Feed (with comments) Last-Modified header only considers latest comment date

[xiven]

Steps to reproduce:

1. Create a wordpress blog
2. Add a post
3. Add a comment to that post
4. Wait a while
5. Add another post
6. Check the Last-Modified header of the blog's feed

Expected result:
The Last-Modified header should have the date/time of the post added in step 5.

Actual result:
The Last-Modified header has the date/time of the comment added in step 3. This behaviour breaks caching and causes feed readers to believe there is no new content even when there might be new posts.

Analysis:
The fault lies in the send_headers function of class-wp.php. For a feed including comments it should use the newest date from both the get_lastcommentmodified and the get_lastpostmodified functions instead of only using the result from the get_lastcommentmodified function.

[mauteri]

Updated patch with code adjustment and unit test.

[hellofromTonya]

5.9 Beta 1 is happening in less than 30 minutes. I'm sorry this ticket didn't get a review and testing in time. As it's a bug that was not introduced in 5.9 cycle, moving it to 6.0 to give it the time and attention it deserves.

[costdev]

Updated patch with some tidying up. Splits tests, adds documentation, @covers annotations, $message parameters.

Trac ticket: https://core.trac.wordpress.org/ticket/47968

[audrasjb]

The above patch looks good to me.
Added a PR to run unit tests on those changes.

[audrasjb]

Tests are passing, self assigning for commit.

[audrasjb]

In 53233:

Feeds: Use latest comment date for the Last-Modified header of comments feed.

Previously, the Last-Modified header of comments feed was using the date/time of the last comment. This behavior was breaking caching and causing feed readers to believe there is no new content even when there might be new posts.

This commit changes this behavior to use the newest date from both get_lastcommentmodified() and get_lastpostmodified() functions instead of only using the result from get_lastcommentmodified().

Props xiven, mauteri, costdev, audrasjb.
Fixes #47968.

Committed in https://core.trac.wordpress.org/changeset/53233

[SergeyBiryukov]

There's a typo in the latest assertion: "...date of the most recent most".

[audrasjb]

ah thanks! I'll fix this right now.

[audrasjb]

In 53241:

Build/Test Tools: Typo correction in rss2 unit tests.

Follow-up to [53233].

Props SergeyBiryukov.
Fixes #47968.