Improve handling of URLs and location redirects in AMP

[westonruter]

• Automatically redirect from /amp/ endpoint to ?amp when amp theme support is present. This is important for plugins to be able to reliably call is_amp_endpoint(). See Discontinue use of amp endpoint in favor of query var when amp theme support is present #1148.
• Add amp_get_current_url() helper function.
• Let amp_add_amphtml_link() replace the poorly-named amp_frontend_add_canonical() function and improve how it is referenced.
• Fix old_slug_redirect_url handling for AMP theme support. This ensures that AMP URLs redirect properly when changing a post's slug.
• Fix handling of re-validating URLs when redirects happen or post slugs change. Fix validation when switching between native and paired modes. Only store canonical URLs for amp_invalid_url posts.
• Restore passing of current user cookies when validating a URL to access private/drafted posts.
• Increase timeout for validate_url requests; show errors when fetching fails
• Fix issue with get_current_screen() not returning an object.

Fixes #1166.

[gravityrail]

Automatically redirect from /amp/ endpoint to ?amp when amp theme support is present. This is important for plugins to be able to reliably call is_amp_endpoint().

I'm not able to reproduce this behaviour. I tried with/without add_theme_support( 'amp' ), with/without trailing /amp/, and I wasn't able to trigger a redirect to ?amp. I see that you have hooked your link redirection code to the filter old_slug_redirect_url but I don't see anywhere in your branch that that filter is called - maybe there's a change missing from this PR?

[westonruter]

@gravityrail humm, the old_slug_redirect_url is for a different case for when the slug changes. The redirection from /amp/ to ?amp is performed here in the ensure_proper_amp_location method:

https://github.com/Automattic/amp-wp/blob/236be4babd249e27b98a60526355d036ef4872d4/includes/class-amp-theme-support.php#L166-L206

I just double-checked and it's working for me:

$ curl -i https://src.wordpress-develop.test/2018/06/06/initial333/amp/
HTTP/1.1 302 Found
Location: https://src.wordpress-develop.test/2018/06/06/initial333/?amp

😕

[gravityrail]

Ah! Now I'm able to reproduce the desired behaviour.

The trick is to enable paired mode, which requires specifying a theme directory to AMP-ify:

add_theme_support( 'amp', array( 'template_dir' => './' ) );

Then you ALSO need to make sure you accept any validation errors that have been generated (otherwise it always redirects from AMP to non-AMP versions of page)

[gravityrail]

This works :)

[artkanna]

Hi! Will Fix old_slug_redirect_url handling for AMP theme support retroactively fix amp redirects for pages that were previously redirected - before the release? Thank you!

[westonruter]

@artkanna Can you give an example of pages that were previously redirected?