relative image xlink:href export to odg broken

[Thomas Holder]

Regression of "Export to odg not work in 0.91 and 0.92"
https://bugs.launchpad.net/inkscape/+bug/1654034

Observed on Arch Linux and macOS 10.12

Inkscape versions:
- master branch: 0.92+devel (6bcef54175, 2018-09-21, custom)
- 0.92.x branch: 0.92.3 (549ef9d338, 2018-09-22, custom)

Steps to reproduce:
1) Create SVG with <image> and relative xlink:href (image not in the current directory)
2) File > Save As... > OpenDocument drawing

Result:
- During export, prints "Could not load image file '...'"
- Image missing from ODG file

The fix for bug 1654034 removed the relative link handling logic:
https://gitlab.com/inkscape/inkscape/commit/d61e166a900e32e6f1640c7c26b1a8a66826e51d

[Thomas Holder (speleo3)]

I think I falsely blamed the bug 1654034 fix, I'm sorry. Reverting the fix doesn't help, and I can reproduce it also with "Inkscape 0.92.2 2405546, 2018-03-11" (from Arch Linux repo).

[Patrick Storz (ede123)]

Is the relative directory a parent directory or a child directory for you?

On Windows I'm seeing the following behavior:
- If the directory is a child directory a relative xlink:href is stored.
  Embedding the file into ODG works as expected.
- If the directory is a parent directory Inkscape only ever saves an
  absolute xlink:href that looks like 'xlink:href="file:///E:/file.png"'.
  Embedding the file into ODG works as expected.
- If the directory is a parent directory and I manually replace the
  xlink:href with a relative one like 'xlink:href="../file.png"'
  embedding the file into ODG works as expected.

So I'm actually *not* able to reproduce this issue on Windows.

That said I put quite some effort into fixing file lookups on Windows around the time I fixed bug #1654034 as the logic seldomly assumed Windows-style paths to be a possibility and could get a lot of them working (although a few border cases are still problematic).
As I didn't test on any other OS I wouldn't rule out some issues remain for those...

[Thomas Holder (speleo3)]

Thanks for testing, Patrick!

I'm attaching a simple test case. It makes sure that the value of xlink:href is not valid as a relative path from the current working directory.

- cd workdir
- inkscape ../datadir/test.svg
- File > Save a Copy... > OpenDocument drawing (*.odg)

As far as I can tell, it was already broken before your changes. I think it used "Inkscape::URI::getFullPath" wrong. I've looked at the Inkscape::URI API but couldn't find appropriate functions yet for this use case.

[Thomas Holder (speleo3)]

might be useful to look at
https://gitlab.com/inkscape/inkscape/blob/master/src/xml/rebase-hrefs.cpp

[Thomas Holder (speleo3)]

Patch:
https://gitlab.com/inkscape/inkscape/merge_requests/333

Attaching a more sophisticated test case which picks up a wrong image (workdir/subdir/triangle.png) without the patch. Also contains an absolute path (file:///tmp/bug-1793940-testcase/datadir/subdir/absolute.png), adjust that before testing.