true that hard forks produce a much cleaner outcome, in terms of
well defined behavior across the entire network.

- Replacing an opcode should not result in undefined behavior.  The
non-upgraded behavior is defined and deterministic.

- IsStandard remains an assistant.  Miners may mine non-standard
transactions.

- "Hard forks require everyone to upgrade and soft forks don't"   Doesn't
require tons of explanation:  Non upgraded clients continue working on the
network even after the rules are upgraded.

All those corrections aside, I do think there has been too much hysteria
surrounding hard forks.  Hard forks, when done right, produce a much
cleaner system for users.








On Mon, Oct 5, 2015 at 6:59 AM, Mike Hearn via bitcoin-dev <
bitcoin-dev@lists.linuxfoundation.org> wrote:

> Putting aside stupid arguments about who is older or who starting using
> the term SPV wallet first, let me try and make a better suggestion than
> what's in the BIP. How about the following:
>
> A new flag is introduced to Core, --scriptchecks=[all,standardonly,none].
> The default is all. When set to "standardonly", non-standard scripts are
> not checked but others are. This is similar to the behaviour during a soft
> fork. In "none" you have something a bit like SPV mode, but still
> calculating the UTXO set. This flag is simple and can be implemented in a
> few lines of code. Then an unused opcode is used for CLTV, so making it a
> hard fork.
>
> This has the following advantages:
>
>    - Nodes that want the pseudo-SPV behaviour of a soft fork can opt in
>    to it if they want it. This prioritises availability (in a sense) over
>    correctness.
>
>    - But otherwise, nodes will prioritise correctness by default, which
>    is how it should be. This isn't PHP where nonsensical code the interpreter
>    doesn't understand just does ...... something. This is financial software
>    where money is at risk. I feel very strongly about this: undefined
>    behaviour is fine *if you opted into getting it. *Otherwise it should
>    be avoided whenever possible.
>
>    - SPV wallets do the right thing by default.
>
>    - IsStandard doesn't silently become a part of the consensus rules.
>
>    - All other software gets simpler. It's not just SPV wallets. Block
>    explorers, for example, can just add a single line to their opcode map.
>    With a soft fork they have to implement the entire soft fork logic just to
>    figure out when an opcode transitioned from OP_NOP to CLTV and make sure
>    they render old scripts differently to new scripts. And they face tricky
>    questions - do they render an opcode as a NOP if the miner who built it was
>    un-upgraded, or do they calculate the flag day and change all of them after
>    that? It's just an explosion of complexity.
>
> Many people by now have accepted that hard forks are simpler, conceptually
> cleaner, and prioritise correctness of results over availability of
> results. I think these arguments are strong.
>
> So let me try addressing the counter-arguments one more time:
>
>    - Hard forks require everyone to upgrade and soft forks don't. I still
>    feel this one has never actually been explained. There is no difference to
>    the level of support required to trigger the change. With the suggestion
>    above, if someone can't or won't upgrade their full node but can no longer
>    verify the change, they can simply restart with -scriptchecks=standardonly
>    and get the soft fork behaviour. Or they can upgrade and get their old
>    security level back.
>
>    - Hard forks are somehow bad or immoral or can lead to "schisms". This
>    is just saying, if we hold a vote, the people who lose the vote might try
>    starting a civil war and refuse to accept the change. That's not a reason
>    to not hold votes.
>
>    But at any rate, they can do that with soft forks too: just decide
>    that any output that contains OP_CLTV doesn't make it into the UTXO set.
>    Eventually coins that trace back to such an output will become unusable in
>    the section of the economy that decided to pick a fight.
>
>
>
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