Intermittent SEGFAULT when precompiling assets as part of the build process

[lexiwitch]

Problem

There is currently an intermittent Segmentation Fault when running the build/test step

bundle exec rake

The output is roughly:

       /tmp/build/vendor/bundle/ruby/2.6.0/gems/sassc-2.2.1/lib/sassc/engine.rb:42: [BUG] Segmentation fault at 0x0000000000000000
       ruby 2.6.5p114 (2019-10-01 revision 67812) [x86_64-linux]
       
-- Control frame information -----------------------------------------------
       c:0048 p:---- s:0351 e:000350 CFUNC  :compile_data_context
       c:0047 p:0298 s:0346 e:000345 METHOD /tmp/build/vendor/bundle/ruby/2.6.0/gems/sassc-2.2.1/lib/sassc/engine.rb:42
       c:0046 p:0006 s:0334 e:000333 BLOCK  /tmp/build/vendor/bundle/ruby/2.6.0/gems/sassc-rails-2.1.2/lib/sassc/rails/template.rb:40
       c:0045 p:0028 s:0331 e:000330 METHOD /tmp/build/vendor/bundle/ruby/2.6.0/gems/sprockets-4.0.0/lib/sprockets/utils.rb:138
    [...]

Cause

This is due to a concurrency race condition bug in the combination sprockets-4.0.0 and sassc-2.2.1, see the following issues for more information: sass/sassc-ruby#167, rails/sprockets#581, rails/sprockets#633, rails/sprockets#630

Possible Remediations:

1. Bump sprockets down to a 3. version (changing it to >3 and <4)
2. Bump sassc down to 2.0.0 or up to 2.4.0 (The bug persists through 2.4.0, it's not known if it persists further)
3. Adding the following to config/initializers/assets.rb or a more relevant assets.rb file:

# Bypass segfault in sassc 2.* + sprockets 4: https://github.com/rails/sprockets/issues/581#issuecomment-486984663
Rails.application.config.assets.configure do |env|
  env.export_concurrent = false
end

Selected Remediation:

Given that we do not explicitly depend on sprockets or sassc in our Gemfile, I decided to go with remediation 3, and move up the list if that does not solve it.

Problems

Because this issue is intermittent, it is not guaranteed that any given remediation will actually have solved the problem until we fail to see it happen long-term

[lexiwitch]

Given that 9b5d956 seems to fix it (https://app.travis-ci.com/github/geeksforsocialchange/PlaceCal/jobs/545219716), I'm pulling production up to master to enable travis to push to dokku for us

[lexiwitch]

Seems to be fixed, will reopen if issue presents in the future.