Fundamentally, what you have described is not a UASF, it is a miner attack on other miners. 51% of hash power has always been able to collude to orphan blocks that contain transactions they have blacklisted (a censorship soft fork). This is clearly a miner attack which would escalate pretty rapidly into a PoW change if sustained for any time.

Miners have always retained the ability to include whatever valid transactions they like. If they don't like P2SH or segwit, they don't have to include them in their blocks. There is a clear difference between opting out of transaction selection vs miners attacking other miners to prevent their voluntary participation in mining valid transactions.

Of course, anything is possible, but game theory and incentives seem to suggest that any tantrums would be at most, short lived, if lived at all. Mining is an extraordinarily expensive endeavour, which is the basis of the strong assumption of Bitcoin that PoW/revenue incentives will keep miners honest. If that assumption is broken, Bitcoin has bigger problems.

-------- Original Message --------
Subject: Re: [bitcoin-dev] Flag day activation of segwit
From: nickodell

The problem with modifying Bitcoin to work around community norms is that it's a two-way street. Other people can do it too.

Let me propose a counter-fork, or a "Double UASF." This is also a BIP9 fork, and it uses, say, bit 2. starttime is 1489449600, and the end time is 1506812400. It enforces every rule that UASF enforces, plus one additional rule. If 60% of blocks in any retargeting period signal for Double UASF, any descendant block that creates or spends a segregated witness output is invalid.

Double UASF signaling never coincides with UASF signaling, because the signaling periods don't overlap. Full nodes happily validate the chain, because Double UASF doesn't break any rules; it just adds new ones. Miners who adopt Double UASF don't need to understand segwit, because all segwit transactions are banned. Miners don't need to commit to a wtxid structure, either. Per BIP 141, "If all transactions in a block do not have witness data, the commitment is optional." Segwit is activated, but useless. Miners who *do* adopt segwit will lose money, as their blocks are orphaned.

Thanks,
--Nick

On Sun, Mar 12, 2017 at 9:50 AM, shaolinfry via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
I recently posted about so called "user activated soft forks" and received a lot of feedback. Much of this was how such methodologies could be applied to segwit which appears to have fallen under the miner veto category I explained in my original proposal, where there is apparently a lot of support for the proposal from the economy, but a few mining pools are vetoing the activation.

It turns out Bitcoin already used flag day activation for P2SH[1], a soft fork which is remarkably similar to segwit. The disadvantage of a UASF for segwit is there is an existing deployment. A UASF would require another wide upgrade cycle (from what I can see, around 80% of existing nodes have upgraded from pre-witness, to NODE_WITNESS capability[2][3]. While absolute node count is meaningless, the uprgrade trend from version to version seems significant.

Also it is quite clear a substantial portion of the ecosystem industry has put in time and resources into segwit adoption, in the form of upgrading wallet code, updating libraries and various other integration work that requires significant time and money. Further more, others have built systems that rely on segwit, having put significant engineering resources into developing systems that require segwit - such as several lightning network system. This is much more significant social proof than running a node.

The delayed activation of segwit is also holding back a raft protocol of innovations such as MAST, Covenants, Schnorr signature schemes and signature aggregation and other script innovations of which, much of the development work is already done.

A better option would be to release code that causes the existing segwit deployment to activate without requiring a completely new deployment nor subject to hash power veto. This could be achieved if the economic majority agree to run code that rejects non-signalling segwit blocks. Then from the perspective of all existing witness nodes, miners trigger the BIP9 activation. Such a rule could come into effect 4-6 weeks before the BIP9 timeout. If a large part of the economic majority publicly say that they will adopt this new client, miners will have to signal bip9 segwit activation in order for their blocks to be valid.

I have drafted a BIP proposal so the community may discuss https://gist.github.com/shaolinfry/743157b0b1ee14e1ddc95031f1057e4c (full text below).

References:
[1]: https://github.com/bitcoin/bitcoin/blob/v0.6.0/src/main.cpp#L1281-L1283
[2]: http://luke.dashjr.org/programs/bitcoin/files/charts/services.html
[3]: https://www.reddit.com/r/Bitcoin/comments/5yyqt1/evidence_of_widespread_segwit_support_near50_of/

Proposal text:

<pre>
  BIP: bip-segwit-flagday
  Title: Flag day activation for segwit deployment
  Author: Shaolin Fry <shaolinfry@protonmail.ch>
  Comments-Summary: No comments yet.
  Comments-URI: https://github.com/bitcoin/bips/wiki/Comments:BIP-????
  Status: Draft
  Type: Informational
  Created: 2017-03-12
  License: BSD-3-Clause
           CC0-1.0
</pre>

==Abstract==

This document specifies a BIP16 like soft fork flag day activation of the segregated witness BIP9 deployment known as "segwit".

==Definitions==

"existing segwit deployment" refer to the BIP9 "segwit" deployment using bit 1, between November 15th 2016 and November 15th 2017 to activate BIP141, BIP143 and BIP147.

==Motivation==

Cause the mandatory activation of the existing segwit deployment before the end of midnight November 15th 2017.

==Specification==

All times are specified according to median past time.

This BIP will be activate between midnight October 1st 2017 (epoch time 1538352000) and midnight November 15th 2017 (epoch time 1510704000) if the existing segwit deployment is not activated before epoch time 1538352000. This BIP will cease to be active when the existing segwit deployment activates.

While this BIP is active, all blocks must set the nVersion header top 3 bits to 001 together with bit field (1<<1) (according to the existing segwit deployment). Blocks that do not signal as required will be rejected.

=== Reference implementation ===

<pre>
// mandatory segwit activation between Oct 1st 2017 and Nov 15th 2017 inclusive
if (pindex->GetMedianTimePast() >= 1538352000 && pindex->GetMedianTimePast() <= 1510704000 && !IsWitnessEnabled(pindex->pprev, chainparams.GetConsensus())) {
    if (!((pindex->nVersion & VERSIONBITS_TOP_MASK) == VERSIONBITS_TOP_BITS) && (pindex->nVersion & VersionBitsMask(params, Consensus::DEPLOYMENT_SEGWIT)) != 0) {
        return state.DoS(2, error("ConnectBlock(): relayed block must signal for segwit, please upgrade"), REJECT_INVALID, "bad-no-segwit");
    }
}
</pre>

==Backwards Compatibility==

This deployment is compatible with the existing "segwit" bit 1 deployment scheduled between midnight November 15th, 2016 and midnight November 15th, 2017.

==Rationale==

Historically, the P2SH soft fork (BIP16) was activated using a predetermined flag day where nodes began enforcing the new rules. P2SH was successfully activated with relatively few issues

By orphaning non-signalling blocks during the last month of the BIP9 bit 1 "segwit" deployment, this BIP can cause the existing "segwit" deployment to activate without needing to release a new deployment.

==References==

[https://github.com/bitcoin/bitcoin/blob/v0.6.0/src/main.cpp#L1281-L1283 P2SH flag day activation].

==Copyright==

This document is placed in the public domain.



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