tests: network state (online/offline) affects epubcheck unit test outcome.

[jayaddison]

Describe the bug

The test_run_epubcheck unit test case passes as expected when:

• Java is considered runnable.
• The epubcheck[.jar] program is found.
• The DO_EPUBCHECK environment variable is set (the value does not matter).
• It can download a remote image contained in the test-root/images.txt file.

(note: we do enable DO_EPUBCHECK in our continous integration in the latex workflow -- and epubcheck is installed into the relevant sphinx-ci container image)

There are a few quirks here: I think that the test behaving differently offline isn't desirable though (or to phrase it another way: connectivity shouldn't be a requirement for most dependable software to test/build, because it's hard to predict when/where it will be rebuilt).

How to Reproduce

Ensure that epubcheck and the local development and test dependencies for sphinx are installed, and then disable network connectivity on the unit test host.

From the sphinx.git checkout directory:

$ DO_EPUBCHECK=1 pytest tests/test_builders/test_build_epub.py -k epubcheck

Environment Information

Platform:              linux; (Linux-6.6.15-rt-amd64-x86_64-with-glibc2.37)
Python version:        3.11.8 (main, Feb  7 2024, 21:52:08) [GCC 13.2.0])
Python implementation: CPython
Sphinx version:        7.3.0+/98d0cc860
Docutils version:      0.20.1
Jinja2 version:        3.1.3
Pygments version:      2.17.2


`epubcheck` v4.2.6 (Debian-packaged)

Sphinx extensions

N/A

Additional context

No response

[jayaddison]

Output from epubcheck (dir paths filtered):

Validating using EPUB version 3.2 rules.
Messages: 0 fatals / 2 errors / 1 warning / 1 info

EPUBCheck completed

ERROR(OPF-014): /tmp/.../root/_build/epub/SphinxTests.epub/images.xhtml(-1,-1): The property "remote-resources" should be declared in the OPF file.
ERROR(RSC-006): /tmp/.../root/_build/epub/SphinxTests.epub/images.xhtml(28,120): Remote resource reference not allowed; resource must be placed in the OCF.
WARNING(NAV-011): /tmp/.../root/_build/epub/SphinxTests.epub/nav.xhtml(303,60): "toc" nav must be in reading order; link target "autodoc.xhtml#autodoc_target.Outer" is before the previous link’s target in document order.
INFO(INF-001): /tmp/.../root/_build/epub/SphinxTests.epub/nav.xhtml(303,60): The previous rule is under review and its severity may change in a future release. See the discussion at https://github.com/w3c/publ-epub-revision/issues/1283