Bug#8841 - CHECKSUM TABLE is broken in MyISAM

There are (at least) two implementations of the checksum
computation. One is in MyISAM for the quick checksum. It
is executed on every row change. The other is in the
SQL layer for the extended checksum. It retrieves all rows
of a table via the respective storage engine.

In former MySQL versions varchars were stored with their 
maximum length, but now with their real length similar to
blobs.

This change had been forgotten to take care of in the
extended checksum calculation. Hence too much data was
checksumed. In MyISAM this change had been taken care of 
already. Only the real data is included in the checksum.

I changed mysql_checksum_table() so that it uses the
length information of true varchar fields instead
of the field length like in former varchar 
implementations.

mysql-test/r/innodb.result

@@ -1457,16 +1457,16 @@ Error	1146	Table 'test.t4' doesn't exist
 checksum table t1, t2, t3, t4;
 Table	Checksum
 test.t1	2948697075
-test.t2	3835700799
-test.t3	3835700799
+test.t2	2948697075
+test.t3	2948697075
 test.t4	NULL
 Warnings:
 Error	1146	Table 'test.t4' doesn't exist
 checksum table t1, t2, t3, t4 extended;
 Table	Checksum
-test.t1	3092701434
-test.t2	3835700799
-test.t3	3835700799
+test.t1	2948697075
+test.t2	2948697075
+test.t3	2948697075
 test.t4	NULL
 Warnings:
 Error	1146	Table 'test.t4' doesn't exist

mysql-test/r/myisam.result

@@ -531,14 +531,14 @@ Error	1146	Table 'test.t3' doesn't exist
 checksum table t1, t2, t3;
 Table	Checksum
 test.t1	2948697075
-test.t2	3092701434
+test.t2	2948697075
 test.t3	NULL
 Warnings:
 Error	1146	Table 'test.t3' doesn't exist
 checksum table t1, t2, t3 extended;
 Table	Checksum
-test.t1	3092701434
-test.t2	3092701434
+test.t1	2948697075
+test.t2	2948697075
 test.t3	NULL
 Warnings:
 Error	1146	Table 'test.t3' doesn't exist

sql/sql_table.cc

@@ -4218,7 +4218,8 @@ bool mysql_checksum_table(THD *thd, TABLE_LIST *tables, HA_CHECK_OPT *check_opt)
 	    for (uint i= 0; i < t->s->fields; i++ )
 	    {
 	      Field *f= t->field[i];
-	      if (f->type() == FIELD_TYPE_BLOB)
+	      if ((f->type() == FIELD_TYPE_BLOB) ||
+                  (f->type() == MYSQL_TYPE_VARCHAR))
 	      {
 		String tmp;
 		f->val_str(&tmp);