Generation3 Copy With Image in Footer

[oleibman]

Fix #3126. A worksheet contained an image in its footer. It could be loaded and saved as another spreadsheet. However, if you tried to load and save that spreadsheet, PhpSpreadsheet would be unable to find the footer image and would therefore throw an exception.

This error was introduced a long time ago, in PhpSpreadsheet 1.3.0. The apparent cause of the problem was PR #435, sometime around June 2018. That change was very useful, but it had problems which exposed themselves only with a third generation copy. An additional contributor to the issue at hand was PR #1690 (December 2020), which again exposed itself with a third generation copy.

The issue from 1690 is easier to explain and deal with. It added a 'ps' suffix to printer settings resources in Xlsx Reader (to avoid name conflicts), but did not limit itself to a single addition (so subseqent generations would have multiple ps's). It also neglected to add the suffix in Reader/Xlsx/PageSetup.

As for 435, it loops through all the worksheet relationships, and uses the last that it finds as the base for header/footer drawings. It has been changed to use only the relationship whose rId matches the worksheet's legacyDrawingHF rId. It also needs a bit extra validation to make sure a drawing exists before adding it to its array of header/footer images. It also meant that Xlsx/Writer/Rels might write an entry with the same rId twice. I have also changed the header/footer image processing to be namespace aware (see PR #3137).

This is:

- [x] a bugfix
- [ ] a new feature
- [ ] refactoring
- [ ] additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate).
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