In `wp_insert_term()`, clean up accidental duplicate terms after insert.

In [30056], the UNIQUE index was removed from the 'slug' column of `wp_terms`.
While we have numerous checks in place to avoid the creation of unwanted
duplicate term+term_taxonomy pairs, it's possible that in certain edge cases -
such as during the lag caused by database replication, or a race condition
involving near-simultaneous creation of more than one term - we'll end up
unwittingly inserting two identical rows.

The current changeset minimizes this risk by introducing a failsafe mechanism
into `wp_insert_term()`. After a term and term_taxonomy are INSERTed, we check
to see whether the term just created is a duplicate of an existing term; if so,
we delete the new one and keep the old one. This prevents problems caused by
replication lag, because SELECT queries that take place after an INSERT will
hit the master database; it mitigates race conditions by enforcing that the
term that was created first (ie, the one with the lowest `term_id`) is
always the "canonical" one.

Props nacin, markjaquith.
See #22023, #5809.

git-svn-id: https://develop.svn.wordpress.org/trunk@30238 602fd350-edb4-49c9-b593-d223f7449a82
Enforce ORDER BY and LIMIT clauses in `term_exists()` queries.

In case of edge cases where a duplicate term might exist in a replicated
database for a split second, these explicit query clauses ensure that
`term_exists()` will always recognize the oldest matched term as the
canonical one. See [30238] for background.

Props pento.
See #22023, #5809.

git-svn-id: https://develop.svn.wordpress.org/trunk@30239 602fd350-edb4-49c9-b593-d223f7449a82
Do not create shared taxonomy terms.

A "shared" term occurs when two entries in the `wp_term_taxonomy` table share a
single `term_id`, and thereby correspond to the same row in `wp_terms`. This
changeset stops the practice of creating shared terms: each new row in
`wp_term_taxonomy` will receive its own row in `wp_terms`. The new strategy
for term creation depends on whether the installation's database schema is up
to date for 4.1:

* If so, terms are allowed to be created with the same slug as an existing term, as long as they are in different taxonomies and do not share a parent. Thus, a new tag with the slug 'wordpress' can exist alongside a category with the slug 'wordpress'.
* If not, new terms will be forced to have unique slugs. Thus, on an installation containing a category with the slug 'wordpress', a new tag 'WordPress' will get the slug 'wordpress-2'.

Fixes #21950. See #5809.

git-svn-id: https://develop.svn.wordpress.org/trunk@30240 602fd350-edb4-49c9-b593-d223f7449a82
Split shared taxonomy terms during term update.

When updating an existing taxonomy term that shares its `term_id` with
another term, we generate a new row in `wp_terms` and associate the updated
term_taxonomy_id with the new term. This separates the terms, such that
updating the name of one term does not change the name of any others.

Note that this term splitting only occurs on installations whose database
schemas have been upgraded to version 30133 or higher. Note also that shared
terms are only split when run through `wp_update_term()`, as on edit-tags.php;
we will wait until a future release of WordPress to force the splitting of all
shared taxonomy terms.

Props boonebgorges, rmccue, greuben, garyc40, wonderboymusic, imath, jesin.
Fixes #5809.

git-svn-id: https://develop.svn.wordpress.org/trunk@30241 602fd350-edb4-49c9-b593-d223f7449a82