Allow resources to be fetched from another location

[azerupi]

Some resources are loaded through a CDN if possible with a local fallback. An option to deactivate the CDNs when rendering is wanted. See #46

An option could be given in the config file, but I think it would fit better as a CLI argument.
On the template side, we could add a cdn variable that evaluates to true or false and use if ... tags in handlebars.

[tjis]

Ideally, we have more flexibility than just a true or false for cdn. When resources cannot be sourced through a cdn, they'll still need to be found in another way.
In debian, we ship javascript libraries in separate packages, which are then symlinked or copied. mdbook cannot have any knowledge of this process though, so I imagine we'll need to configure our intentions somehow.
A simple solution would be to make the resource url configurable somewhere, but default to the cdn.

[azerupi]

So I am a little lost with what would be actually required. Currently, the strategy is to fetch from a CDN to have cache benefits and parallel loading. If the CDN fails we send the local version. This strategy does not really benefit books that are not hosted online though. But I am not sure it hurts either.

If someone could clearly formulate the problem that needs to be solved and what they expect mdBook to do or allow, that would help us immensely! :)

[tjis]

Alright. The problem is as follows.
Debian packages aren't supposed to make network connections without the user's consent. Also, packages aren't supposed to contain embedded resources that could be provided by other packages instead. Therefore, both the CDN and the fallback are problematic for Debian.
We are running into this issue while packaging rust, which uses mdBook to build the various rust books. In order to make the generated books comply with debian policy, we strip the embedded resources from mdBook and alter the templates so that the generated documentation complies.
But this is far from ideal. Not only might future mdBook changes break our present patch, it also means we have to ship a patched copy of mdBook with the rust source package, rather than using a pristine packaged version of mdBook and build-depend on that.

The solution to our problem would be if mdbook could be supplied with alternative means to retrieve third-party dependencies, rather than having the current CDN+fallback method hardcoded. Then debian could ship rust with the books configured to use the debian locations.

If this is something your project is willing to consider, I could provide a patch and we could work from there.

[azerupi]

Ooh I see now, thank you for the explanation!
I would totally be ok with adding an option to allow this. I am not sure if this is common enough to put a key in the config file, but controlling this with environment variables seems like a good plan to begin with. :)

[Michael-F-Bryan]

I've created a new PR (#541) for updating config values using environment variables which is the first step required for fixing this issue. The HTML renderer needs to be cleaned up and restructured now that #507 has landed, so hopefully the second half of this issue (the concept of a configurable Resource) will just "fall out".