Check for existence of a MIME type database at build time.

Because we're required to build a C extension in order to do so
(don't ask, its a long story), use that C extension to make the
path provided at build time available at run time.

ext/mimemagic/extconf.rb

+require "mkmf"
+
+def locate_mime_database
+  # User provided path.
+  unless ENV["FREEDESKTOP_MIME_TYPES_PATH"].nil?
+    path = File.expand_path(ENV["FREEDESKTOP_MIME_TYPES_PATH"])
+    unless File.exist?(path)
+      raise "The path #{path} was provided for the MIME types database, but no file exists at that path."
+    end
+    return path
+  end
+
+  # Default path on Linux installs for the MIME types database.
+  return "/usr/share/mime/packages/freedesktop.org.xml" if File.exist?("/usr/share/mime/packages/freedesktop.org.xml")
+
+  raise "No database of MIME types could be found. Ensure you have either installed the shared-mime-types package for your distribution, or obtain a version of freedesktop.org.xml, and set FREEDESKTOP_MIME_TYPES_PATH to the location of that file."
+end
+
+mime_database_path = locate_mime_database
+$defs.push("-DMIMEDB_PATH=\\\"#{mime_database_path}\\\"")
+create_header
+create_makefile("mimemagic/mimemagic")
\ No newline at end of file

ext/mimemagic/mimemagic.c

+#include <ruby.h>
+#include "extconf.h"
+
+void Init_mimemagic(void) {
+    VALUE cMimeMagic;
+
+    cMimeMagic = rb_const_get(rb_cObject, rb_intern("MimeMagic"));
+
+    rb_define_const(cMimeMagic, "DATABASE_PATH", rb_str_new(MIMEDB_PATH, strlen(MIMEDB_PATH)));
+}

lib/mimemagic/tables.rb

@@ -8,7 +8,6 @@ class MimeMagic
   TYPES = {}
   MAGIC = []
 
-
   def self.str2int(s)
     return s.to_i(16) if s[0..1].downcase == '0x'
     return s.to_i(8) if s[0..0].downcase == '0'
@@ -55,22 +54,10 @@ class MimeMagic
     }.compact
   end
 
-  def self.locate_mime_database
-    # User provided path.
-    return ENV["FREEDESKTOP_MIME_TYPES_PATH"] unless ENV["FREEDESKTOP_MIME_TYPES_PATH"].nil?
-
-    # Default path on Linux installs for the MIME types database.
-    return "/usr/share/mime/packages/freedesktop.org.xml" if File.exist?("/usr/share/mime/packages/freedesktop.org.xml")
-
-    nil
-  end
-
   def self.open_mime_database
-    path = locate_mime_database
-    return File.open(path) unless path.nil?
-
-    # Couldn't find it locally, pull it from the Internet.
-    raise "MIME types database could not be found. Pulling from the internet is currently unsupported."
+    require "mimemagic/mimemagic"
+    path = MimeMagic::DATABASE_PATH
+    File.open(path)
   end
 
   def self.parse_database

lib/mimemagic/version.rb

 class MimeMagic
   # MimeMagic version string
   # @api public
-  VERSION = '0.4.0'
+  VERSION = '0.3.7'
 end

mimemagic.gemspec

@@ -12,11 +12,12 @@ Gem::Specification.new do |s|
 
   s.files         = `git ls-files`.split("\n").reject { |f| f.match(%r{^(test|script)/}) }
   s.require_paths = %w(lib)
+  s.extensions = %w(ext/mimemagic/extconf.rb)
 
   s.summary = 'Fast mime detection by extension or content'
   s.description = 'Fast mime detection by extension or content in pure ruby (Uses freedesktop.org.xml shared-mime-info database)'
   s.homepage = 'https://github.com/minad/mimemagic'
-  s.license = 'GPL-2.0'
+  s.license = 'MIT'
 
   s.add_development_dependency('minitest', '~> 5.14')
   s.add_development_dependency('rake', '~> 13.0')