Implement configuration for automatic local bundle loading.

[crlf0710]

This commit adds an optional local parameter in config.toml, when specified as true, tectonic will try to load bundle from a zip file path specified in the usual url parameter.

[pkgw]

Thanks for submitting this as well!

I like the general idea of this PR, but having a boolean local flag that causes the url field to be treated differently seems like a design that would be surprising and confusing for users. How about an approach where file:// URLs are loaded as local Zip bundles?

[crlf0710]

Ok, so i changed it to try to detect an file:// prefix, if the prefix exists, it strips it off and try to load the rest as a path to a zip file. On my Windows machine this works fine.

[codecov]

Codecov Report

Merging #211 into master will decrease coverage by <.01%.
The diff coverage is 7.4%.

@@            Coverage Diff             @@
##           master     #211      +/-   ##
==========================================
- Coverage    37.9%   37.89%   -0.01%     
==========================================
  Files         131      131              
  Lines       60307    60339      +32     
==========================================
+ Hits        22857    22864       +7     
- Misses      37450    37475      +25

Impacted Files	Coverage Δ
tests/tex-outputs.rs	83.5% <100%> (+0.52%)	⬆️
src/config.rs	11.9% <3.84%> (+0.99%)	⬆️

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[crlf0710]

@pkgw Please review this when you have time. What can i do to improve code coverage in this case?

[pkgw]

I am a little concerned about URL encoding here. A proper URL might contain a sequence like %20 to encode a space, but this approach won't convert it as appropriate. I think the solution here is to parse the URL for real — Tectonic already depends on the url crate, so that should be used. Then you can check if the scheme is file, and use the handy to_file_path function.

[pkgw]

OK, this looks very good. There is one change I think should be made to make the URL parsing more correct but based on my read of the relevant API docs, I think it should be easy to implement.

It might be hard to boost the code coverage here. A small unit test could maybe cover how different URLs are parsed, but since these functions want to do the actual I/O, I suspect that one would need to build a lot of infrastructure to have a test cover all of the code. It is OK if you can't figure out a way to get code coverage for your changes. If you can devise a small new test case that increases coverage in some other part of the codebase, even if it is totally unrelated to this change, that would be great, but I'll be happy to merge this even if the coverage doesn't go up.

[pkgw]

Thanks for addressing this! I had to merge in changes from another PR to master, but will merge this once the CI passes.

[pkgw]

OK, as expected, code coverage remains a challenge, but let's get this merged.