BSIP: 0031
    Title: Update Short Position's Margin Call Price After Partially Called Or Settled
    Author: Abit More <https://github.com/abitmore>
    Status: Draft
    Type: Protocol
    Created: 2018-02-16
    Discussion: https://github.com/bitshares/bitshares-core/issues/343,
                https://github.com/bitshares/bitshares-core/issues/649
    Replaces: -
    Worker: To be done

# Abstract

Currently, when a short position get partially called or settled, the call
price won't change, that said, even if its actual collateral ratio is higher
than others, higher than minimum required, it will still be selling collateral
at a low price, taking precedence over other short positions.

This behavior is causing several issues:
* it's somehow unfair, thus brought bad experience to shorters, and
* it prevents black swan event from being triggered in time when needed,
  because the collateral ratio of the 2nd even overall short positions may
  be too low but not being checked, thus risks the pegging system.

This BSIP proposes a mechanism to improve this situation.

# Motivation

Make the exchange system more user-friendly.

# Rationale

To attract more users, the system should be fair, should be well balanced.

It's common sense that short positions with least collateral ratio should
get margin called first. This can be achieved if always update the margin
call price after every fill.

# Specifications

In `fill_order( const call_order_object& ...)` function of `database` class,
update `call_price` field of `call_order_object` after debt or collateral
changed to a non-zero value.

In addtion, after `call_price` get updated, the iterators initialized with
`by_price` index may be invalidated, so need to review / revise involved code,
E.G. `check_call_orders(...)` function of `database` class.

# Discussion

[to be added if any]

# Summary for Shareholders

[to be added if any]

# Copyright

This document is placed in the public domain.

# See Also

* https://github.com/bitshares/bitshares-core/issues/343
* https://github.com/bitshares/bitshares-core/issues/649
* https://bitsharestalk.org/index.php?topic=25926.0