Merge branch 'master' into update_default_hsts_max_age

35 files changed, 468 insertions, 435 deletions

diff --git a/activesupport/lib/active_support/cache.rb b/activesupport/lib/active_support/cache.rb
index 2d038dba77..1ea2d0bbf2 100644
--- a/activesupport/lib/active_support/cache.rb
+++ b/activesupport/lib/active_support/cache.rb
@@ -160,6 +160,23 @@ module ActiveSupport
       attr_reader :silence, :options
       alias :silence? :silence
 
+      class << self
+        private
+          def retrieve_pool_options(options)
+            {}.tap do |pool_options|
+              pool_options[:size] = options.delete(:pool_size) if options[:pool_size]
+              pool_options[:timeout] = options.delete(:pool_timeout) if options[:pool_timeout]
+            end
+          end
+
+          def ensure_connection_pool_added!
+            require "connection_pool"
+          rescue LoadError => e
+            $stderr.puts "You don't have connection_pool installed in your application. Please add it to your Gemfile and run bundle install"
+            raise e
+          end
+      end
+
       # Creates a new cache. The options will be passed to any write method calls
       # except for <tt>:namespace</tt> which can be used to set the global
       # namespace for the cache.
diff --git a/activesupport/lib/active_support/cache/mem_cache_store.rb b/activesupport/lib/active_support/cache/mem_cache_store.rb
index 00a3670d64..2840781dde 100644
--- a/activesupport/lib/active_support/cache/mem_cache_store.rb
+++ b/activesupport/lib/active_support/cache/mem_cache_store.rb
@@ -63,7 +63,14 @@ module ActiveSupport
         addresses = addresses.flatten
         options = addresses.extract_options!
         addresses = ["localhost:11211"] if addresses.empty?
-        Dalli::Client.new(addresses, options)
+        pool_options = retrieve_pool_options(options)
+
+        if pool_options.empty?
+          Dalli::Client.new(addresses, options)
+        else
+          ensure_connection_pool_added!
+          ConnectionPool.new(pool_options) { Dalli::Client.new(addresses, options.merge(threadsafe: false)) }
+        end
       end
 
       # Creates a new MemCacheStore object, with the given memcached server
@@ -99,7 +106,7 @@ module ActiveSupport
         options = merged_options(options)
         instrument(:increment, name, amount: amount) do
           rescue_error_with nil do
-            @data.incr(normalize_key(name, options), amount, options[:expires_in])
+            @data.with { |c| c.incr(normalize_key(name, options), amount, options[:expires_in]) }
           end
         end
       end
@@ -112,7 +119,7 @@ module ActiveSupport
         options = merged_options(options)
         instrument(:decrement, name, amount: amount) do
           rescue_error_with nil do
-            @data.decr(normalize_key(name, options), amount, options[:expires_in])
+            @data.with { |c| c.decr(normalize_key(name, options), amount, options[:expires_in]) }
           end
         end
       end
@@ -120,18 +127,18 @@ module ActiveSupport
       # Clear the entire cache on all memcached servers. This method should
       # be used with care when shared cache is being used.
       def clear(options = nil)
-        rescue_error_with(nil) { @data.flush_all }
+        rescue_error_with(nil) { @data.with { |c| c.flush_all } }
       end
 
       # Get the statistics from the memcached servers.
       def stats
-        @data.stats
+        @data.with { |c| c.stats }
       end
 
       private
         # Read an entry from the cache.
         def read_entry(key, options)
-          rescue_error_with(nil) { deserialize_entry(@data.get(key, options)) }
+          rescue_error_with(nil) { deserialize_entry(@data.with { |c| c.get(key, options) }) }
         end
 
         # Write an entry to the cache.
@@ -144,7 +151,7 @@ module ActiveSupport
             expires_in += 5.minutes
           end
           rescue_error_with false do
-            @data.send(method, key, value, expires_in, options)
+            @data.with { |c| c.send(method, key, value, expires_in, options) }
           end
         end
 
@@ -152,7 +159,7 @@ module ActiveSupport
         def read_multi_entries(names, options)
           keys_to_names = Hash[names.map { |name| [normalize_key(name, options), name] }]
 
-          raw_values = @data.get_multi(keys_to_names.keys)
+          raw_values = @data.with { |c| c.get_multi(keys_to_names.keys) }
           values = {}
 
           raw_values.each do |key, value|
@@ -168,7 +175,7 @@ module ActiveSupport
 
         # Delete an entry from the cache.
         def delete_entry(key, options)
-          rescue_error_with(false) { @data.delete(key) }
+          rescue_error_with(false) { @data.with { |c| c.delete(key) } }
         end
 
         # Memcache keys are binaries. So we need to force their encoding to binary
diff --git a/activesupport/lib/active_support/cache/redis_cache_store.rb b/activesupport/lib/active_support/cache/redis_cache_store.rb
index e7f015bff5..64bc77cf22 100644
--- a/activesupport/lib/active_support/cache/redis_cache_store.rb
+++ b/activesupport/lib/active_support/cache/redis_cache_store.rb
@@ -20,6 +20,15 @@ require "active_support/core_ext/marshal"
 
 module ActiveSupport
   module Cache
+    module ConnectionPoolLike
+      def with
+        yield self
+      end
+    end
+
+    ::Redis.include(ConnectionPoolLike)
+    ::Redis::Distributed.include(ConnectionPoolLike)
+
     # Redis cache store.
     #
     # Deployment note: Take care to use a *dedicated Redis cache* rather
@@ -69,7 +78,7 @@ module ActiveSupport
 
           def write_entry(key, entry, options)
             if options[:raw] && local_cache
-              raw_entry = Entry.new(entry.value.to_s)
+              raw_entry = Entry.new(serialize_entry(entry, raw: true))
               raw_entry.expires_at = entry.expires_at
               super(key, raw_entry, options)
             else
@@ -80,7 +89,7 @@ module ActiveSupport
           def write_multi_entries(entries, options)
             if options[:raw] && local_cache
               raw_entries = entries.map do |key, entry|
-                raw_entry = Entry.new(entry.value.to_s)
+                raw_entry = Entry.new(serialize_entry(entry, raw: true))
                 raw_entry.expires_at = entry.expires_at
               end.to_h
 
@@ -149,7 +158,7 @@ module ActiveSupport
       #
       # Compression is enabled by default with a 1kB threshold, so cached
       # values larger than 1kB are automatically compressed. Disable by
-      # passing <tt>cache: false</tt> or change the threshold by passing
+      # passing <tt>compress: false</tt> or change the threshold by passing
       # <tt>compress_threshold: 4.kilobytes</tt>.
       #
       # No expiry is set on cache entries by default. Redis is expected to
@@ -172,7 +181,16 @@ module ActiveSupport
       end
 
       def redis
-        @redis ||= self.class.build_redis(**redis_options)
+        @redis ||= begin
+          pool_options = self.class.send(:retrieve_pool_options, redis_options)
+
+          if pool_options.any?
+            self.class.send(:ensure_connection_pool_added!)
+            ::ConnectionPool.new(pool_options) { self.class.build_redis(**redis_options) }
+          else
+            self.class.build_redis(**redis_options)
+          end
+        end
       end
 
       def inspect
@@ -186,7 +204,11 @@ module ActiveSupport
       # fetched values.
       def read_multi(*names)
         if mget_capable?
-          read_multi_mget(*names)
+          instrument(:read_multi, names, options) do |payload|
+            read_multi_mget(*names).tap do |results|
+              payload[:hits] = results.keys
+            end
+          end
         else
           super
         end
@@ -211,7 +233,7 @@ module ActiveSupport
         instrument :delete_matched, matcher do
           case matcher
           when String
-            redis.eval DELETE_GLOB_LUA, [], [namespace_key(matcher, options)]
+            redis.with { |c| c.eval DELETE_GLOB_LUA, [], [namespace_key(matcher, options)] }
           else
             raise ArgumentError, "Only Redis glob strings are supported: #{matcher.inspect}"
           end
@@ -228,7 +250,9 @@ module ActiveSupport
       # Failsafe: Raises errors.
       def increment(name, amount = 1, options = nil)
         instrument :increment, name, amount: amount do
-          redis.incrby normalize_key(name, options), amount
+          failsafe :increment do
+            redis.with { |c| c.incrby normalize_key(name, options), amount }
+          end
         end
       end
 
