MDEV-5858 MySQL Bug#12744991 - DECIMAL_ROUND(X,D) GIVES WRONG RESULTS WHEN D == N*(-9)

don't use mysql-5.6 change.
correct fix: zero-out rounded tail after the number was shifted because
of the carry digit (otherwise the carry digit will be zeroed out too).

mysql-test/r/func_math.result

@@ -728,6 +728,21 @@ select (1.175494351E-37 div 1.7976931348623157E+308);
 Warnings:
 Warning	1916	Got overflow when converting '' to DECIMAL. Value truncated.
 #
+# Bug#12744991 - DECIMAL_ROUND(X,D) GIVES WRONG RESULTS WHEN D == N*(-9)
+#
+select round(999999999, -9);
+round(999999999, -9)
+1000000000
+select round(999999999.0, -9);
+round(999999999.0, -9)
+1000000000
+select round(999999999999999999, -18);
+round(999999999999999999, -18)
+1000000000000000000
+select round(999999999999999999.0, -18);
+round(999999999999999999.0, -18)
+1000000000000000000
+#
 # Bug#12537160 ASSERTION FAILED:
 # STOP0 <= &TO->BUF[TO->LEN] WITH LARGE NUMBER.
 #

mysql-test/t/func_math.test

@@ -538,6 +538,15 @@ SELECT 1 div null;
 --echo #
 select (1.175494351E-37 div 1.7976931348623157E+308);
 
+--echo #
+--echo # Bug#12744991 - DECIMAL_ROUND(X,D) GIVES WRONG RESULTS WHEN D == N*(-9)
+--echo #
+
+select round(999999999, -9);
+select round(999999999.0, -9);
+select round(999999999999999999, -18);
+select round(999999999999999999.0, -18);
+
 --echo #
 --echo # Bug#12537160 ASSERTION FAILED:
 --echo # STOP0 <= &TO->BUF[TO->LEN] WITH LARGE NUMBER.

strings/decimal.c

@@ -1594,24 +1594,6 @@ decimal_round(const decimal_t *from, decimal_t *to, int scale,
       x+=10;
     *buf1=powers10[pos]*(x-y);
   }
-  /*
-    In case we're rounding e.g. 1.5e9 to 2.0e9, the decimal_digit_t's inside
-    the buffer are as follows.
-
-    Before <1, 5e8>
-    After  <2, 5e8>
-
-    Hence we need to set the 2nd field to 0.
-    The same holds if we round 1.5e-9 to 2e-9.
-   */
-  if (frac0 < frac1)
-  {
-    dec1 *buf= to->buf + ((scale == 0 && intg0 == 0) ? 1 : intg0 + frac0);
-    dec1 *end= to->buf + len;
-
-    while (buf < end)
-      *buf++=0;
-  }
   if (*buf1 >= DIG_BASE)
   {
     carry=1;
@@ -1633,6 +1615,7 @@ decimal_round(const decimal_t *from, decimal_t *to, int scale,
       }
       *buf1=1;
       to->intg++;
+      intg0++;
     }
   }
   else
@@ -1654,6 +1637,24 @@ decimal_round(const decimal_t *from, decimal_t *to, int scale,
       }
     }
   }
+  /*
+    In case we're rounding e.g. 1.5e9 to 2.0e9, the decimal_digit_t's inside
+    the buffer are as follows.
+
+    Before <1, 5e8>
+    After  <2, 5e8>
+
+    Hence we need to set the 2nd field to 0.
+    The same holds if we round 1.5e-9 to 2e-9.
+   */
+  if (frac0 < frac1)
+  {
+    dec1 *buf= to->buf + ((scale == 0 && intg0 == 0) ? 1 : intg0 + frac0);
+    dec1 *end= to->buf + len;
+
+    while (buf < end)
+      *buf++=0;
+  }
 
   /* Here we  check 999.9 -> 1000 case when we need to increase intg */
   first_dig= to->intg % DIG_PER_DEC1;