Merge pull request #17102 from matthewd/load-interlock

Concurrent load interlock (rm Rack::Lock)

8 files changed, 315 insertions, 53 deletions

diff --git a/actionpack/lib/action_dispatch.rb b/actionpack/lib/action_dispatch.rb
index dcd3ee0644..f6336c8c7a 100644
--- a/actionpack/lib/action_dispatch.rb
+++ b/actionpack/lib/action_dispatch.rb
@@ -52,6 +52,7 @@ module ActionDispatch
     autoload :DebugExceptions
     autoload :ExceptionWrapper
     autoload :Flash
+    autoload :LoadInterlock
     autoload :ParamsParser
     autoload :PublicExceptions
     autoload :Reloader
diff --git a/actionpack/lib/action_dispatch/middleware/load_interlock.rb b/actionpack/lib/action_dispatch/middleware/load_interlock.rb
new file mode 100644
index 0000000000..07f498319c
--- /dev/null
+++ b/actionpack/lib/action_dispatch/middleware/load_interlock.rb
@@ -0,0 +1,21 @@
+require 'active_support/dependencies'
+require 'rack/body_proxy'
+
+module ActionDispatch
+  class LoadInterlock
+    def initialize(app)
+      @app = app
+    end
+
+    def call(env)
+      interlock = ActiveSupport::Dependencies.interlock
+      interlock.start_running
+      response = @app.call(env)
+      body = Rack::BodyProxy.new(response[2]) { interlock.done_running }
+      response[2] = body
+      response
+    ensure
+      interlock.done_running unless body
+    end
+  end
+end
diff --git a/activesupport/lib/active_support/concurrency/share_lock.rb b/activesupport/lib/active_support/concurrency/share_lock.rb
new file mode 100644
index 0000000000..f1c6230084
--- /dev/null
+++ b/activesupport/lib/active_support/concurrency/share_lock.rb
@@ -0,0 +1,138 @@
+require 'thread'
+require 'monitor'
+
+module ActiveSupport
+  module Concurrency
+    # A share/exclusive lock, otherwise known as a read/write lock.
+    #
+    # https://en.wikipedia.org/wiki/Readers%E2%80%93writer_lock
+    #--
+    # Note that a pending Exclusive lock attempt does not block incoming
+    # Share requests (i.e., we are "read-preferring"). That seems
+    # consistent with the behavior of +loose_upgrades+, but may be the
+    # wrong choice otherwise: it nominally reduces the possibility of
+    # deadlock by risking starvation instead.
+    class ShareLock
+      include MonitorMixin
+
+      # We track Thread objects, instead of just using counters, because
+      # we need exclusive locks to be reentrant, and we need to be able
+      # to upgrade share locks to exclusive.
+
+
+      # If +loose_upgrades+ is false (the default), then a thread that
+      # is waiting on an Exclusive lock will continue to hold any Share
+      # lock that it has already established. This is safer, but can
+      # lead to deadlock.
+      #
+      # If +loose_upgrades+ is true, a thread waiting on an Exclusive
+      # lock will temporarily relinquish its Share lock. Being less
+      # strict, this behavior prevents some classes of deadlocks. For
+      # many resources, loose upgrades are sufficient: if a thread is
+      # awaiting a lock, it is not running any other code.
+      attr_reader :loose_upgrades
+
+      def initialize(loose_upgrades = false)
+        @loose_upgrades = loose_upgrades
+
+        super()
+
+        @cv = new_cond
+
+        @sharing = Hash.new(0)
+        @exclusive_thread = nil
+        @exclusive_depth = 0
+      end
+
+      # Returns false if +no_wait+ is specified and the lock is not
+      # immediately available. Otherwise, returns true after the lock
+      # has been acquired.
+      def start_exclusive(no_wait=false)
+        synchronize do
+          unless @exclusive_thread == Thread.current
+            return false if no_wait && busy?
+
+            loose_shares = nil
+            if @loose_upgrades
+              loose_shares = @sharing.delete(Thread.current)
+            end
+
+            @cv.wait_while { busy? } if busy?
+
+            @exclusive_thread = Thread.current
+            @sharing[Thread.current] = loose_shares if loose_shares
+          end
+          @exclusive_depth += 1
+
+          true
+        end
+      end
+
+      # Relinquish the exclusive lock. Must only be called by the thread
+      # that called start_exclusive (and currently holds the lock).
+      def stop_exclusive
+        synchronize do
+          raise "invalid unlock" if @exclusive_thread != Thread.current
+
+          @exclusive_depth -= 1
+          if @exclusive_depth == 0
+            @exclusive_thread = nil
+            @cv.broadcast
+          end
+        end
+      end
+
+      def start_sharing
+        synchronize do
+          if @exclusive_thread && @exclusive_thread != Thread.current
+            @cv.wait_while { @exclusive_thread }
+          end
+          @sharing[Thread.current] += 1
+        end
+      end
+
+      def stop_sharing
+        synchronize do
+          if @sharing[Thread.current] > 1
+            @sharing[Thread.current] -= 1
+          else
+            @sharing.delete Thread.current
+            @cv.broadcast
+          end
+        end
+      end
+
+      # Execute the supplied block while holding the Exclusive lock. If
+      # +no_wait+ is set and the lock is not immediately available,
+      # returns +nil+ without yielding. Otherwise, returns the result of
+      # the block.
+      def exclusive(no_wait=false)
+        if start_exclusive(no_wait)
+          begin
+            yield
+          ensure
+            stop_exclusive
+          end
+        end
+      end
+
+      # Execute the supplied block while holding the Share lock.
+      def sharing
+        start_sharing
+        begin
+          yield
+        ensure
+          stop_sharing
+        end
+      end
+
+      private
+
+      # Must be called within synchronize
+      def busy?
+        (@exclusive_thread && @exclusive_thread != Thread.current) ||
+          @sharing.size > (@sharing[Thread.current] > 0 ? 1 : 0)
+      end
+    end
+  end
+end
diff --git a/activesupport/lib/active_support/dependencies.rb b/activesupport/lib/active_support/dependencies.rb
index 664cc15a29..770c845435 100644
--- a/activesupport/lib/active_support/dependencies.rb
+++ b/activesupport/lib/active_support/dependencies.rb
@@ -12,12 +12,33 @@ require 'active_support/core_ext/kernel/reporting'
 require 'active_support/core_ext/load_error'
 require 'active_support/core_ext/name_error'
 require 'active_support/core_ext/string/starts_ends_with'
+require "active_support/dependencies/interlock"
 require 'active_support/inflector'
 
