Health token adjustment: Deal with odd I80F48 behavior

The identity
  a * b = -((-a) * b)
does not hold for I80F48, probably it's rounding to -inf.

This meant that withdrawing the full native token balance could
fail because the magnitude of -position * price was bigger than
position * price.

programs/mango-v4/src/state/health.rs

@@ -652,7 +652,12 @@ impl HealthCache {
     pub fn adjust_token_balance(&mut self, token_index: TokenIndex, change: I80F48) -> Result<()> {
         let entry_index = self.token_entry_index(token_index)?;
         let mut entry = &mut self.token_infos[entry_index];
-        entry.balance = cm!(entry.balance + change * entry.oracle_price);
+
+        // Work around the fact that -((-x) * y) == x * y does not hold for I80F48:
+        // We need to make sure that if balance is before * price, then change = -before
+        // brings it to exactly zero.
+        let removed_contribution = (-change) * entry.oracle_price;
+        entry.balance = cm!(entry.balance - removed_contribution);
         Ok(())
     }
 

programs/mango-v4/src/util.rs

@@ -38,3 +38,17 @@ pub fn format_zero_terminated_utf8_bytes(
             .trim_matches(char::from(0)),
     )
 }
+
+#[cfg(test)]
+mod tests {
+    use fixed::types::I80F48;
+
+    #[test]
+    pub fn test_i80f48_mul_rounding() {
+        // It's not desired, but I80F48 seems to round to -inf
+        let price = I80F48::from_num(0.04);
+        let x = I80F48::from_bits(96590783907000000);
+        assert_eq!((x * price).to_string(), "13.726375969298193");
+        assert_eq!(((-x) * price).to_string(), "-13.726375969298196");
+    }
+}