Bug #20569 Garbage in DECIMAL results from some mathematical functions

  Adding decimal "digits" in multiplication resulted in signed overflow and
producing wrong results.

  Fixed by using large enough buffers and intermediary result types :
dec2 (currently longlong) to hold result of adding decimal "digits" 
(currently int32). 

mysql-test/r/select.result

@@ -3395,3 +3395,6 @@ a	t1.b + 0	t1.c + 0	a	t2.b + 0	c	d
 1	0	1	1	0	1	NULL
 2	0	1	NULL	NULL	NULL	NULL
 drop table t1,t2;
+SELECT 0.9888889889 * 1.011111411911;
+0.9888889889 * 1.011111411911
+0.9998769417899202067879

mysql-test/t/select.test

@@ -2901,3 +2901,8 @@ from t1 left outer join t2 on t1.a = t2.c and t2.b <> 1
 where t1.b <> 1 order by t1.a;
 
 drop table t1,t2;
+
+#
+# Bug #20569: Garbage in DECIMAL results from some mathematical functions
+#
+SELECT 0.9888889889 * 1.011111411911;

strings/decimal.c

@@ -170,6 +170,7 @@ static const dec1 frac_max[DIG_PER_DEC1-1]={
 #define ADD(to, from1, from2, carry)  /* assume carry <= 1 */           \
         do                                                              \
         {                                                               \
+          DBUG_ASSERT((carry) <= 1);                                    \
           dec1 a=(from1)+(from2)+(carry);                               \
           if (((carry)= a >= DIG_BASE)) /* no division here! */         \
             a-=DIG_BASE;                                                \
@@ -179,7 +180,7 @@ static const dec1 frac_max[DIG_PER_DEC1-1]={
 #define ADD2(to, from1, from2, carry)                                   \
         do                                                              \
         {                                                               \
-          dec1 a=(from1)+(from2)+(carry);                               \
+          dec2 a=((dec2)(from1))+(from2)+(carry);                       \
           if (((carry)= a >= DIG_BASE))                                 \
             a-=DIG_BASE;                                                \
           if (unlikely(a >= DIG_BASE))                                  \
@@ -187,7 +188,7 @@ static const dec1 frac_max[DIG_PER_DEC1-1]={
             a-=DIG_BASE;                                                \
             carry++;                                                    \
           }                                                             \
-          (to)=a;                                                       \
+          (to)=(dec1) a;                                                \
         } while(0)
 
 #define SUB(to, from1, from2, carry) /* to=from1-from2 */               \
@@ -1998,7 +1999,13 @@ int decimal_mul(decimal_t *from1, decimal_t *from2, decimal_t *to)
       ADD2(*buf0, *buf0, lo, carry);
       carry+=hi;
     }
-    for (; carry; buf0--)
+    if (carry)
+    {
+      if (buf0 < to->buf)
+        return E_DEC_OVERFLOW;
+      ADD2(*buf0, *buf0, 0, carry);
+    }
+    for (buf0--; carry; buf0--)
     {
       if (buf0 < to->buf)
         return E_DEC_OVERFLOW;