Remove bundled theme files from $_old_files

[SergeyBiryukov]

Per @desrosj in comment:4:ticket:56934:

I'm wondering about including the deleted theme files here. Except 6.0, I don't see any additional wp-content/themes/* entries in the list.

As far as I know, themes are not included with update packages unless it's the first release being included (and they are not intentionally skipped with the CORE_UPGRADE_SKIP_NEW_BUNDLED constant).

...

In my testing, it seems that files within wp-content/themes/* are not flagged during Core upgrades and can probably be left out from this list.

Based on the above, it looks like bundled theme files should be removed from the list and a note should be added to clarify that for future updates.

This PR:

• [x] Adds a note not to include bundled theme files in $_old_files.
• [x] Removes bundled theme files entries.

Trac ticket: https://core.trac.wordpress.org/ticket/56936

[costdev]

​PR 3632 is ready for testing and review.

[desrosj]

I've updated the ​Dry Run section of the handbook to note that theme files need not be noted in $_old_files.

[desrosj]

I did some more testing before making this update and wanted to detail what I found which confirms that these files should be excluded.

Including bundled theme files in the $_old_files list could actually break sites when Core is set to auto-update major versions and the theme with a file included in the list has auto-updates off.

The reason is that all files in the list are deleted when Core updates, regardless of where they are located. So if an earlier version of the theme depends on a file removed in a future version, and that file is listed in $_old_files, it will be deleted and sites with this theme active will potentially break.

This scenario has thankfully not yet happened because the files that were included in this list so far were only license files and SASS files.

[desrosj]

In 54849:

Upgrade/Install: Remove bundled theme files from $_old_files.

Because themes are updated independently of Core updates, any deleted files from bundled themes should not be included in the $_old_files list.

Any file included in this list is deleted on update, which could cause problems for sites with a given theme active if the removed files were required in earlier versions of that theme and that theme is not updated at the same time.

Props desrosj, costdev, SergeyBiryukov.
Fixes #56936.

[desrosj]

I think this is worth consideration for backporting to the 6.1 branch, but don't feel strongly about that.

Backporting to the 6.0 branch is unnecessary.

Merged in https://core.trac.wordpress.org/changeset/54849.

[SergeyBiryukov]

Replying to desrosj:

I think this is worth consideration for backporting to the 6.1 branch, but don't feel strongly about that.

I don't feel strongly either, but it's a straightforward fix and backporting would not hurt, so might as well do that :)

[desrosj]

In 54966:

Upgrade/Install: Remove bundled theme files from $_old_files.

Because themes are updated independently of Core updates, any deleted files from bundled themes should not be included in the $_old_files list.

Any file included in this list is deleted on update, which could cause problems for sites with a given theme active if the removed files were required in earlier versions of that theme and that theme is not updated at the same time.

Props desrosj, costdev, SergeyBiryukov.
Merges [54849] to the 6.1 branch.
Fixes #56936.

[SergeyBiryukov]

Moving back to 6.2, as there are no plans for 6.1.2 at this time.