Create a fake mimemagic gem

Issue: gitlab.com/gitlab-org/gitlab/-/issues/325851
MR: gitlab.com/gitlab-org/gitlab/-/merge_requests/57443

There is a current issue affecting mimemagic where it was
using a magic number dataset which turned out to be under
a GPL licence, which in turn meant the maintainer of mimemagic
had to switch to the same licence. We can not use this licence.
There are still dependencies in our gemfile which require mimemagic,
and instead of removing all these, we create a fake gem that we
require.

Gemfile

@@ -276,6 +276,9 @@ gem 'charlock_holmes', '~> 0.7.7'
 # Detect mime content type from content
 gem 'ruby-magic', '~> 0.3'
 
+# Fake version of the gem to trick bundler
+gem 'mimemagic', '0.3.5', path: 'vendor/shims/mimemagic', require: false
+
 # Faster blank
 gem 'fast_blank'
 

Gemfile.lock

+PATH
+  remote: vendor/shims/mimemagic
+  specs:
+    mimemagic (0.3.5)
+
 GEM
   remote: https://rubygems.org/
   specs:
@@ -728,7 +733,6 @@ GEM
     mime-types (3.3.1)
       mime-types-data (~> 3.2015)
     mime-types-data (3.2020.0512)
-    mimemagic (0.3.5)
     mini_histogram (0.3.1)
     mini_magick (4.10.1)
     mini_mime (1.0.2)
@@ -1482,6 +1486,7 @@ DEPENDENCIES
   marginalia (~> 1.10.0)
   memory_profiler (~> 0.9)
   method_source (~> 1.0)
+  mimemagic (= 0.3.5)!
   mini_magick (~> 4.10.1)
   minitest (~> 5.11.0)
   multi_json (~> 1.14.1)

changelogs/unreleased/mimemagic_shim.yml

+---
+title: Switch to using a fake mimemagic gem
+merge_request: 57443
+author:
+type: other

vendor/shims/mimemagic/Gemfile

+source "https://rubygems.org"
+
+# Specify your gem's dependencies in mimemagic.gemspec
+gemspec
+
+gem "rake", "~> 12.0"

vendor/shims/mimemagic/LICENSE.txt

+The MIT License (MIT)
+
+Copyright (c) 2021 GitLab B.V.
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.

vendor/shims/mimemagic/README.md

+This is a fake gem to prevent mimemagic from being included into GitLab.

vendor/shims/mimemagic/Rakefile

+require "bundler/gem_tasks"
+task :default => :spec

vendor/shims/mimemagic/lib/mimemagic.rb

+require "mimemagic/version"
+
+module MimeMagic
+  class Error < StandardError; end
+  raise Error, 'This gem should never be required'
+end

vendor/shims/mimemagic/lib/mimemagic/version.rb

+module MimeMagic
+  VERSION = "0.3.5"
+end

vendor/shims/mimemagic/mimemagic.gemspec

+require_relative 'lib/mimemagic/version'
+
+Gem::Specification.new do |spec|
+  spec.name          = "mimemagic"
+  spec.version       = MimeMagic::VERSION
+  spec.authors       = ["Marc Shaw"]
+  spec.email         = ["mshaw@gitlab.com"]
+
+  spec.summary       = %q{MimeMagic shim}
+  spec.description   = %q{A shim for mimemagic}
+  spec.license       = "MIT"
+  spec.required_ruby_version = Gem::Requirement.new(">= 2.3.0")
+
+  # Specify which files should be added to the gem when it is released.
+  # The `git ls-files -z` loads the files in the RubyGem that have been added into git.
+  spec.files         = Dir.chdir(File.expand_path('..', __FILE__)) do
+    `git ls-files -z`.split("\x0").reject { |f| f.match(%r{^(test|spec|features)/}) }
+  end
+  spec.require_paths = ["lib"]
+end