Add a top level eager_load! method for Rails compatibility.

Allows TZInfo to be eager loaded in Rails with:

config.eager_load_namespaces << TZInfo

Calls the eager_load! method (renamed from preload!) of the currently
selected DataSource.

lib/tzinfo.rb

@@ -3,6 +3,18 @@
 
 # The top level module for TZInfo.
 module TZInfo
+  class << self
+    # Instructs the current {DataSource} to load all timezone and country data
+    # into memory (initializing the {DataSource} first if not previously
+    # accessed or set).
+    #
+    # This may be desirable in production environments to improve copy-on-write
+    # performance and to avoid flushing the constant cache every time a new
+    # timezone or country is loaded from {DataSources::RubyDataSource}.
+    def eager_load!
+      DataSource.get.eager_load!
+    end
+  end
 end
 
 # Object#untaint is a deprecated no-op in Ruby >= 2.7 and will be removed in

lib/tzinfo/data_source.rb

@@ -247,11 +247,12 @@ def country_codes
       raise_invalid_data_source('country_codes')
     end
 
-    # Loads all timezone and country data into memory. This may be desirable in
-    # production environments to improve copy-on-write performance and to
-    # avoid flushing the constant cache every time a new timezone or country
-    # is loaded from {DataSources::RubyDataSource}.
-    def preload!
+    # Loads all timezone and country data into memory.
+    #
+    # This may be desirable in production environments to improve copy-on-write
+    # performance and to avoid flushing the constant cache every time a new
+    # timezone or country is loaded from {DataSources::RubyDataSource}.
+    def eager_load!
       timezone_identifiers.each {|identifier| load_timezone_info(identifier) }
       country_codes.each {|code| load_country_info(code) }
       nil

test/tc_data_source.rb

@@ -87,7 +87,7 @@ def call_lookup_country_info(hash, code, encoding = Encoding::UTF_8)
     end
   end
 
-  class PreloadTestDataSource < GetTimezoneIdentifiersTestDataSource
+  class EagerLoadTestDataSource < GetTimezoneIdentifiersTestDataSource
     attr_reader :country_codes_called
     attr_reader :loaded_timezones
     attr_reader :loaded_countries
@@ -581,13 +581,13 @@ def test_lookup_country_info_case
     end
   end
 
-  def test_preload
+  def test_eager_load
     data_timezone_identifiers = ['Data/Zone1', 'Data/Zone2']
     linked_timezone_identifiers = ['Linked/Zone1', 'Linked/Zone2']
     all_timezone_identifiers = data_timezone_identifiers + linked_timezone_identifiers
     country_codes = ['AA', 'BB', 'CC']
-    ds = PreloadTestDataSource.new(data_timezone_identifiers, linked_timezone_identifiers, country_codes)
-    assert_nil(ds.preload!)
+    ds = EagerLoadTestDataSource.new(data_timezone_identifiers, linked_timezone_identifiers, country_codes)
+    assert_nil(ds.eager_load!)
     assert_equal(1, ds.data_timezone_identifiers_called)
     assert_equal(1, ds.linked_timezone_identifiers_called)
     assert_equal(all_timezone_identifiers, ds.loaded_timezones)

test/tc_tzinfo.rb

@@ -0,0 +1,21 @@
+# encoding: UTF-8
+# frozen_string_literal: true
+
+require_relative 'test_utils'
+
+class TCTZInfo < Minitest::Test
+  def test_eager_load
+    test_data_source = Minitest::Mock.new
+    test_data_source.expect(:kind_of?, true, [DataSource])
+    test_data_source.expect(:eager_load!, nil)
+
+    orig_data_source = DataSource.get
+    DataSource.set(test_data_source)
+    begin
+      assert_nil(TZInfo.eager_load!)
+      test_data_source.verify
+    ensure
+      DataSource.set(orig_data_source)
+    end
+  end
+end