@@ -242,7 +266,9 @@ module ActiveSupport
       # Failsafe: Raises errors.
       def decrement(name, amount = 1, options = nil)
         instrument :decrement, name, amount: amount do
-          redis.decrby normalize_key(name, options), amount
+          failsafe :decrement do
+            redis.with { |c| c.decrby normalize_key(name, options), amount }
+          end
         end
       end
 
@@ -263,7 +289,7 @@ module ActiveSupport
           if namespace = merged_options(options)[namespace]
             delete_matched "*", namespace: namespace
           else
-            redis.flushdb
+            redis.with { |c| c.flushdb }
           end
         end
       end
@@ -294,7 +320,15 @@ module ActiveSupport
         # Read an entry from the cache.
         def read_entry(key, options = nil)
           failsafe :read_entry do
-            deserialize_entry redis.get(key)
+            deserialize_entry redis.with { |c| c.get(key) }
+          end
+        end
+
+        def read_multi_entries(names, _options)
+          if mget_capable?
+            read_multi_mget(*names)
+          else
+            super
           end
         end
 
@@ -303,7 +337,10 @@ module ActiveSupport
           options = merged_options(options)
 
           keys = names.map { |name| normalize_key(name, options) }
-          values = redis.mget(*keys)
+
+          values = failsafe(:read_multi_mget, returning: {}) do
+            redis.with { |c| c.mget(*keys) }
+          end
 
           names.zip(values).each_with_object({}) do |(name, value), results|
             if value
@@ -319,7 +356,7 @@ module ActiveSupport
         #
         # Requires Redis 2.6.12+ for extended SET options.
         def write_entry(key, entry, unless_exist: false, raw: false, expires_in: nil, race_condition_ttl: nil, **options)
-          value = raw ? entry.value.to_s : serialize_entry(entry)
+          serialized_entry = serialize_entry(entry, raw: raw)
 
           # If race condition TTL is in use, ensure that cache entries
           # stick around a bit longer after they would have expired
@@ -328,15 +365,15 @@ module ActiveSupport
             expires_in += 5.minutes
           end
 
-          failsafe :write_entry do
+          failsafe :write_entry, returning: false do
             if unless_exist || expires_in
               modifiers = {}
               modifiers[:nx] = unless_exist
               modifiers[:px] = (1000 * expires_in.to_f).ceil if expires_in
 
-              redis.set key, value, modifiers
+              redis.with { |c| c.set key, serialized_entry, modifiers }
             else
-              redis.set key, value
+              redis.with { |c| c.set key, serialized_entry }
             end
           end
         end
@@ -344,7 +381,7 @@ module ActiveSupport
         # Delete an entry from the cache.
         def delete_entry(key, options)
           failsafe :delete_entry, returning: false do
-            redis.del key
+            redis.with { |c| c.del key }
           end
         end
 
@@ -353,7 +390,7 @@ module ActiveSupport
           if entries.any?
             if mset_capable? && expires_in.nil?
               failsafe :write_multi_entries do
-                redis.mapped_mset(entries)
+                redis.with { |c| c.mapped_mset(serialize_entries(entries, raw: options[:raw])) }
               end
             else
               super
@@ -363,7 +400,7 @@ module ActiveSupport
 
         # Truncate keys that exceed 1kB.
         def normalize_key(key, options)
-          truncate_key super
+          truncate_key super.b
         end
 
         def truncate_key(key)
@@ -376,15 +413,25 @@ module ActiveSupport
           end
         end
 
-        def deserialize_entry(raw_value)
-          if raw_value
-            entry = Marshal.load(raw_value) rescue raw_value
+        def deserialize_entry(serialized_entry)
+          if serialized_entry
+            entry = Marshal.load(serialized_entry) rescue serialized_entry
             entry.is_a?(Entry) ? entry : Entry.new(entry)
           end
         end
 
-        def serialize_entry(entry)
-          Marshal.dump(entry)
+        def serialize_entry(entry, raw: false)
+          if raw
+            entry.value.to_s
+          else
+            Marshal.dump(entry)
+          end
+        end
+
+        def serialize_entries(entries, raw: false)
+          entries.transform_values do |entry|
+            serialize_entry entry, raw: raw
+          end
         end
 
         def failsafe(method, returning: nil)
diff --git a/activesupport/lib/active_support/cache/strategy/local_cache.rb b/activesupport/lib/active_support/cache/strategy/local_cache.rb
index aaa9638fa8..e17308f83e 100644
--- a/activesupport/lib/active_support/cache/strategy/local_cache.rb
+++ b/activesupport/lib/active_support/cache/strategy/local_cache.rb
@@ -54,6 +54,10 @@ module ActiveSupport
             @data[key]
           end
 
+          def read_multi_entries(keys, options)
+            Hash[keys.map { |name| [name, read_entry(name, options)] }.keep_if { |_name, value| value }]
+          end
+
           def write_entry(key, value, options)
             @data[key] = value
             true
@@ -116,6 +120,19 @@ module ActiveSupport
             end
           end
 
+          def read_multi_entries(keys, options)
+            return super unless local_cache
+
+            local_entries = local_cache.read_multi_entries(keys, options)
+            missed_keys = keys - local_entries.keys
+
+            if missed_keys.any?
+              local_entries.merge!(super(missed_keys, options))
+            else
+              local_entries
+            end
+          end
+
           def write_entry(key, entry, options)
             if options[:unless_exist]
               local_cache.delete_entry(key, options) if local_cache
diff --git a/activesupport/lib/active_support/core_ext/class/attribute.rb b/activesupport/lib/active_support/core_ext/class/attribute.rb
index 7928efb871..fa33ff945f 100644
--- a/activesupport/lib/active_support/core_ext/class/attribute.rb
+++ b/activesupport/lib/active_support/core_ext/class/attribute.rb
@@ -98,7 +98,7 @@ class Class
       singleton_class.silence_redefinition_of_method("#{name}?")
       define_singleton_method("#{name}?") { !!public_send(name) } if instance_predicate
 
-      ivar = "@#{name}"
+      ivar = "@#{name}".to_sym
 
       singleton_class.silence_redefinition_of_method("#{name}=")
       define_singleton_method("#{name}=") do |val|
diff --git a/activesupport/lib/active_support/core_ext/hash.rb b/activesupport/lib/active_support/core_ext/hash.rb
index e19aeaa983..325581a60a 100644
--- a/activesupport/lib/active_support/core_ext/hash.rb
+++ b/activesupport/lib/active_support/core_ext/hash.rb
@@ -8,4 +8,3 @@ require "active_support/core_ext/hash/indifferent_access"
 require "active_support/core_ext/hash/keys"
 require "active_support/core_ext/hash/reverse_merge"
 require "active_support/core_ext/hash/slice"
-require "active_support/core_ext/hash/transform_values"
diff --git a/activesupport/lib/active_support/core_ext/hash/conversions.rb b/activesupport/lib/active_support/core_ext/hash/conversions.rb
index 11d28d12a1..5b48254646 100644
--- a/activesupport/lib/active_support/core_ext/hash/conversions.rb
+++ b/activesupport/lib/active_support/core_ext/hash/conversions.rb
@@ -165,7 +165,7 @@ module ActiveSupport
           Hash[params.map { |k, v| [k.to_s.tr("-", "_"), normalize_keys(v)] } ]
         when Array
           params.map { |v| normalize_keys(v) }
-          else
+        else
           params
         end
       end
@@ -178,7 +178,7 @@ module ActiveSupport
           process_array(value)
         when String
           value
-          else
+        else
           raise "can't typecast #{value.class.name} - #{value.inspect}"
         end
       end
diff --git a/activesupport/lib/active_support/core_ext/hash/transform_values.rb b/activesupport/lib/active_support/core_ext/hash/transform_values.rb
index 4b19c9fc1f..fc15130c9e 100644
--- a/activesupport/lib/active_support/core_ext/hash/transform_values.rb
+++ b/activesupport/lib/active_support/core_ext/hash/transform_values.rb
@@ -1,32 +1,5 @@
 # frozen_string_literal: true
 
