Prediction markets can suffer a black swan

[pmconrad]

@abitmore noticed that when a price feed for a prediction market is published, the usual black swan conditions are checked. The black swan checks disregard the is_prediction_market flag, and if the conditions are met, global settlement is triggered for the prediction market.

This behaviour does not make sense:

• PM tokens are always backed by the same number of backing tokens. A PM will be settled in the range [0,1], which means there is always sufficient collateral available.
• The actual feed price and MCR seem to be otherwise unused in a PM, so there is no reason to act upon them.

(This has happened on mainnet and testnet, which caused the recent problems.)

• http://docs.bitshares.eu/bitshares/tutorials/pm-close-manual.html should also be corrected, as right now it indicates that a price feed must be published for settling a PM.
• the pm-create-manual document needs to be revised as well

[abitmore]

I thought it's a feature, so not only the issuer but also specified feed producers can global settle the PM.

[pmconrad]

Hm. But the black swan settles at the ratio of the least collateralized short, which is always 1:1 in a PM. So this is a rather limited feature.

[abitmore]

Agreed. Since we don't have a specification document, we can only guess. We may need to redesign this feature.

[abitmore]

Need a BSIP.

[abitmore]

After done more investigation, I agree this is a bug. We can fix it by disabling price feeds for prediction markets, don't need a BSIP.

The feature that global settle a prediction market via price feeds is debatable and deserves a BSIP.

By the way, need a better business model for the asset creator. Because the token can worth zero in the end, percentage market fee is not enough. This can be discussed in a BSIP as well.