[Performance] Reuse preview directory listing

[Glandos]

Superseed #34910

Use directory listing in both functions to gain 25% speed on run where every preview already exist.

I went from "./occ preview:generate-all" from 159 seconds down to 17 seconds on a directory full of images with already generated previews.

[artonge]

Am I right that this PR "only" moves the getDirectoryListing call from getSmallImagePreview to generatePreviews?
All other code changes are to:

• pass previewFiles to all the methods
• reuse some logic to make getSmallImagePreview smaller with getCachedPreview and generateProviderPreview

[Glandos]

Am I right that this PR "only" moves the getDirectoryListing call from getSmallImagePreview to generatePreviews? All other code changes are to:

* pass `previewFiles` to all the methods

* reuse some logic to make `getSmallImagePreview` smaller with `getCachedPreview` and `generateProviderPreview`

Yes, the rationale is that calling getDirectoryListing multiple times is slow and useless, since the directory won't change between two invocation, given the proximity of the code. So instead of passing previewDirectory, the list, output from getDirectoryListing($previewDirectory) is passed along.

I've noticed that getSmallImagePreview was added with Imaginary support, but the code was duplicated. Since my modifications modify the code, I gathered them in one function.

[PVince81]

👍 nice one!

[PVince81]

@Glandos can you rebase ? our bot cannot rebase on forks

this should help bring the branch up to date and help wake up the CI jobs that got stuck

[Glandos]

@PVince81 commented on Jan 11, 2023, 9:01 PM GMT+1:

@Glandos can you rebase ? our bot cannot rebase on forks

this should help bring the branch up to date and help wake up the CI jobs that got stuck

Originally posted by @PVince81 in #35129 (comment)

This is now rebased on current master.

[juliushaertl]

Failures seemed unrelated but restarted cypress to be sure ;)

[PVince81]

hmmm, is background rendering using the previews endpoint ?

 /home/runner/work/server/server/cypress/snapshots/actual/theming/user-background     (1280x720)
     .cy.ts/User default background settings -- Default cloud background is not rende               
     red (failed).png