-class Hash
-  # Returns a new hash with the results of running +block+ once for every value.
-  # The keys are unchanged.
-  #
-  #   { a: 1, b: 2, c: 3 }.transform_values { |x| x * 2 } # => { a: 2, b: 4, c: 6 }
-  #
-  # If you do not provide a +block+, it will return an Enumerator
-  # for chaining with other methods:
-  #
-  #   { a: 1, b: 2 }.transform_values.with_index { |v, i| [v, i].join.to_i } # => { a: 10, b: 21 }
-  def transform_values
-    return enum_for(:transform_values) { size } unless block_given?
-    return {} if empty?
-    result = self.class.new
-    each do |key, value|
-      result[key] = yield(value)
-    end
-    result
-  end unless method_defined? :transform_values
+require "active_support/deprecation"
 
-  # Destructively converts all values using the +block+ operations.
-  # Same as +transform_values+ but modifies +self+.
-  def transform_values!
-    return enum_for(:transform_values!) { size } unless block_given?
-    each do |key, value|
-      self[key] = yield(value)
-    end
-  end unless method_defined? :transform_values!
-  # TODO: Remove this file when supporting only Ruby 2.4+.
-end
+ActiveSupport::Deprecation.warn "Ruby 2.4+ (required by Rails 6) provides Hash#transform_values natively, so requiring active_support/core_ext/hash/transform_values is no longer necessary. Requiring it will raise LoadError in Rails 6.1."
diff --git a/activesupport/lib/active_support/core_ext/module/redefine_method.rb b/activesupport/lib/active_support/core_ext/module/redefine_method.rb
index a0a6622ca4..5bd8e6e973 100644
--- a/activesupport/lib/active_support/core_ext/module/redefine_method.rb
+++ b/activesupport/lib/active_support/core_ext/module/redefine_method.rb
@@ -1,23 +1,14 @@
 # frozen_string_literal: true
 
 class Module
-  if RUBY_VERSION >= "2.3"
-    # Marks the named method as intended to be redefined, if it exists.
-    # Suppresses the Ruby method redefinition warning. Prefer
-    # #redefine_method where possible.
-    def silence_redefinition_of_method(method)
-      if method_defined?(method) || private_method_defined?(method)
-        # This suppresses the "method redefined" warning; the self-alias
-        # looks odd, but means we don't need to generate a unique name
-        alias_method method, method
-      end
-    end
-  else
-    def silence_redefinition_of_method(method)
-      if method_defined?(method) || private_method_defined?(method)
-        alias_method :__rails_redefine, method
-        remove_method :__rails_redefine
-      end
+  # Marks the named method as intended to be redefined, if it exists.
+  # Suppresses the Ruby method redefinition warning. Prefer
+  # #redefine_method where possible.
+  def silence_redefinition_of_method(method)
+    if method_defined?(method) || private_method_defined?(method)
+      # This suppresses the "method redefined" warning; the self-alias
+      # looks odd, but means we don't need to generate a unique name
+      alias_method method, method
     end
   end
 
diff --git a/activesupport/lib/active_support/core_ext/numeric/conversions.rb b/activesupport/lib/active_support/core_ext/numeric/conversions.rb
index f6c2713986..7fcd0d0311 100644
--- a/activesupport/lib/active_support/core_ext/numeric/conversions.rb
+++ b/activesupport/lib/active_support/core_ext/numeric/conversions.rb
@@ -129,12 +129,6 @@ module ActiveSupport::NumericWithFormat
   end
 end
 
-# Ruby 2.4+ unifies Fixnum & Bignum into Integer.
-if 0.class == Integer
-  Integer.prepend ActiveSupport::NumericWithFormat
-else
-  Fixnum.prepend ActiveSupport::NumericWithFormat
-  Bignum.prepend ActiveSupport::NumericWithFormat
-end
+Integer.prepend ActiveSupport::NumericWithFormat
 Float.prepend ActiveSupport::NumericWithFormat
 BigDecimal.prepend ActiveSupport::NumericWithFormat
diff --git a/activesupport/lib/active_support/core_ext/object/duplicable.rb b/activesupport/lib/active_support/core_ext/object/duplicable.rb
index 9bb99087bc..c78ee6bbfc 100644
--- a/activesupport/lib/active_support/core_ext/object/duplicable.rb
+++ b/activesupport/lib/active_support/core_ext/object/duplicable.rb
@@ -75,8 +75,11 @@ end
 
 class Symbol
   begin
-    :symbol.dup # Ruby 2.4.x.
-    "symbol_from_string".to_sym.dup # Some symbols can't `dup` in Ruby 2.4.0.
+    :symbol.dup
+
+    # Some symbols couldn't be duped in Ruby 2.4.0 only, due to a bug.
+    # This feature check catches any regression.
+    "symbol_from_string".to_sym.dup
   rescue TypeError
 
     # Symbols are not duplicable:
diff --git a/activesupport/lib/active_support/core_ext/regexp.rb b/activesupport/lib/active_support/core_ext/regexp.rb
index efbd708aee..d92943c7ae 100644
--- a/activesupport/lib/active_support/core_ext/regexp.rb
+++ b/activesupport/lib/active_support/core_ext/regexp.rb
@@ -4,8 +4,4 @@ class Regexp #:nodoc:
   def multiline?
     options & MULTILINE == MULTILINE
   end
-
-  def match?(string, pos = 0)
-    !!match(string, pos)
-  end unless //.respond_to?(:match?)
 end
diff --git a/activesupport/lib/active_support/core_ext/string/filters.rb b/activesupport/lib/active_support/core_ext/string/filters.rb
index 66e721eea3..df0e79afa8 100644
--- a/activesupport/lib/active_support/core_ext/string/filters.rb
+++ b/activesupport/lib/active_support/core_ext/string/filters.rb
@@ -78,6 +78,47 @@ class String
     "#{self[0, stop]}#{omission}"
   end
 
