RE: There is already code to warn the user if they are on a minority chain more than 6 blocks long-- see GetWarnings():
Quote
WARNING: Displayed transactions may not be correct! You may need to upgrade, or other nodes may need to upgrade.
It just warns, it doesn't stop you from generating/sending transactions. Perhaps if the longer fork is more than 120 blocks ahead it should.
ByteCoin: we can prevent the scenario you describe (generate a block with a NOP1/EVAL transaction whose validity is different in old and new clients) by:
1. Requiring that EVAL transactions be valid if the EVAL is interpreted as a no-op. New clients can replace EVALs with no-ops and re-evaluate them to make sure that is true.
2. Waiting until a majority of hashing power has upgraded before turning on EVAL.
gmaxwell: I really like the idea of implementing key recovery to save space in the block chain. It can also be done in a backwards-compatible way if only CHECKSIGs "hidden inside" of EVALs support it (CHECKSIG would be aware of whether it was being executed inside of an OP_EVAL, and if it was it could use a compressed version of the public key instead of the full public key).
All the ideas about figuring out what percentage of miners have upgraded: seems like that deserves its own discussion thread.