[imorland]

Fixes: no issue filed.

Changes proposed in this pull request:

#5319 lets a theme set the forum's logos and icons, but only as fixed files. A theme with switchable brand sets, seasonal ones for example, still has to write the image settings itself, where the admin can overwrite them. And while a theme sets an image, code reading the image settings directly still gets the admin's old upload.

• Callables: every image argument of Extend\Theme (logo(), logoDarkMode(), emailLogo(), favicon() and its SVG, appIcon() and its background) also accepts a callable that returns one, with its parameters resolved from the container. Returning null leaves the image to the admin. Returning a different file draws the image again.
• The image settings: while a theme sets an image, logo_path, logo_dark_mode_path, favicon_path, email_logo_path and email_logo_size hold the drawn file, when it is the kind of file an upload stores there. An SVG logo leaves logo_path as it was. The admin's values come back when the theme stops setting the image. fof/seo, fof/pwa and ianm/twofactor read these settings.
• Replaced files are kept for 30 days, for cached pages, the cached manifest and /favicon.ico redirect, and emails already sent, and taken back if the image returns to them. They were deleted at once.
• Signatures use file contents, not the absolute path and modification time, so servers deployed separately agree on what was drawn instead of drawing over each other.

Docs: flarum/docs#694

Reviewers should focus on:

• Callables run each time ThemeImages is built, about twice per forum page, so they should be cheap and give the same answer within a request.
• has() now brings the stored state up to date first. Nothing did once a theme stopped setting every image, so its files were never removed.
• A file's hash is kept in the cache store, keyed by its path, size and modification time, so the file is only read again when one of those changes.
• The theme_images state format changed. It is unreleased, so a 2.x snapshot install just draws its theme images once more.
• ThemeImagesTest allows repeated settings writes: drawing writes each image's settings once.

Necessity

• Has the problem that is being solved here been clearly explained?
• If applicable, have various options for solving this problem been considered?
• For core PRs, does this need to be in core, or could it be in an extension?
• Are we willing to maintain this for years / potentially forever?

Confirmed

• Frontend changes: tested on a local Flarum installation.
• Frontend changes: tests are green (run yarn test in js/).
• Frontend changes: tests have been added, or are not appropriate here.
• Backend changes: tests are green (run composer test).
• Backend changes: tests have been added, or are not appropriate here.
• Where applicable, changes are suitable for all supported database drivers (MySQL, MariaDB, PostgreSQL, SQLite). (No database changes.)
• Core developer confirmed locally this works as intended.