+  # Truncates +text+ to at most <tt>bytesize</tt> bytes in length without
+  # breaking string encoding by splitting multibyte characters or breaking
+  # grapheme clusters ("perceptual characters") by truncating at combining
+  # characters.
+  #
+  #   >> "🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪".size
+  #   => 20
+  #   >> "🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪".bytesize
+  #   => 80
+  #   >> "🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪🔪".truncate_bytes(20)
+  #   => "🔪🔪🔪🔪…"
+  #
+  # The truncated text ends with the <tt>:omission</tt> string, defaulting
+  # to "…", for a total length not exceeding <tt>bytesize</tt>.
+  def truncate_bytes(truncate_at, omission: "…")
+    omission ||= ""
+
+    case
+    when bytesize <= truncate_at
+      dup
+    when omission.bytesize > truncate_at
+      raise ArgumentError, "Omission #{omission.inspect} is #{omission.bytesize}, larger than the truncation length of #{truncate_at} bytes"
+    when omission.bytesize == truncate_at
+      omission.dup
+    else
+      self.class.new.tap do |cut|
+        cut_at = truncate_at - omission.bytesize
+
+        scan(/\X/) do |grapheme|
+          if cut.bytesize + grapheme.bytesize <= cut_at
+            cut << grapheme
+          else
+            break
+          end
+        end
+
+        cut << omission
+      end
+    end
+  end
+
   # Truncates a given +text+ after a given number of words (<tt>words_count</tt>):
   #
   #   'Once upon a time in a world far far away'.truncate_words(4)
diff --git a/activesupport/lib/active_support/core_ext/string/multibyte.rb b/activesupport/lib/active_support/core_ext/string/multibyte.rb
index 07c0d16398..6cceb46507 100644
--- a/activesupport/lib/active_support/core_ext/string/multibyte.rb
+++ b/activesupport/lib/active_support/core_ext/string/multibyte.rb
@@ -11,12 +11,13 @@ class String
   # encapsulates the original string. A Unicode safe version of all the String methods are defined on this proxy
   # class. If the proxy class doesn't respond to a certain method, it's forwarded to the encapsulated string.
   #
-  #   >> "lj".upcase
-  #   => "lj"
   #   >> "lj".mb_chars.upcase.to_s
   #   => "LJ"
   #
-  # NOTE: An above example is useful for pre Ruby 2.4. Ruby 2.4 supports Unicode case mappings.
+  # NOTE: Ruby 2.4 and later support native Unicode case mappings:
+  #
+  #   >> "lj".upcase
+  #   => "LJ"
   #
   # == Method chaining
   #
diff --git a/activesupport/lib/active_support/core_ext/string/strip.rb b/activesupport/lib/active_support/core_ext/string/strip.rb
index cc26274e4a..6f9834bb16 100644
--- a/activesupport/lib/active_support/core_ext/string/strip.rb
+++ b/activesupport/lib/active_support/core_ext/string/strip.rb
@@ -20,6 +20,8 @@ class String
   # Technically, it looks for the least indented non-empty line
   # in the whole string, and removes that amount of leading whitespace.
   def strip_heredoc
-    gsub(/^#{scan(/^[ \t]*(?=\S)/).min}/, "".freeze)
+    gsub(/^#{scan(/^[ \t]*(?=\S)/).min}/, "".freeze).tap do |stripped|
+      stripped.freeze if frozen?
+    end
   end
 end
diff --git a/activesupport/lib/active_support/dependencies.rb b/activesupport/lib/active_support/dependencies.rb
index 82c10b3079..0f59558bb5 100644
--- a/activesupport/lib/active_support/dependencies.rb
+++ b/activesupport/lib/active_support/dependencies.rb
@@ -447,6 +447,7 @@ module ActiveSupport #:nodoc:
       mod = Module.new
       into.const_set const_name, mod
       autoloaded_constants << qualified_name unless autoload_once_paths.include?(base_path)
+      autoloaded_constants.uniq!
       mod
     end
 
@@ -670,7 +671,7 @@ module ActiveSupport #:nodoc:
       when Module
         desc.name ||
           raise(ArgumentError, "Anonymous modules have no name to be referenced by")
-        else raise TypeError, "Not a valid constant descriptor: #{desc.inspect}"
+      else raise TypeError, "Not a valid constant descriptor: #{desc.inspect}"
       end
     end
 
diff --git a/activesupport/lib/active_support/deprecation.rb b/activesupport/lib/active_support/deprecation.rb
index a1ad2ca465..7271ab565b 100644
--- a/activesupport/lib/active_support/deprecation.rb
+++ b/activesupport/lib/active_support/deprecation.rb
@@ -35,7 +35,7 @@ module ActiveSupport
     # and the second is a library name.
     #
     #   ActiveSupport::Deprecation.new('2.0', 'MyLibrary')
-    def initialize(deprecation_horizon = "6.0", gem_name = "Rails")
+    def initialize(deprecation_horizon = "6.1", gem_name = "Rails")
       self.gem_name = gem_name
       self.deprecation_horizon = deprecation_horizon
       # By default, warnings are not silenced and debugging is off.
diff --git a/activesupport/lib/active_support/deprecation/behaviors.rb b/activesupport/lib/active_support/deprecation/behaviors.rb
index 581db5f449..66d6f3225a 100644
--- a/activesupport/lib/active_support/deprecation/behaviors.rb
+++ b/activesupport/lib/active_support/deprecation/behaviors.rb
@@ -85,7 +85,7 @@ module ActiveSupport
       #   ActiveSupport::Deprecation.behavior = :stderr
       #   ActiveSupport::Deprecation.behavior = [:stderr, :log]
       #   ActiveSupport::Deprecation.behavior = MyCustomHandler
-      #   ActiveSupport::Deprecation.behavior = ->(message, callstack) {
+      #   ActiveSupport::Deprecation.behavior = ->(message, callstack, deprecation_horizon, gem_name) {
       #     # custom stuff
       #   }
       def behavior=(behavior)
diff --git a/activesupport/lib/active_support/deprecation/reporting.rb b/activesupport/lib/active_support/deprecation/reporting.rb
index 242e21b782..2c004f4c9e 100644
--- a/activesupport/lib/active_support/deprecation/reporting.rb
+++ b/activesupport/lib/active_support/deprecation/reporting.rb
@@ -61,7 +61,7 @@ module ActiveSupport
           case message
           when Symbol then "#{warning} (use #{message} instead)"
           when String then "#{warning} (#{message})"
-            else warning
+          else warning
           end
         end
 
diff --git a/activesupport/lib/active_support/duration.rb b/activesupport/lib/active_support/duration.rb
index fe1058762b..88897f811e 100644
--- a/activesupport/lib/active_support/duration.rb
+++ b/activesupport/lib/active_support/duration.rb
@@ -383,6 +383,14 @@ module ActiveSupport
       to_i
     end
 
+    def init_with(coder) #:nodoc:
+      initialize(coder["value"], coder["parts"])
+    end
+
+    def encode_with(coder) #:nodoc:
+      coder.map = { "value" => @value, "parts" => @parts }
+    end
+
     # Build ISO 8601 Duration string for this duration.
     # The +precision+ parameter can be used to limit seconds' precision of duration.
     def iso8601(precision: nil)
diff --git a/activesupport/lib/active_support/duration/iso8601_serializer.rb b/activesupport/lib/active_support/duration/iso8601_serializer.rb
index bb177ae5b7..84ae29c1ec 100644
--- a/activesupport/lib/active_support/duration/iso8601_serializer.rb
+++ b/activesupport/lib/active_support/duration/iso8601_serializer.rb
@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require "active_support/core_ext/object/blank"
-require "active_support/core_ext/hash/transform_values"
 
 module ActiveSupport
   class Duration
diff --git a/activesupport/lib/active_support/gem_version.rb b/activesupport/lib/active_support/gem_version.rb
index 1e09adbb52..c951ad16a3 100644
--- a/activesupport/lib/active_support/gem_version.rb
+++ b/activesupport/lib/active_support/gem_version.rb
@@ -7,10 +7,10 @@ module ActiveSupport
   end
 
   module VERSION
-    MAJOR = 5
-    MINOR = 2
+    MAJOR = 6
+    MINOR = 0
     TINY  = 0
-    PRE   = "beta2"
+    PRE   = "alpha"
 
     STRING = [MAJOR, MINOR, TINY, PRE].compact.join(".")
   end
diff --git a/activesupport/lib/active_support/hash_with_indifferent_access.rb b/activesupport/lib/active_support/hash_with_indifferent_access.rb
index 2e2ed8a25d..e4afc8af93 100644
--- a/activesupport/lib/active_support/hash_with_indifferent_access.rb
+++ b/activesupport/lib/active_support/hash_with_indifferent_access.rb
@@ -177,20 +177,18 @@ module ActiveSupport
       super(convert_key(key), *extras)
     end
 
