Enable loud hard-fork detection

[nathan-at-least]

This feature may be a fairly light configuration of an existing feature: #109 is for documenting the existing feature, and it links to this bitcoin-development message which alludes to the feature:

https://www.mail-archive.com/bitcoin-development@lists.sourceforge.net/msg06273.html

[zookoatleastauthoritycom]

#171 was a duplicate. It seems to me that if you, as a Zcoin node, have detected that you're on the short side of a fork, then it is unsafe to tell your user that they received any Zcoin. Any Zcoin that you tell your user they've received, at that point, could be invalidated out from under them if they subsequently upgrade to new software which switches to the longer branch of the chain. So as far as I can tell, once you've detected this condition, the only safe thing to do is to warn the user and refuse to process anymore incoming transactions ostensibly offered to your user.

[zookoatleastauthoritycom]

sipa says that this is currently implemented in bitcoin core:

sipa zooko: seems i am mistaken, and the fork warning will trigger safe mode
zooko sipa: Hm...
zooko sipa: are you looking at the code? Could you give me a pointer to this trigger connection?
sipa GetWarnings in main.cpp
sipa if that returns non-"" for RPC, init.cpp OnRPCCommand will disallow the RPC command
sipa if the RPC command does not have okSafeMode in rpcserver.cpp

[nathan-at-least]

See also: https://en.bitcoin.it/wiki/User:Theymos/Alert_codes

HT @zookoatleastauthoritycom

[nathan-at-least]

It seems possible to notice being on either side of a fork (assuming you receive the block headers).

Presumably the core codebase doesn't propagate headers on other forks, so it may be hard to see a shorter fork, since those blocks will not propagate to your node.

[daira]

I'd like to revisit whether this should be in 1.0, because I think that putting it in 1.0 would make it much easier to safely implement hard-forking post-1.0 changes.

[ebfull]

+1 on revisiting this.

[zookozcash]

This seems to be related to #688. Perhaps #688 is an implementation of this which causes the node to freeze — ignoring all transactions — in case it detects a hard fork?

[zookozcash]

Unfortunately upstream Bitcoin-Core is talking about removing this from that codebase due to it being unsatisfactory in some way: https://botbot.me/freenode/bitcoin-core-dev/2016-03-31/?msg=63274564&page=2 They've been saying, in IRC chat, that they want to put back a "fixed" implementation of it in the future. I don't understand what's wrong with the current implementation or what fixed implementation they'd want.

[zookozcash]

Whoops, my mistake. The thing that they're talking about disabling is not the hard-fork-detector, it is the network-partition-detector: https://botbot.me/freenode/bitcoin-core-dev/2016-03-31/?msg=63275812&page=2 Thanks to sipa for clarifying!

[defuse]

#1009 makes it run the -alertnotify command when there's a hard fork and adds an RPC test that checks the hard fork detection.