MDEV-23105 Cast number string with many leading zeros to decimal gives unexpected result

Skip leading zeros when converting a string to decimal_t.

mysql-test/r/type_newdecimal.result

@@ -2342,6 +2342,50 @@ t1	CREATE TABLE `t1` (
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 DROP TABLE t1;
 #
+# MDEV-23105 Cast number string with many leading zeros to decimal gives unexpected result
+#
+SELECT CAST(0000000000000000000000000000000000000000000000000000000000000000000000000000000020.01 AS DECIMAL(15,2)) as val;
+val
+20.01
+SET sql_mode='';
+CREATE TABLE t1 (a TEXT);
+INSERT INTO t1 VALUES (CONCAT(REPEAT('0',100),1));
+INSERT INTO t1 VALUES (CONCAT(REPEAT('0',100),1,'.0'));
+INSERT INTO t1 VALUES (CONCAT(REPEAT('0',100),1,'.9'));
+INSERT INTO t1 VALUES (CONCAT(REPEAT('0',100),1,'.99'));
+INSERT INTO t1 VALUES (CONCAT(REPEAT('0',100),1,'.994'));
+INSERT INTO t1 VALUES (CONCAT(REPEAT('0',100),1,'.995'));
+INSERT INTO t1 VALUES (CONCAT(REPEAT('0',100),1,'.999'));
+CREATE TABLE t2 (a TEXT, d DECIMAL(15,2));
+INSERT IGNORE INTO t2 (a,d) SELECT a, a FROM t1;
+Warnings:
+Note	1265	Data truncated for column 'd' at row 5
+Note	1265	Data truncated for column 'd' at row 6
+Note	1265	Data truncated for column 'd' at row 7
+INSERT IGNORE INTO t2 (a,d) SELECT CONCAT('-',a), CONCAT('-',a) FROM t1;
+Warnings:
+Note	1265	Data truncated for column 'd' at row 5
+Note	1265	Data truncated for column 'd' at row 6
+Note	1265	Data truncated for column 'd' at row 7
+SELECT d, a FROM t2 ORDER BY d,a;
+d	a
+-2.00	-00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001.995
+-2.00	-00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001.999
+-1.99	-00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001.99
+-1.99	-00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001.994
+-1.90	-00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001.9
+-1.00	-00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001
+-1.00	-00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001.0
+1.00	00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001
+1.00	00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001.0
+1.90	00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001.9
+1.99	00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001.99
+1.99	00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001.994
+2.00	00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001.995
+2.00	00000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000001.999
+DROP TABLE t1, t2;
+SET sql_mode=DEFAULT;
+#
 # End of 10.1 tests
 #
 #

mysql-test/t/type_newdecimal.test

@@ -1821,6 +1821,28 @@ CREATE TABLE t1 (a DECIMAL(2,1) DEFAULT 0.10001e0);
 SHOW CREATE TABLE t1;
 DROP TABLE t1;
 
+--echo #
+--echo # MDEV-23105 Cast number string with many leading zeros to decimal gives unexpected result
+--echo #
+
+SELECT CAST(0000000000000000000000000000000000000000000000000000000000000000000000000000000020.01 AS DECIMAL(15,2)) as val;
+
+SET sql_mode='';
+CREATE TABLE t1 (a TEXT);
+INSERT INTO t1 VALUES (CONCAT(REPEAT('0',100),1));
+INSERT INTO t1 VALUES (CONCAT(REPEAT('0',100),1,'.0'));
+INSERT INTO t1 VALUES (CONCAT(REPEAT('0',100),1,'.9'));
+INSERT INTO t1 VALUES (CONCAT(REPEAT('0',100),1,'.99'));
+INSERT INTO t1 VALUES (CONCAT(REPEAT('0',100),1,'.994'));
+INSERT INTO t1 VALUES (CONCAT(REPEAT('0',100),1,'.995'));
+INSERT INTO t1 VALUES (CONCAT(REPEAT('0',100),1,'.999'));
+CREATE TABLE t2 (a TEXT, d DECIMAL(15,2));
+INSERT IGNORE INTO t2 (a,d) SELECT a, a FROM t1;
+INSERT IGNORE INTO t2 (a,d) SELECT CONCAT('-',a), CONCAT('-',a) FROM t1;
+SELECT d, a FROM t2 ORDER BY d,a;
+DROP TABLE t1, t2;
+SET sql_mode=DEFAULT;
+
 --echo #
 --echo # End of 10.1 tests
 --echo #

strings/decimal.c

@@ -812,6 +812,24 @@ internal_str2dec(const char *from, decimal_t *to, char **end, my_bool fixed)
   while (s < end_of_string && my_isdigit(&my_charset_latin1, *s))
     s++;
   intg= (int) (s-s1);
+  /*
+    If the integer part is long enough and it has multiple leading zeros,
+    let's trim them, so this expression can return 1 without overflowing:
+      CAST(CONCAT(REPEAT('0',90),'1') AS DECIMAL(10))
+  */
+  if (intg > DIG_PER_DEC1 && s1[0] == '0' && s1[1] == '0')
+  {
+    /*
+      Keep at least one digit, to avoid an empty string.
+      So we trim '0000' to '0' rather than to ''.
+      Otherwise the below code (converting digits to to->buf)
+      would fail on a fatal error.
+    */
+    const char *iend= s - 1;
+    for ( ; s1 < iend && *s1 == '0'; s1++)
+    { }
+    intg= (int) (s-s1);
+  }
   if (s < end_of_string && *s=='.')
   {
     endp= s+1;