-    if Hash.new.respond_to?(:dig)
-      # Same as <tt>Hash#dig</tt> where the key passed as argument can be
-      # either a string or a symbol:
-      #
-      #   counters = ActiveSupport::HashWithIndifferentAccess.new
-      #   counters[:foo] = { bar: 1 }
-      #
-      #   counters.dig('foo', 'bar')     # => 1
-      #   counters.dig(:foo, :bar)       # => 1
-      #   counters.dig(:zoo)             # => nil
-      def dig(*args)
-        args[0] = convert_key(args[0]) if args.size > 0
-        super(*args)
-      end
+    # Same as <tt>Hash#dig</tt> where the key passed as argument can be
+    # either a string or a symbol:
+    #
+    #   counters = ActiveSupport::HashWithIndifferentAccess.new
+    #   counters[:foo] = { bar: 1 }
+    #
+    #   counters.dig('foo', 'bar')     # => 1
+    #   counters.dig(:foo, :bar)       # => 1
+    #   counters.dig(:zoo)             # => nil
+    def dig(*args)
+      args[0] = convert_key(args[0]) if args.size > 0
+      super(*args)
     end
 
     # Same as <tt>Hash#default</tt> where the key passed as argument can be
@@ -228,7 +226,7 @@ module ActiveSupport
     #   hash.fetch_values('a', 'c') # => KeyError: key not found: "c"
     def fetch_values(*indices, &block)
       indices.collect { |key| fetch(key, &block) }
-    end if Hash.method_defined?(:fetch_values)
+    end
 
     # Returns a shallow copy of the hash.
     #
@@ -311,6 +309,14 @@ module ActiveSupport
       dup.tap { |hash| hash.transform_keys!(*args, &block) }
     end
 
+    def transform_keys!
+      return enum_for(:transform_keys!) { size } unless block_given?
+      keys.each do |key|
+        self[yield(key)] = delete(key)
+      end
+      self
+    end
+
     def slice(*keys)
       keys.map! { |key| convert_key(key) }
       self.class.new(super)
diff --git a/activesupport/lib/active_support/inflector/inflections.rb b/activesupport/lib/active_support/inflector/inflections.rb
index 0450a4be4c..7e5dff1d6d 100644
--- a/activesupport/lib/active_support/inflector/inflections.rb
+++ b/activesupport/lib/active_support/inflector/inflections.rb
@@ -227,7 +227,7 @@ module ActiveSupport
         case scope
         when :all
           @plurals, @singulars, @uncountables, @humans = [], [], Uncountables.new, []
-          else
+        else
           instance_variable_set "@#{scope}", []
         end
       end
diff --git a/activesupport/lib/active_support/inflector/methods.rb b/activesupport/lib/active_support/inflector/methods.rb
index 60eeaa77cb..7e782e2a93 100644
--- a/activesupport/lib/active_support/inflector/methods.rb
+++ b/activesupport/lib/active_support/inflector/methods.rb
@@ -350,7 +350,7 @@ module ActiveSupport
         when 1; "st"
         when 2; "nd"
         when 3; "rd"
-          else    "th"
+        else    "th"
         end
       end
     end
diff --git a/activesupport/lib/active_support/multibyte/unicode.rb b/activesupport/lib/active_support/multibyte/unicode.rb
index a64223c0e0..4f0e1165ef 100644
--- a/activesupport/lib/active_support/multibyte/unicode.rb
+++ b/activesupport/lib/active_support/multibyte/unicode.rb
@@ -11,7 +11,7 @@ module ActiveSupport
       NORMALIZATION_FORMS = [:c, :kc, :d, :kd]
 
       # The Unicode version that is supported by the implementation
-      UNICODE_VERSION = "9.0.0"
+      UNICODE_VERSION = RbConfig::CONFIG["UNICODE_VERSION"]
 
       # The default normalization used for operations that require
       # normalization. It can be set to any of the normalizations
@@ -21,96 +21,13 @@ module ActiveSupport
       attr_accessor :default_normalization_form
       @default_normalization_form = :kc
 
-      # Hangul character boundaries and properties
-      HANGUL_SBASE = 0xAC00
-      HANGUL_LBASE = 0x1100
-      HANGUL_VBASE = 0x1161
-      HANGUL_TBASE = 0x11A7
-      HANGUL_LCOUNT = 19
-      HANGUL_VCOUNT = 21
-      HANGUL_TCOUNT = 28
-      HANGUL_NCOUNT = HANGUL_VCOUNT * HANGUL_TCOUNT
-      HANGUL_SCOUNT = 11172
-      HANGUL_SLAST = HANGUL_SBASE + HANGUL_SCOUNT
-
-      # Detect whether the codepoint is in a certain character class. Returns
-      # +true+ when it's in the specified character class and +false+ otherwise.
-      # Valid character classes are: <tt>:cr</tt>, <tt>:lf</tt>, <tt>:l</tt>,
-      # <tt>:v</tt>, <tt>:lv</tt>, <tt>:lvt</tt> and <tt>:t</tt>.
-      #
-      # Primarily used by the grapheme cluster support.
-      def in_char_class?(codepoint, classes)
-        classes.detect { |c| database.boundary[c] === codepoint } ? true : false
-      end
-
       # Unpack the string at grapheme boundaries. Returns a list of character
       # lists.
       #
       #   Unicode.unpack_graphemes('क्षि') # => [[2325, 2381], [2359], [2367]]
       #   Unicode.unpack_graphemes('Café') # => [[67], [97], [102], [233]]
       def unpack_graphemes(string)
-        codepoints = string.codepoints.to_a
-        unpacked = []
-        pos = 0
-        marker = 0
-        eoc = codepoints.length
-        while (pos < eoc)
-          pos += 1
-          previous = codepoints[pos - 1]
-          current = codepoints[pos]
-
-          # See http://unicode.org/reports/tr29/#Grapheme_Cluster_Boundary_Rules
-          should_break =
-            if pos == eoc
-              true
-            # GB3. CR X LF
-            elsif previous == database.boundary[:cr] && current == database.boundary[:lf]
-              false
-            # GB4. (Control|CR|LF) ÷
-            elsif previous && in_char_class?(previous, [:control, :cr, :lf])
-              true
-            # GB5. ÷ (Control|CR|LF)
-            elsif in_char_class?(current, [:control, :cr, :lf])
-              true
-            # GB6. L X (L|V|LV|LVT)
-            elsif database.boundary[:l] === previous && in_char_class?(current, [:l, :v, :lv, :lvt])
-              false
-            # GB7. (LV|V) X (V|T)
-            elsif in_char_class?(previous, [:lv, :v]) && in_char_class?(current, [:v, :t])
-              false
-            # GB8. (LVT|T) X (T)
-            elsif in_char_class?(previous, [:lvt, :t]) && database.boundary[:t] === current
-              false
-            # GB9. X (Extend | ZWJ)
-            elsif in_char_class?(current, [:extend, :zwj])
-              false
-            # GB9a. X SpacingMark
-            elsif database.boundary[:spacingmark] === current
-              false
-            # GB9b. Prepend X
-            elsif database.boundary[:prepend] === previous
-              false
-            # GB10. (E_Base | EBG) Extend* X E_Modifier
-            elsif (marker...pos).any? { |i| in_char_class?(codepoints[i], [:e_base, :e_base_gaz]) && codepoints[i + 1...pos].all? { |c| database.boundary[:extend] === c } } && database.boundary[:e_modifier] === current
-              false
-            # GB11. ZWJ X (Glue_After_Zwj | EBG)
-            elsif database.boundary[:zwj] === previous && in_char_class?(current, [:glue_after_zwj, :e_base_gaz])
-              false
-            # GB12. ^ (RI RI)* RI X RI
-            # GB13. [^RI] (RI RI)* RI X RI
-            elsif codepoints[marker..pos].all? { |c| database.boundary[:regional_indicator] === c } && codepoints[marker..pos].count { |c| database.boundary[:regional_indicator] === c }.even?
-              false
-            # GB999. Any ÷ Any
-            else
-              true
-            end
-
-          if should_break
-            unpacked << codepoints[marker..pos - 1]
-            marker = pos
-          end
-        end
-        unpacked
+        string.scan(/\X/).map(&:codepoints)
       end
 
