Add BSIP 31

bsip-0031.md

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+    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.
+
+# Rational
+
+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