 module ActiveSupport #:nodoc:
   module Dependencies #:nodoc:
     extend self
 
+    mattr_accessor :interlock
+    self.interlock = Interlock.new
+
+    # :doc:
+
+    # Execute the supplied block without interference from any
+    # concurrent loads
+    def self.run_interlock
+      Dependencies.interlock.running { yield }
+    end
+
+    # Execute the supplied block while holding an exclusive lock,
+    # preventing any other thread from being inside a #run_interlock
+    # block at the same time
+    def self.load_interlock
+      Dependencies.interlock.loading { yield }
+    end
+
+    # :nodoc:
+
     # Should we turn on Ruby warnings on the first load of dependent files?
     mattr_accessor :warnings_on_first_load
     self.warnings_on_first_load = false
@@ -234,10 +255,12 @@ module ActiveSupport #:nodoc:
       end
 
       def load_dependency(file)
-        if Dependencies.load? && ActiveSupport::Dependencies.constant_watch_stack.watching?
-          Dependencies.new_constants_in(Object) { yield }
-        else
-          yield
+        Dependencies.load_interlock do
+          if Dependencies.load? && ActiveSupport::Dependencies.constant_watch_stack.watching?
+            Dependencies.new_constants_in(Object) { yield }
+          else
+            yield
+          end
         end
       rescue Exception => exception  # errors from loading file
         exception.blame_file! file if exception.respond_to? :blame_file!
@@ -325,9 +348,11 @@ module ActiveSupport #:nodoc:
 
     def clear
       log_call
-      loaded.clear
-      loading.clear
-      remove_unloadable_constants!
+      Dependencies.load_interlock do
+        loaded.clear
+        loading.clear
+        remove_unloadable_constants!
+      end
     end
 
     def require_or_load(file_name, const_path = nil)
@@ -336,39 +361,44 @@ module ActiveSupport #:nodoc:
       expanded = File.expand_path(file_name)
       return if loaded.include?(expanded)
 