       # Reverse operation of unpack_graphemes.
@@ -120,100 +37,18 @@ module ActiveSupport
         unpacked.flatten.pack("U*")
       end
 
-      # Re-order codepoints so the string becomes canonical.
-      def reorder_characters(codepoints)
-        length = codepoints.length - 1
-        pos = 0
-        while pos < length do
-          cp1, cp2 = database.codepoints[codepoints[pos]], database.codepoints[codepoints[pos + 1]]
-          if (cp1.combining_class > cp2.combining_class) && (cp2.combining_class > 0)
-            codepoints[pos..pos + 1] = cp2.code, cp1.code
-            pos += (pos > 0 ? -1 : 1)
-          else
-            pos += 1
-          end
-        end
-        codepoints
-      end
-
       # Decompose composed characters to the decomposed form.
       def decompose(type, codepoints)
-        codepoints.inject([]) do |decomposed, cp|
-          # if it's a hangul syllable starter character
-          if HANGUL_SBASE <= cp && cp < HANGUL_SLAST
-            sindex = cp - HANGUL_SBASE
-            ncp = [] # new codepoints
-            ncp << HANGUL_LBASE + sindex / HANGUL_NCOUNT
-            ncp << HANGUL_VBASE + (sindex % HANGUL_NCOUNT) / HANGUL_TCOUNT
-            tindex = sindex % HANGUL_TCOUNT
-            ncp << (HANGUL_TBASE + tindex) unless tindex == 0
-            decomposed.concat ncp
-          # if the codepoint is decomposable in with the current decomposition type
-          elsif (ncp = database.codepoints[cp].decomp_mapping) && (!database.codepoints[cp].decomp_type || type == :compatibility)
-            decomposed.concat decompose(type, ncp.dup)
-          else
-            decomposed << cp
-          end
+        if type == :compatibility
+          codepoints.pack("U*").unicode_normalize(:nfkd).codepoints
+        else
+          codepoints.pack("U*").unicode_normalize(:nfd).codepoints
         end
       end
 
       # Compose decomposed characters to the composed form.
       def compose(codepoints)
-        pos = 0
-        eoa = codepoints.length - 1
-        starter_pos = 0
-        starter_char = codepoints[0]
-        previous_combining_class = -1
-        while pos < eoa
-          pos += 1
-          lindex = starter_char - HANGUL_LBASE
-          # -- Hangul
-          if 0 <= lindex && lindex < HANGUL_LCOUNT
-            vindex = codepoints[starter_pos + 1] - HANGUL_VBASE rescue vindex = -1
-            if 0 <= vindex && vindex < HANGUL_VCOUNT
-              tindex = codepoints[starter_pos + 2] - HANGUL_TBASE rescue tindex = -1
-              if 0 <= tindex && tindex < HANGUL_TCOUNT
-                j = starter_pos + 2
-                eoa -= 2
-              else
-                tindex = 0
-                j = starter_pos + 1
-                eoa -= 1
-              end
-              codepoints[starter_pos..j] = (lindex * HANGUL_VCOUNT + vindex) * HANGUL_TCOUNT + tindex + HANGUL_SBASE
-            end
-            starter_pos += 1
-            starter_char = codepoints[starter_pos]
-          # -- Other characters
-          else
-            current_char = codepoints[pos]
-            current = database.codepoints[current_char]
-            if current.combining_class > previous_combining_class
-              if ref = database.composition_map[starter_char]
-                composition = ref[current_char]
-              else
-                composition = nil
-              end
-              unless composition.nil?
-                codepoints[starter_pos] = composition
-                starter_char = composition
-                codepoints.delete_at pos
-                eoa -= 1
-                pos -= 1
-                previous_combining_class = -1
-              else
-                previous_combining_class = current.combining_class
-              end
-            else
-              previous_combining_class = current.combining_class
-            end
-            if current.combining_class == 0
-              starter_pos = pos
-              starter_char = codepoints[pos]
-            end
-          end
-        end
-        codepoints
+        codepoints.pack("U*").unicode_normalize(:nfc).codepoints
       end
 
       # Rubinius' String#scrub, however, doesn't support ASCII-incompatible chars.
@@ -266,129 +101,37 @@ module ActiveSupport
       def normalize(string, form = nil)
         form ||= @default_normalization_form
         # See http://www.unicode.org/reports/tr15, Table 1
-        codepoints = string.codepoints.to_a
         case form
         when :d
-          reorder_characters(decompose(:canonical, codepoints))
+          string.unicode_normalize(:nfd)
         when :c
-          compose(reorder_characters(decompose(:canonical, codepoints)))
+          string.unicode_normalize(:nfc)
         when :kd
-          reorder_characters(decompose(:compatibility, codepoints))
+          string.unicode_normalize(:nfkd)
         when :kc
-          compose(reorder_characters(decompose(:compatibility, codepoints)))
-          else
+          string.unicode_normalize(:nfkc)
+        else
           raise ArgumentError, "#{form} is not a valid normalization variant", caller
-        end.pack("U*".freeze)
+        end
       end
 
       def downcase(string)
-        apply_mapping string, :lowercase_mapping
+        string.downcase
       end
 
       def upcase(string)
-        apply_mapping string, :uppercase_mapping
+        string.upcase
       end
 
       def swapcase(string)
-        apply_mapping string, :swapcase_mapping
-      end
-
-      # Holds data about a codepoint in the Unicode database.
-      class Codepoint
-        attr_accessor :code, :combining_class, :decomp_type, :decomp_mapping, :uppercase_mapping, :lowercase_mapping
-
-        # Initializing Codepoint object with default values
-        def initialize
-          @combining_class = 0
-          @uppercase_mapping = 0
-          @lowercase_mapping = 0
-        end
-
-        def swapcase_mapping
-          uppercase_mapping > 0 ? uppercase_mapping : lowercase_mapping
-        end
-      end
-
-      # Holds static data from the Unicode database.
-      class UnicodeDatabase
-        ATTRIBUTES = :codepoints, :composition_exclusion, :composition_map, :boundary, :cp1252
-
-        attr_writer(*ATTRIBUTES)
-
-        def initialize
-          @codepoints = Hash.new(Codepoint.new)
-          @composition_exclusion = []
-          @composition_map = {}
-          @boundary = {}
-          @cp1252 = {}
-        end
-
-        # Lazy load the Unicode database so it's only loaded when it's actually used
-        ATTRIBUTES.each do |attr_name|
-          class_eval(<<-EOS, __FILE__, __LINE__ + 1)
-            def #{attr_name}     # def codepoints
-              load               #   load
-              @#{attr_name}      #   @codepoints
-            end                  # end
-          EOS
-        end
-
-        # Loads the Unicode database and returns all the internal objects of
-        # UnicodeDatabase.
-        def load
-          begin
-            @codepoints, @composition_exclusion, @composition_map, @boundary, @cp1252 = File.open(self.class.filename, "rb") { |f| Marshal.load f.read }
-          rescue => e
-            raise IOError.new("Couldn't load the Unicode tables for UTF8Handler (#{e.message}), ActiveSupport::Multibyte is unusable")
-          end
-
-          # Redefine the === method so we can write shorter rules for grapheme cluster breaks
-          @boundary.each_key do |k|
-            @boundary[k].instance_eval do
-              def ===(other)
-                detect { |i| i === other } ? true : false
-              end
-            end if @boundary[k].kind_of?(Array)
-          end
-
-          # define attr_reader methods for the instance variables
-          class << self
-            attr_reader(*ATTRIBUTES)
-          end
-        end
-
-        # Returns the directory in which the data files are stored.
-        def self.dirname
-          File.expand_path("../values", __dir__)
-        end
-
-        # Returns the filename for the data file for this version.
-        def self.filename
-          File.expand_path File.join(dirname, "unicode_tables.dat")
-        end
+        string.swapcase
       end
 
