Fix offset check for insert position overflow

[dbussink]

When multibyte characters are used, the behavior for the insert function is incorrect. It inserts a character even if the given position is beyond the input string. See the following example:

mysql> select insert('Å', 2, 1, 'a');
+--------------------------+
| insert('Å', 2, 1, 'a')   |
+--------------------------+
| Åa                       |
+--------------------------+
1 row in set (0.00 sec)

According to the documentation and if compared to the behavior for single byte characters, this should not have inserted the lowercase a.

The reason for this is that the code first steps for byte lengths with the following check:

  if ((start < 1) || (start > orig_len))
    return res;  // Wrong param; skip insert

In the case of a multibyte character, the start position won't be beyond the length since the byte length of the input string here is 3 bytes. This checks is a shortcut though and after this the correct offset is computed with charpos.

One thing overlooked here though is that the following changes the meaning of the start value:

  --start;  // Internal start from '0'

It's no longer a value starting with 1, but now with 0. This means that we can't do the same start > orig_len because now start will be one less.

Hence the check here after the multibyte aware position is calculated, should use start >= orig_len instead of start > orig_len.

A test for this bug is also added.

[mysql-oca-bot]

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Thanks

[dbussink]

I confirm the code being submitted is offered under the terms of the OCA, and that I am authorized to contribute it.

[mysql-oca-bot]

Hi, thank you for your contribution. Your code has been assigned to an internal queue. Please follow
bug http://bugs.mysql.com/bug.php?id=113962 for updates.
Thanks