To install ffmpeg and mupdf on CI should be enough to add those lines to travis.yml:

addons: 
  apt: 
    sources: 
      - sourceline: 'ppa:mc3man/trusty-media' 
    packages: 
      - ffmpeg 
      - mupdf 

@dhh ah, so users are really meant to be able to do things like ActiveStore.previewers.prepend SpecificPreviewer? And not just append previewers?

I think this is a more widely-applicable observation for Active Storage, so may not be worth dealing with in this PR, but things like previewers need to go through config

I'm slightly surprised to see no warning about MuPDF's licensing. When I read this PR I hoped it was a more liberally licensed alternative to GhostScript for PDF processing, only to discover it's also Affero GPL. Unless the app developer buys a commercial license from Artifex, by using MuPDF they need to make their source code available per the license requirements. AFAIK most (if not all) Rails dependencies use MIT or similarly liberal licenses?

Not an expert on this topic,
Section 13 of AGPL

13. Remote Network Interaction; Use with the GNU General Public License.

Notwithstanding any other provision of this License, if you modify the Program, your modified version must prominently offer all users interacting with it remotely through a computer network (if your version supports such interaction) an opportunity to receive the Corresponding Source of your version by providing access to the Corresponding Source from a network server at no charge, through some standard or customary means of facilitating copying of software. This Corresponding Source shall include the Corresponding Source for any work covered by version 3 of the GNU General Public License that is incorporated pursuant to the following paragraph.

Notwithstanding any other provision of this License, you have permission to link or combine any covered work with a work licensed under version 3 of the GNU General Public License into a single combined work, and to convey the resulting work. The terms of this License will continue to apply to the part which is the covered work, but the work with which it is combined will remain governed by version 3 of the GNU General Public License.

My understanding, please correct me if I am wrong
Without a commercial license of mupdf

1. You can't distribute your rails application with mupdf bundled with less restrictive or closed source license

2. You can't change the source of code of mupdf and use it in your own rails app server(a typical saas app setup)

If you are using mupdf without altering the source and re distribution is not a concern, then you can you mupdf without a commercial license.

But still a warning about license will be great

I went thru AGPL meaning few years ago.

TL;DR (sourced from wiki - https://en.wikipedia.org/wiki/Affero_General_Public_License)

This provision requires that the full source code be made available to any network user of the AGPL-licensed work, typically a web application.

If you use mupdf in Rails app, any application user gains right to obtain Rails app source code.

Also https://choosealicense.com/licenses/agpl-3.0/ helps here:

Network use is distribution (Users who interact with the software via network are given the right to receive a copy of the source code.)

Disclose source (Source code must be made available when the software is distributed.)

IMHO @bdewater is right. It will be nice to find some alternative to mupdf to be optional dependency by default.

Seems like we are using mutool from shell here and it will be MereAggregation as per GPL terms, So I believe this usage won't make an issue in terms of licensing

Artifex is clear that use of MuPDF in a closed-source Rails application requires a commercial license. See their licensing page:

Generally speaking, “distribution” is the key word. Some examples of distribution/network use requiring a commercial license include:

[...]

Running Ghostscript or MuPDF on your network servers and permitting non-employees access to interact with these products as part of your non-AGPL application. Example: utilizing Ghostscript to interpret and display a PDF on a cloud application.

I have a version of the PDF previewer that uses Apache PDFBox working locally. The main drawback is that it requires Java. If there are no objections, I’ll push this change.

After some discussion, we’ve added a statement to the docs indicating that app developers should look into the licensing requirements that apply to them, since neither ffmpeg nor mupdf is released under the MIT License. This review of Artifex’s interpretation of the AGPL seems relevant.