       private
 
-        def apply_mapping(string, mapping)
-          database.codepoints
-          string.each_codepoint.map do |codepoint|
-            cp = database.codepoints[codepoint]
-            if cp && (ncp = cp.send(mapping)) && ncp > 0
-              ncp
-            else
-              codepoint
-            end
-          end.pack("U*")
-        end
-
         def recode_windows1252_chars(string)
           string.encode(Encoding::UTF_8, Encoding::Windows_1252, invalid: :replace, undef: :replace)
         end
-
-        def database
-          @database ||= UnicodeDatabase.new
-        end
     end
   end
 end
diff --git a/activesupport/lib/active_support/rails.rb b/activesupport/lib/active_support/rails.rb
index 5c34a0abb3..8b727a69ec 100644
--- a/activesupport/lib/active_support/rails.rb
+++ b/activesupport/lib/active_support/rails.rb
@@ -27,9 +27,3 @@ require "active_support/core_ext/module/delegation"
 
 # Defines ActiveSupport::Deprecation.
 require "active_support/deprecation"
-
-# Defines Regexp#match?.
-#
-# This should be removed when Rails needs Ruby 2.4 or later, and the require
-# added where other Regexp extensions are being used (easy to grep).
-require "active_support/core_ext/regexp"
diff --git a/activesupport/lib/active_support/subscriber.rb b/activesupport/lib/active_support/subscriber.rb
index d6dd5474d0..8ad39f7a05 100644
--- a/activesupport/lib/active_support/subscriber.rb
+++ b/activesupport/lib/active_support/subscriber.rb
@@ -54,25 +54,20 @@ module ActiveSupport
         @@subscribers ||= []
       end
 
-      # TODO Change this to private once we've dropped Ruby 2.2 support.
-      # Workaround for Ruby 2.2 "private attribute?" warning.
-      protected
-
-      attr_reader :subscriber, :notifier, :namespace
-
       private
+        attr_reader :subscriber, :notifier, :namespace
 
-      def add_event_subscriber(event) # :doc:
-        return if %w{ start finish }.include?(event.to_s)
+        def add_event_subscriber(event) # :doc:
+          return if %w{ start finish }.include?(event.to_s)
 
-        pattern = "#{event}.#{namespace}"
+          pattern = "#{event}.#{namespace}"
 
-        # Don't add multiple subscribers (eg. if methods are redefined).
-        return if subscriber.patterns.include?(pattern)
+          # Don't add multiple subscribers (eg. if methods are redefined).
+          return if subscriber.patterns.include?(pattern)
 
-        subscriber.patterns << pattern
-        notifier.subscribe(pattern, subscriber)
-      end
+          subscriber.patterns << pattern
+          notifier.subscribe(pattern, subscriber)
+        end
     end
 
     attr_reader :patterns # :nodoc:
diff --git a/activesupport/lib/active_support/test_case.rb b/activesupport/lib/active_support/test_case.rb
index d1f7e6ea09..a698b4e61e 100644
--- a/activesupport/lib/active_support/test_case.rb
+++ b/activesupport/lib/active_support/test_case.rb
@@ -11,6 +11,7 @@ require "active_support/testing/isolation"
 require "active_support/testing/constant_lookup"
 require "active_support/testing/time_helpers"
 require "active_support/testing/file_fixtures"
+require "active_support/testing/parallelization"
 
 module ActiveSupport
   class TestCase < ::Minitest::Test
@@ -39,6 +40,91 @@ module ActiveSupport
       def test_order
         ActiveSupport.test_order ||= :random
       end
+
+      # Parallelizes the test suite.
+      #
+      # Takes a `workers` argument that controls how many times the process
+      # is forked. For each process a new database will be created suffixed
+      # with the worker number.
+      #
+      #   test-database-0
+      #   test-database-1
+      #
+      # If `ENV["PARALLEL_WORKERS"]` is set the workers argument will be ignored
+      # and the environment variable will be used instead. This is useful for CI
+      # environments, or other environments where you may need more workers than
+      # you do for local testing.
+      #
+      # If the number of workers is set to `1` or fewer, the tests will not be
+      # parallelized.
+      #
+      # The default parallelization method is to fork processes. If you'd like to
+      # use threads instead you can pass `with: :threads` to the `parallelize`
+      # method. Note the threaded parallelization does not create multiple
+      # database and will not work with system tests at this time.
+      #
+      #   parallelize(workers: 2, with: :threads)
+      #
+      # The threaded parallelization uses Minitest's parallel executor directly.
+      # The processes parallelization uses a Ruby Drb server.
+      def parallelize(workers: 2, with: :processes)
+        workers = ENV["PARALLEL_WORKERS"].to_i if ENV["PARALLEL_WORKERS"]
+
+        return if workers <= 1
+
+        executor = case with
+                   when :processes
+                     Testing::Parallelization.new(workers)
+                   when :threads
+                     Minitest::Parallel::Executor.new(workers)
+                   else
+                     raise ArgumentError, "#{with} is not a supported parallelization executor."
+        end
+
+        self.lock_threads = false if defined?(self.lock_threads) && with == :threads
+
+        Minitest.parallel_executor = executor
+
+        parallelize_me!
+      end
+
+      # Set up hook for parallel testing. This can be used if you have multiple
+      # databases or any behavior that needs to be run after the process is forked
+      # but before the tests run.
+      #
+      # Note: this feature is not available with the threaded parallelization.
+      #
+      # In your +test_helper.rb+ add the following:
+      #
+      #   class ActiveSupport::TestCase
+      #     parallelize_setup do
+      #       # create databases
+      #     end
+      #   end
+      def parallelize_setup(&block)
+        ActiveSupport::Testing::Parallelization.after_fork_hook do |worker|
+          yield worker
+        end
+      end
+
+      # Clean up hook for parallel testing. This can be used to drop databases
+      # if your app uses multiple write/read databases or other clean up before
+      # the tests finish. This runs before the forked process is closed.
+      #
+      # Note: this feature is not available with the threaded parallelization.
+      #
+      # In your +test_helper.rb+ add the following:
+      #
+      #   class ActiveSupport::TestCase
+      #     parallelize_teardown do
+      #       # drop databases
+      #     end
+      #   end
+      def parallelize_teardown(&block)
+        ActiveSupport::Testing::Parallelization.run_cleanup_hook do |worker|
+          yield worker
+        end
+      end
     end
 
     alias_method :method_name, :name
diff --git a/activesupport/lib/active_support/testing/assertions.rb b/activesupport/lib/active_support/testing/assertions.rb
index 6f69c48674..a891ff616d 100644
--- a/activesupport/lib/active_support/testing/assertions.rb
+++ b/activesupport/lib/active_support/testing/assertions.rb
@@ -58,6 +58,12 @@ module ActiveSupport
       #     post :create, params: { article: {...} }
       #   end
       #
+      # A hash of expressions/numeric differences can also be passed in and evaluated.
+      #
+      #   assert_difference ->{ Article.count } => 1, ->{ Notification.count } => 2 do
+      #     post :create, params: { article: {...} }
+      #   end
+      #
       # A lambda or a list of lambdas can be passed in and evaluated:
       #
       #   assert_difference ->{ Article.count }, 2 do
@@ -73,20 +79,28 @@ module ActiveSupport
       #   assert_difference 'Article.count', -1, 'An Article should be destroyed' do
       #     post :delete, params: { id: ... }
       #   end
-      def assert_difference(expression, difference = 1, message = nil, &block)
-        expressions = Array(expression)
-
-        exps = expressions.map { |e|
+      def assert_difference(expression, *args, &block)
+        expressions =
+          if expression.is_a?(Hash)
+            message = args[0]
+            expression
+          else
+            difference = args[0] || 1
+            message = args[1]
+            Hash[Array(expression).map { |e| [e, difference] }]
+          end
+
+        exps = expressions.keys.map { |e|
           e.respond_to?(:call) ? e : lambda { eval(e, block.binding) }
         }
         before = exps.map(&:call)
 
