Pull Request for Issue #24535 (part).

Summary of Changes

This PR fixes all datetime columns of the com_tags database table #__tags so there will not be any Invalid value '0000-00-00 00:00:00' for datetime error anymore on MySQL 5.7 or later when strict mode is enabled.

Similar to my other PR's for the datetime/timestamp columns of other components, this PR makes columns publish_up, publish_down and checked_out_time nullable and have default value NULL.

In addition this PR removes the default values from columns created_time and modified_time. This will enforce to insert new records with values for these columns being provided and throw an SQL error if some of these values is not specified, i.e. such errors will not be hidden anymore.

Old data will be updated as little as possible. The created_time column will be not touched at all. We can assume that for core components there is no data with values '0000-00-00 00:00:00' on MySQL or '1970-01-01 00:00:00' on PostgreSQL, and data created by 3rd party components should not be modified. The modified_time column will be set to the value of the created_time column only if it has value '0000-00-00 00:00:00' on MySQL or '1970-01-01 00:00:00' on PostgreSQL. Our PHP already sets the modified time to the created time when saving new records, i.e. modified = created means never modified.

Testing Instructions

Testers please report back the database kind (MySQL or PostgreSQL) on which you have tested.

If you have both MySQL and PostgreSQL, please test on both if possible.

Test 1: New installation

1. Apply the patch on a clean 4.0-dev branch using git or merging manually, or apply it on an installation of current 4.0-dev using patchtester.
2. If used patchtester on an existing installation in step 1, delete file configuration.php and delete all Joomla database tables in PhpMyAdmin or PhpPgAdmin (depending on your database type).
3. Do a new installation, login to backend, confirm the statistics dialog, go to global config and set error reporting to maximum or development in server settings.
4. Play around with tags, create some, modify some, filter in the list display, ...
5. After any action, check in your database the relevant datetime/timestamp without timezone columns.

Result: See section "Expected result" below.

Test 2: Update sql script

1. Install a clean clean 4.0-dev, login to backend, confirm the statistics dialog, go to global config and set error reporting to maximum or development in server settings.
2. Apply the changes from this PR e.g. manually or with patch tester.
3. Open PhpMyAdmin or PhpPgAdmin (depending on your database type), select your database and then go to the SQL command input.
4. On MySQL copy the first line of file installation/sql/mysql/joomla.sql into the SQL command window but don't execute the commands yet:

SET SQL_MODE = "NO_AUTO_VALUE_ON_ZERO";

This switches off strict mode to the SQL will run on MySQL 5.7 or later.

5. Copy the content of file administrator/components/com_admin/sql/updates/mysql/4.0.0-2019-09-27.sql or administrator/components/com_admin/sql/updates/postgresql/4.0.0-2019-09-27.sql (depending on your database type) into the SQL command window, in case of MySQL below the previously pasted commands, but don't execute the commands yet.
6. Replace #__ by your database prefix in the SQL statements pasted before in the SQL input window.
7. Put the cursor to the beginning of the 1st SQL statement in the SQL input window and now execute all SQL commands.
8. Play around with tags, create some, modify some, filter in the list display, ...
9. After any action, check in your database the relevant datetime/timestamp without timezone columns.

Result: See section "Expected result" below.

Expected result

New Installation

Tags work as well as without this PR. In a MySQL database there are no columns of data type datetime having value '0000-00-00 00:00:00' in table #__tags, and there is no invalid default value anymore in MySQL >= 5.7 with strict mode on. On PostgreSQL there are no columns of data type timestamp without time zone having value '0000-00-00 00:00:00' in this table.

There is one exception: When checking in a tag using com_checkin, the checked_out_time is set to '0000-00-00 00:00:00' on MySQL and '1970-01-01 00:00:00' on PosgreSQL. This will be changed with a separate, future PR for com_checkin. Checking in an item with the lock icon in list display works, i.e. there checked_out_time is set to NULL.

Update sql script

The statements are processed without error. The expected result is the same as for a new installation.

Actual result

New Installation

On MySQL same as expected, but the default value of database columns of data type datetime having value '0000-00-00 00:00:00' in table #__tags is invalid in MySQL >= 5.7 with strict mode on, and there might be values '0000-00-00 00:00:00'. On postgreSQL same as expected, but there might be values '1970-01-01 00:00:00' in this table.

Documentation Changes Required

Maybe core developer docs and extension developer docs should be updated to encourage them not to use '0000-00-00 00:00:00' on MySQL anymore but use real NULL and not abuse '1970-01-01 00:00:00' on PostgreSQL as a speudo null date anymore and use real NULL values also there.