Fix conversion of floating point values to character fields when the

absolute value of the float is less than 1, and also fix calculation of
length for negative values. (Bug #7774)

mysql-test/r/type_float.result

@@ -179,3 +179,18 @@ f
 9.999
 9.999
 drop table if exists t1;
+create table t1 (c char(20));
+insert into t1 values (5e-28);
+select * from t1;
+c
+5e-28
+drop table t1;
+create table t1 (c char(6));
+insert into t1 values (2e5),(2e6),(2e-4),(2e-5);
+select * from t1;
+c
+200000
+2e+06
+0.0002
+2e-05
+drop table t1;

mysql-test/r/type_float.result.es

@@ -179,3 +179,18 @@ f
 9.999
 9.999
 drop table if exists t1;
+create table t1 (c char(20));
+insert into t1 values (5e-28);
+select * from t1;
+c
+5e-28
+drop table t1;
+create table t1 (c char(6));
+insert into t1 values (2e5),(2e6),(2e-4),(2e-5);
+select * from t1;
+c
+200000
+2e+06
+0.0002
+2e-05
+drop table t1;

mysql-test/t/type_float.test

@@ -103,3 +103,13 @@ create table t1 (f double(4,3));
 insert into t1 values (-11.0),(-11),("-11"),(11.0),(11),("11");
 select * from t1;
 drop table if exists t1;
+
+# Check conversion of floats to character field (Bug #7774)
+create table t1 (c char(20));
+insert into t1 values (5e-28);
+select * from t1;
+drop table t1;
+create table t1 (c char(6));
+insert into t1 values (2e5),(2e6),(2e-4),(2e-5);
+select * from t1;
+drop table t1;

sql/field.cc

@@ -4301,13 +4301,20 @@ int Field_str::store(double nr)
   char buff[DOUBLE_TO_STRING_CONVERSION_BUFFER_SIZE];
   uint length;
   bool use_scientific_notation= TRUE;
-  use_scientific_notation= TRUE;
-  if (field_length < 32 && fabs(nr) < log_10[field_length]-1)
+  /*
+    Check fabs(nr) against longest value that can be stored in field,
+    which depends on whether the value is < 1 or not, and negative or not
+  */
+  double anr= fabs(nr);
+  int neg= (nr < 0.0) ? 1 : 0;
+  if (field_length < 32 &&
+      (anr < 1.0 ? anr > 1/(log_10[max(0,field_length-neg-2)]) /* -2 for "0." */
+                 : anr < log_10[field_length-neg]-1))
     use_scientific_notation= FALSE;
 
   length= (uint) my_sprintf(buff, (buff, "%-.*g",
                                    (use_scientific_notation ?
-                                    max(0, (int)field_length-5) :
+                                    max(0, (int)field_length-neg-5) :
                                     field_length),
                                    nr));
   /*