-      # Record that we've seen this file *before* loading it to avoid an
-      # infinite loop with mutual dependencies.
-      loaded << expanded
-      loading << expanded
+      Dependencies.load_interlock do
+        # Maybe it got loaded while we were waiting for our lock:
+        return if loaded.include?(expanded)
 
-      begin
-        if load?
-          log "loading #{file_name}"
-
-          # Enable warnings if this file has not been loaded before and
-          # warnings_on_first_load is set.
-          load_args = ["#{file_name}.rb"]
-          load_args << const_path unless const_path.nil?
+        # Record that we've seen this file *before* loading it to avoid an
+        # infinite loop with mutual dependencies.
+        loaded << expanded
+        loading << expanded
 
-          if !warnings_on_first_load or history.include?(expanded)
-            result = load_file(*load_args)
+        begin
+          if load?
+            log "loading #{file_name}"
+
+            # Enable warnings if this file has not been loaded before and
+            # warnings_on_first_load is set.
+            load_args = ["#{file_name}.rb"]
+            load_args << const_path unless const_path.nil?
+
+            if !warnings_on_first_load or history.include?(expanded)
+              result = load_file(*load_args)
+            else
+              enable_warnings { result = load_file(*load_args) }
+            end
           else
-            enable_warnings { result = load_file(*load_args) }
+            log "requiring #{file_name}"
+            result = require file_name
           end
-        else
-          log "requiring #{file_name}"
-          result = require file_name
+        rescue Exception
+          loaded.delete expanded
+          raise
+        ensure
+          loading.pop
         end
-      rescue Exception
-        loaded.delete expanded
-        raise
-      ensure
-        loading.pop
-      end
 
-      # Record history *after* loading so first load gets warnings.
-      history << expanded
-      result
+        # Record history *after* loading so first load gets warnings.
+        history << expanded
+        result
+      end
     end
 
     # Is the provided constant path defined?
diff --git a/activesupport/lib/active_support/dependencies/interlock.rb b/activesupport/lib/active_support/dependencies/interlock.rb
new file mode 100644
index 0000000000..148212c951
--- /dev/null
+++ b/activesupport/lib/active_support/dependencies/interlock.rb
@@ -0,0 +1,41 @@
+require 'active_support/concurrency/share_lock'
+
+module ActiveSupport #:nodoc:
+  module Dependencies #:nodoc:
+    class Interlock
+      def initialize # :nodoc:
+        @lock = ActiveSupport::Concurrency::ShareLock.new(true)
+      end
+
+      def loading
+        @lock.exclusive do
+          yield
+        end
+      end
+
+      # Attempt to obtain a "loading" (exclusive) lock. If possible,
+      # execute the supplied block while holding the lock. If there is
+      # concurrent activity, return immediately (without executing the
+      # block) instead of waiting.
+      def attempt_loading
+        @lock.exclusive(true) do
+          yield
+        end
+      end
+
+      def start_running
+        @lock.start_sharing
+      end
+
+      def done_running
+        @lock.stop_sharing
+      end
+
+      def running
+        @lock.sharing do
+          yield
+        end
+      end
+    end
+  end
+end
diff --git a/railties/lib/rails/application/default_middleware_stack.rb b/railties/lib/rails/application/default_middleware_stack.rb
index 6f9ccec137..88eade5c5a 100644
--- a/railties/lib/rails/application/default_middleware_stack.rb
+++ b/railties/lib/rails/application/default_middleware_stack.rb
@@ -26,7 +26,27 @@ module Rails
             middleware.use ::Rack::Cache, rack_cache
           end
 
-          middleware.use ::Rack::Lock unless allow_concurrency?
+          if config.allow_concurrency == false
+            # User has explicitly opted out of concurrent request
+            # handling: presumably their code is not threadsafe
+
+            middleware.use ::Rack::Lock
+
+          elsif config.allow_concurrency == :unsafe
+            # Do nothing, even if we know this is dangerous. This is the
+            # historical behaviour for true.
+
+          else
+            # Default concurrency setting: enabled, but safe
+
+            unless config.cache_classes && config.eager_load
+              # Without cache_classes + eager_load, the load interlock
+              # is required for proper operation
+
+              middleware.use ::ActionDispatch::LoadInterlock
+            end
+          end
+
           middleware.use ::Rack::Runtime
           middleware.use ::Rack::MethodOverride unless config.api_only
           middleware.use ::ActionDispatch::RequestId
@@ -65,14 +85,6 @@ module Rails
           config.reload_classes_only_on_change != true || app.reloaders.map(&:updated?).any?
         end
 
