rails-latex gem: ActionView::Template::Error (rails-latex failed ...) with Heroku - ruby-on-rails

Question

Rails Version: Rails 6.0.2.1
Ruby Version: ruby 2.6.3p62 (2019-04-16 revision 67580) [x86_64-linux]
When I try to generate a PDF with the gem rails-latex in Heroku, I get the following error: ActionView::Template::Error rails-latex failed (Full error message below).
This error does not arise locally.
What I tried
There is already a stackoverflow-question with the same problem: Debugging rails-latex gem on Heroku | ActionView::Template::Error rails-latex failed
#blackbrokkoli writes:
If you have a similar problem, I recommend deleting all tex
dependencies, get a MWE and then slowly add them back in to identify
the evildoers.
But I don't know what he means with "MWE" and I doubt that I am able to "identify the evildoers" by myself.
There is a thread called "Deploying on Heroku" suggesting that one needs to install a buildpack.
I tried to follow the instructions for the "Multipacks"-option.
The command cat .buildpacks returns
cat: .buildpacks: No such file or directory
The command heroku config:add BUILDPACK_URL=git://github.com/ddollar/heroku-buildpack-multi.git returns:
Setting BUILDPACK_URL and restarting
BUILDPACK_URL: git://github.com/ddollar/heroku-buildpack-multi.git
But after that, I don't know how to proceed further.
I tried to figure out how to add buildpacks with Heroku with the heroku buildpacks:add command. But I couldn't figure out, how I can add a third-party buildpack with this command.
Does anyone have any suggestions on how to fix this error?
Error message in heroku logs
2020-02-07T05:50:55.909360+00:00 app[web.1]: I, [2020-02-07T05:50:55.909264 #4] INFO -- : [1c4e2355-293c-4cb1-b591-b97e4ba4ecf3] Started GET "/articles/2.pdf" for 87.245.109.31 at 2020-
02-07 05:50:55 +0000
2020-02-07T05:50:55.910207+00:00 app[web.1]: I, [2020-02-07T05:50:55.910130 #4] INFO -- : [1c4e2355-293c-4cb1-b591-b97e4ba4ecf3] Processing by ArticlesController#show as PDF
2020-02-07T05:50:55.910257+00:00 app[web.1]: I, [2020-02-07T05:50:55.910202 #4] INFO -- : [1c4e2355-293c-4cb1-b591-b97e4ba4ecf3] Parameters: {"id"=>"2"}
2020-02-07T05:50:55.913042+00:00 app[web.1]: D, [2020-02-07T05:50:55.912964 #4] DEBUG -- : [1c4e2355-293c-4cb1-b591-b97e4ba4ecf3] Article Load (1.1ms) SELECT "articles".* FROM "articl
es" WHERE "articles"."id" = $1 LIMIT $2 [["id", 2], ["LIMIT", 1]]
2020-02-07T05:50:55.914866+00:00 app[web.1]: I, [2020-02-07T05:50:55.914789 #4] INFO -- : [1c4e2355-293c-4cb1-b591-b97e4ba4ecf3] Rendering articles/show.pdf.erb within layouts/applica
tion
2020-02-07T05:50:55.915357+00:00 app[web.1]: I, [2020-02-07T05:50:55.915290 #4] INFO -- : [1c4e2355-293c-4cb1-b591-b97e4ba4ecf3] Rendered articles/show.pdf.erb within layouts/applicat
ion (Duration: 0.4ms | Allocations: 101)
2020-02-07T05:50:55.916214+00:00 app[web.1]: I, [2020-02-07T05:50:55.916141 #4] INFO -- : [1c4e2355-293c-4cb1-b591-b97e4ba4ecf3] Running 'pdflatex -halt-on-error -shell-escape -interact
ion=batchmode input' in /app/tmp/rails-latex/4-521112460259582216 1 times...
2020-02-07T05:50:55.923855+00:00 app[web.1]: I, [2020-02-07T05:50:55.923739 #4] INFO -- : [1c4e2355-293c-4cb1-b591-b97e4ba4ecf3] Running 'bibtex input' in /app/tmp/rails-latex/4-5211124
60259582216 1 times...
2020-02-07T05:50:55.929983+00:00 app[web.1]: I, [2020-02-07T05:50:55.929846 #4] INFO -- : [1c4e2355-293c-4cb1-b591-b97e4ba4ecf3] Running 'pdflatex -halt-on-error -shell-escape -interact
ion=batchmode input' in /app/tmp/rails-latex/4-521112460259582216 2 times...
2020-02-07T05:50:55.936216+00:00 app[web.1]: I, [2020-02-07T05:50:55.936104 #4] INFO -- : [1c4e2355-293c-4cb1-b591-b97e4ba4ecf3] Completed 500 Internal Server Error in 26ms (ActiveRecor
d: 1.1ms | Allocations: 1610)
2020-02-07T05:50:55.937843+00:00 app[web.1]: F, [2020-02-07T05:50:55.937756 #4] FATAL -- : [1c4e2355-293c-4cb1-b591-b97e4ba4ecf3]
2020-02-07T05:50:55.937845+00:00 app[web.1]: [1c4e2355-293c-4cb1-b591-b97e4ba4ecf3] ActionView::Template::Error (rails-latex failed: See /app/tmp/rails-latex/4-521112460259582216/input.l
og for details):
2020-02-07T05:50:55.937845+00:00 app[web.1]: [1c4e2355-293c-4cb1-b591-b97e4ba4ecf3] 15:
2020-02-07T05:50:55.937846+00:00 app[web.1]: [1c4e2355-293c-4cb1-b591-b97e4ba4ecf3] 16: \end{document}
2020-02-07T05:50:55.937846+00:00 app[web.1]: [1c4e2355-293c-4cb1-b591-b97e4ba4ecf3]
2020-02-07T05:50:55.937847+00:00 app[web.1]: [1c4e2355-293c-4cb1-b591-b97e4ba4ecf3] app/views/layouts/application.pdf.erbtex:18
#Syed Asim Hashmi suggested to use this command to install a third-party buildpack:
heroku buildpacks:add git://github.com/Thermondo/heroku-buildpack-tex.git#VERSION_NUMBER
This is what I get:
heroku buildpacks:add git://github.com/Thermondo/heroku-buildpack-te
x.git#VERSION_NUMBER
› Error: Could not find the buildpack:
› git://github.com/Thermondo/heroku-buildpack-tex.git#VERSION_NUMB
› ER. To specify a buildpack, please format it like the following:
› namespace/name (e.g. heroku/ruby). Also names can only contain
› letters, numbers, '_', and '-'.
I tried to add the buildpack via the settings on the heroku website:
I restarted heroku with heroku restart
but I receive the same error message.

Answer 1

rails-latex uses third-party buildpack
https://github.com/Thermondo/heroku-buildpack-tex
You can install it by running this command:
heroku buildpacks:add git://github.com/Thermondo/heroku-buildpack-tex.git#VERSION_NUMBER
Alternatively you can also add the above git repository into your buildpacks by going to your dyno settings.
One thing to keep in mind is that The Heroku filesystem is ephemeral -
that means that any changes to the filesystem whilst the dyno is
running only last until that dyno is shut down or restarted
You can find more information regarding third party buildpacks here
More information regarding heroku filesystem here
Hope it helps.