         retval = yield
 
-        expressions.zip(exps).each_with_index do |(code, e), i|
-          error  = "#{code.inspect} didn't change by #{difference}"
+        expressions.zip(exps, before) do |(code, diff), exp, before_value|
+          error  = "#{code.inspect} didn't change by #{diff}"
           error  = "#{message}.\n#{error}" if message
-          assert_equal(before[i] + difference, e.call, error)
+          assert_equal(before_value + diff, exp.call, error)
         end
 
         retval
diff --git a/activesupport/lib/active_support/testing/parallelization.rb b/activesupport/lib/active_support/testing/parallelization.rb
new file mode 100644
index 0000000000..59c8486f41
--- /dev/null
+++ b/activesupport/lib/active_support/testing/parallelization.rb
@@ -0,0 +1,102 @@
+# frozen_string_literal: true
+
+require "drb"
+require "drb/unix"
+
+module ActiveSupport
+  module Testing
+    class Parallelization # :nodoc:
+      class Server
+        include DRb::DRbUndumped
+
+        def initialize
+          @queue = Queue.new
+        end
+
+        def record(reporter, result)
+          reporter.synchronize do
+            reporter.record(result)
+          end
+        end
+
+        def <<(o)
+          @queue << o
+        end
+
+        def pop; @queue.pop; end
+      end
+
+      @after_fork_hooks = []
+
+      def self.after_fork_hook(&blk)
+        @after_fork_hooks << blk
+      end
+
+      def self.after_fork_hooks
+        @after_fork_hooks
+      end
+
+      @run_cleanup_hooks = []
+
+      def self.run_cleanup_hook(&blk)
+        @run_cleanup_hooks << blk
+      end
+
+      def self.run_cleanup_hooks
+        @run_cleanup_hooks
+      end
+
+      def initialize(queue_size)
+        @queue_size = queue_size
+        @queue      = Server.new
+        @pool       = []
+
+        @url = DRb.start_service("drbunix:", @queue).uri
+      end
+
+      def after_fork(worker)
+        self.class.after_fork_hooks.each do |cb|
+          cb.call(worker)
+        end
+      end
+
+      def run_cleanup(worker)
+        self.class.run_cleanup_hooks.each do |cb|
+          cb.call(worker)
+        end
+      end
+
+      def start
+        @pool = @queue_size.times.map do |worker|
+          fork do
+            DRb.stop_service
+
+            after_fork(worker)
+
+            queue = DRbObject.new_with_uri(@url)
+
+            while job = queue.pop
+              klass    = job[0]
+              method   = job[1]
+              reporter = job[2]
+              result   = Minitest.run_one_method(klass, method)
+
+              queue.record(reporter, result)
+            end
+
+            run_cleanup(worker)
+          end
+        end
+      end
+
+      def <<(work)
+        @queue << work
+      end
+
+      def shutdown
+        @queue_size.times { @queue << nil }
+        @pool.each { |pid| Process.waitpid pid }
+      end
+    end
+  end
+end
diff --git a/activesupport/lib/active_support/testing/time_helpers.rb b/activesupport/lib/active_support/testing/time_helpers.rb
index 998a51a34c..801ea2909b 100644
--- a/activesupport/lib/active_support/testing/time_helpers.rb
+++ b/activesupport/lib/active_support/testing/time_helpers.rb
@@ -1,7 +1,6 @@
 # frozen_string_literal: true
 
 require "active_support/core_ext/module/redefine_method"
-require "active_support/core_ext/string/strip" # for strip_heredoc
 require "active_support/core_ext/time/calculations"
 require "concurrent/map"
 
@@ -112,7 +111,7 @@ module ActiveSupport
       #   Time.current # => Sat, 09 Nov 2013 15:34:49 EST -05:00
       def travel_to(date_or_time)
         if block_given? && simple_stubs.stubbing(Time, :now)
-          travel_to_nested_block_call = <<-MSG.strip_heredoc
+          travel_to_nested_block_call = <<~MSG
 
       Calling `travel_to` with a block, when we have previously already made a call to `travel_to`, can lead to confusing time stubbing.
 
diff --git a/activesupport/lib/active_support/values/time_zone.rb b/activesupport/lib/active_support/values/time_zone.rb
index 4d81ac939e..9dfaddb825 100644
--- a/activesupport/lib/active_support/values/time_zone.rb
+++ b/activesupport/lib/active_support/values/time_zone.rb
@@ -2,7 +2,6 @@
 
 require "tzinfo"
 require "concurrent/map"
-require "active_support/core_ext/object/blank"
 
 module ActiveSupport
   # The TimeZone class serves as a wrapper around TZInfo::Timezone instances.
@@ -238,7 +237,7 @@ module ActiveSupport
         when Numeric, ActiveSupport::Duration
           arg *= 3600 if arg.abs <= 13
           all.find { |z| z.utc_offset == arg.to_i }
-          else
+        else
           raise ArgumentError, "invalid argument to TimeZone[]: #{arg.inspect}"
         end
       end
@@ -268,11 +267,14 @@ module ActiveSupport
           country = TZInfo::Country.get(code)
           country.zone_identifiers.map do |tz_id|
             if MAPPING.value?(tz_id)
-              self[MAPPING.key(tz_id)]
+              MAPPING.inject([]) do |memo, (key, value)|
+                memo << self[key] if value == tz_id
+                memo
+              end
             else
               create(tz_id, nil, TZInfo::Timezone.new(tz_id))
             end
-          end.sort!
+          end.flatten(1).sort!
         end
 
         def zones_map
diff --git a/activesupport/lib/active_support/values/unicode_tables.dat b/activesupport/lib/active_support/values/unicode_tables.dat
deleted file mode 100644
index f7d9c48bbe..0000000000
--- a/activesupport/lib/active_support/values/unicode_tables.dat
+++ /dev/null
diff --git a/activesupport/lib/active_support/xml_mini.rb b/activesupport/lib/active_support/xml_mini.rb
index ee2048cb54..e42eee07a3 100644
--- a/activesupport/lib/active_support/xml_mini.rb
+++ b/activesupport/lib/active_support/xml_mini.rb
@@ -48,10 +48,6 @@ module ActiveSupport
         "Array"      => "array",
         "Hash"       => "hash"
       }
-
-      # No need to map these on Ruby 2.4+
-      TYPE_NAMES["Fixnum"] = "integer" unless 0.class == Integer
-      TYPE_NAMES["Bignum"] = "integer" unless 0.class == Integer
     end
 
     FORMATTING = {
@@ -83,7 +79,7 @@ module ActiveSupport
         end,
         "boolean"      => Proc.new { |boolean| %w(1 true).include?(boolean.to_s.strip) },
         "string"       => Proc.new { |string|  string.to_s },
-        "yaml"         => Proc.new { |yaml|    YAML::load(yaml) rescue yaml },
+        "yaml"         => Proc.new { |yaml|    YAML.load(yaml) rescue yaml },
         "base64Binary" => Proc.new { |bin|     ::Base64.decode64(bin) },
         "binary"       => Proc.new { |bin, entity| _parse_binary(bin, entity) },
         "file"         => Proc.new { |file, entity| _parse_file(file, entity) }