-        def allow_concurrency?
-          if config.allow_concurrency.nil?
-            config.cache_classes && config.eager_load
-          else
-            config.allow_concurrency
-          end
-        end
-
         def load_rack_cache
           rack_cache = config.action_dispatch.rack_cache
           return unless rack_cache
diff --git a/railties/lib/rails/application/finisher.rb b/railties/lib/rails/application/finisher.rb
index 0599e988d9..f8f92792a7 100644
--- a/railties/lib/rails/application/finisher.rb
+++ b/railties/lib/rails/application/finisher.rb
@@ -86,8 +86,10 @@ module Rails
       # added in the hook are taken into account.
       initializer :set_clear_dependencies_hook, group: :all do
         callback = lambda do
-          ActiveSupport::DescendantsTracker.clear
-          ActiveSupport::Dependencies.clear
+          ActiveSupport::Dependencies.interlock.attempt_loading do
+            ActiveSupport::DescendantsTracker.clear
+            ActiveSupport::Dependencies.clear
+          end
         end
 
         if config.reload_classes_only_on_change
diff --git a/railties/test/application/middleware_test.rb b/railties/test/application/middleware_test.rb
index ce92ebbf66..d298e8d632 100644
--- a/railties/test/application/middleware_test.rb
+++ b/railties/test/application/middleware_test.rb
@@ -26,7 +26,7 @@ module ApplicationTests
       assert_equal [
         "Rack::Sendfile",
         "ActionDispatch::Static",
-        "Rack::Lock",
+        "ActionDispatch::LoadInterlock",
         "ActiveSupport::Cache::Strategy::LocalCache",
         "Rack::Runtime",
         "Rack::MethodOverride",
@@ -58,7 +58,7 @@ module ApplicationTests
       assert_equal [
         "Rack::Sendfile",
         "ActionDispatch::Static",
-        "Rack::Lock",
+        "ActionDispatch::LoadInterlock",
         "ActiveSupport::Cache::Strategy::LocalCache",
         "Rack::Runtime",
         "ActionDispatch::RequestId",
@@ -121,23 +121,40 @@ module ApplicationTests
       assert !middleware.include?("ActiveRecord::Migration::CheckPending")
     end
 
-    test "includes lock if cache_classes is set but eager_load is not" do
+    test "includes interlock if cache_classes is set but eager_load is not" do
       add_to_config "config.cache_classes = true"
       boot!
-      assert middleware.include?("Rack::Lock")
+      assert_not_includes middleware, "Rack::Lock"
+      assert_includes middleware, "ActionDispatch::LoadInterlock"
+    end
+
+    test "includes interlock if cache_classes is off" do
+      add_to_config "config.cache_classes = false"
+      boot!
+      assert_not_includes middleware, "Rack::Lock"
+      assert_includes middleware, "ActionDispatch::LoadInterlock"
     end
 
     test "does not include lock if cache_classes is set and so is eager_load" do
       add_to_config "config.cache_classes = true"
       add_to_config "config.eager_load = true"
       boot!
-      assert !middleware.include?("Rack::Lock")
+      assert_not_includes middleware, "Rack::Lock"
+      assert_not_includes middleware, "ActionDispatch::LoadInterlock"
+    end
+
+    test "does not include lock if allow_concurrency is set to :unsafe" do
+      add_to_config "config.allow_concurrency = :unsafe"
+      boot!
+      assert_not_includes middleware, "Rack::Lock"
+      assert_not_includes middleware, "ActionDispatch::LoadInterlock"
     end
 
-    test "does not include lock if allow_concurrency is set" do
-      add_to_config "config.allow_concurrency = true"
+    test "includes lock if allow_concurrency is disabled" do
+      add_to_config "config.allow_concurrency = false"
       boot!
-      assert !middleware.include?("Rack::Lock")
+      assert_includes middleware, "Rack::Lock"
+      assert_not_includes middleware, "ActionDispatch::LoadInterlock"
     end
 
     test "removes static asset server if serve_static_files is disabled" do