Don't Double URL Query Args in Canonical Redirect

[filosofo]

Currently, when using "default" permalinks and requesting something that triggers canonical redirect, the query portion of the request URL can double up the same request arguments, which leads to redirect_canonical()'s returning falsely different URLs.

In other words, suppose you have a custom post type of custom_post_thing with an object that has a slug a-custom-post-thingy-title and an ID of 123.

If you request /?p=123 redirect_canonical() figures out that its correct, canonical URL is actually

/?custom_post_thing=a-custom-post-thingy-title

However, when redirect_canonical() is called again at line 362, it appends the same query value to the fetched URL, like so:

/?custom_post_thing=a-custom-post-thingy-title&custom_post_thing=a-custom-post-thingy-title

The result is that

/?custom_post_thing=a-custom-post-thingy-title&custom_post_thing=a-custom-post-thingy-title

and

/?custom_post_thing=a-custom-post-thingy-title

differ as strings even though they're the same in actual meaning, and the redirect fails and a 404 results.

(I have seen this happen in other redirect situations--it's not limited to custom post types.)

The patch parses the redirect URL's query value and uses add_query_arg() to add it, rather than the current naive string concatenation. The result is that arguments do not get doubled, and the redirect succeeds.

[filosofo]

It's also worth pointing out that the current behavior results in differing behavior between default permalinks and pretty permalinks. With pretty permalinks canonical will correctly redirect a request like /?p=123 above; with the default ones, it fails to redirect and produces a 404.

[filosofo]

Note: #15775 uses the patch here. Committing #15775 will fix this ticket as well.

[markjaquith]

(In [17030]) Do not add URL query args twice in certain Canonical Redirect situations. Use add_query_arg(). props filosofo. fixes #15774