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Emacs buildpack for Heroku

This is a Heroku buildpack which installs GNU Emacs version 26.1.


To install Emacs for your Heroku app called <myapp>, run:

$ heroku buildpacks:add                                 \  \
    -a <myapp>

After the next time you deploy your app, emacs will be available on the PATH.

How does it work?

Ideally, we could just use heroku-buildpack-apt to install Emacs. Unfortunately, this doesn't work, because the Ubuntu package for Emacs is configured to be installed in /usr, while Heroku apps are run from /app (and therefore any packages accompanying an app must live inside /app).

You might think that we could just compile Emacs from source instead. However, Heroku uses Docker, and you can't compile Emacs inside a Docker container without changing certain security options that Heroku doesn't support.

The solution? I compile Emacs myself inside a Docker container based on the Heroku image, running with the necessary security options on my own computer, install Emacs to /app in the container, and then copy out the tarball to upload to GitHub releases on this repository. (Why GitHub Releases? It's an easy way to host a large binary which doesn't require the cooperation of another service.) At deploy time, the buildpack just downloads and extracts this tarball. Simple!

Here is how I compile Emacs to obtain the tarball you see on GitHub Releases:

$ docker run -it --security-opt seccomp=unconfined heroku/heroku:18-build
$ cd /tmp
$ wget
$ tar -xf emacs-26.1.tar.xz
$ pushd emacs-26.1
$ ./configure --without-all --without-x --with-gnutls=yes --prefix=/app/emacs
$ make -j9
$ make install-strip prefix=/app/emacs
$ popd
$ tar -C /app/emacs -cf emacs-for-heroku-26.1.tar.gz .
$ exit
$ docker cp <container-id>:/tmp/emacs-for-heroku-26.1.tar.gz <destination>


This repository was originally forked from kosh04/heroku-buildpack-emacs, but since then literally all of the code and documentation was rewritten, so I marked it as a source repository and added a license.

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