Merge pull request #23398 from matthewd/interlock

Address remaining known issues in Interlock

4 files changed, 119 insertions, 23 deletions

diff --git a/activesupport/lib/active_support/concurrency/share_lock.rb b/activesupport/lib/active_support/concurrency/share_lock.rb
index ca48164c54..8e4ca272ba 100644
--- a/activesupport/lib/active_support/concurrency/share_lock.rb
+++ b/activesupport/lib/active_support/concurrency/share_lock.rb
@@ -48,17 +48,11 @@ module ActiveSupport
       def start_exclusive(purpose: nil, compatible: [], no_wait: false)
         synchronize do
           unless @exclusive_thread == Thread.current
-            if busy?(purpose)
+            if busy_for_exclusive?(purpose)
               return false if no_wait
 
-              loose_shares = @sharing.delete(Thread.current)
-              @waiting[Thread.current] = compatible if loose_shares
-
-              begin
-                @cv.wait_while { busy?(purpose) }
-              ensure
-                @waiting.delete Thread.current
-                @sharing[Thread.current] = loose_shares if loose_shares
+              yield_shares(purpose, compatible) do
+                @cv.wait_while { busy_for_exclusive?(purpose) }
               end
             end
             @exclusive_thread = Thread.current
@@ -71,22 +65,26 @@ module ActiveSupport
 
       # Relinquish the exclusive lock. Must only be called by the thread
       # that called start_exclusive (and currently holds the lock).
-      def stop_exclusive
+      def stop_exclusive(compatible: [])
         synchronize do
           raise "invalid unlock" if @exclusive_thread != Thread.current
 
           @exclusive_depth -= 1
           if @exclusive_depth == 0
             @exclusive_thread = nil
-            @cv.broadcast
+
+            yield_shares(nil, compatible) do
+              @cv.broadcast
+              @cv.wait_while { @exclusive_thread || eligible_waiters?(compatible) }
+            end
           end
         end
       end
 
-      def start_sharing
+      def start_sharing(purpose: :share)
         synchronize do
-          if @exclusive_thread && @exclusive_thread != Thread.current
-            @cv.wait_while { @exclusive_thread }
+          if @sharing[Thread.current] == 0 && @exclusive_thread != Thread.current && busy_for_sharing?(purpose)
+            @cv.wait_while { busy_for_sharing?(purpose) }
           end
           @sharing[Thread.current] += 1
         end
@@ -109,12 +107,12 @@ module ActiveSupport
       # the block.
       #
       # See +start_exclusive+ for other options.
-      def exclusive(purpose: nil, compatible: [], no_wait: false)
+      def exclusive(purpose: nil, compatible: [], after_compatible: [], no_wait: false)
         if start_exclusive(purpose: purpose, compatible: compatible, no_wait: no_wait)
           begin
             yield
           ensure
-            stop_exclusive
+            stop_exclusive(compatible: after_compatible)
           end
         end
       end
@@ -132,11 +130,31 @@ module ActiveSupport
       private
 
       # Must be called within synchronize
-      def busy?(purpose)
-        (@exclusive_thread && @exclusive_thread != Thread.current) ||
-          @waiting.any? { |k, v| k != Thread.current && !v.include?(purpose) } ||
+      def busy_for_exclusive?(purpose)
+        busy_for_sharing?(purpose) ||
           @sharing.size > (@sharing[Thread.current] > 0 ? 1 : 0)
       end
+
+      def busy_for_sharing?(purpose)
+        (@exclusive_thread && @exclusive_thread != Thread.current) ||
+          @waiting.any? { |t, (_, c)| t != Thread.current && !c.include?(purpose) }
+      end
+
+      def eligible_waiters?(compatible)
+        @waiting.any? { |t, (p, _)| compatible.include?(p) && @waiting.all? { |t2, (_, c2)| t == t2 || c2.include?(p) } }
+      end
+
+      def yield_shares(purpose, compatible)
+        loose_shares = @sharing.delete(Thread.current)
+        @waiting[Thread.current] = [purpose, compatible] if loose_shares
+
+        begin
+          yield
+        ensure
+          @waiting.delete Thread.current
+          @sharing[Thread.current] = loose_shares if loose_shares
+        end
+      end
     end
   end
 end
diff --git a/activesupport/lib/active_support/dependencies/interlock.rb b/activesupport/lib/active_support/dependencies/interlock.rb
index fbeb904684..b6a1b25eee 100644
--- a/activesupport/lib/active_support/dependencies/interlock.rb
+++ b/activesupport/lib/active_support/dependencies/interlock.rb
@@ -8,13 +8,13 @@ module ActiveSupport #:nodoc:
       end
 
       def loading
-        @lock.exclusive(purpose: :load, compatible: [:load]) do
+        @lock.exclusive(purpose: :load, compatible: [:load], after_compatible: [:load]) do
           yield
         end
       end
 
       def unloading
-        @lock.exclusive(purpose: :unload, compatible: [:load, :unload]) do
+        @lock.exclusive(purpose: :unload, compatible: [:load, :unload], after_compatible: [:load, :unload]) do
           yield
         end
       end
@@ -24,7 +24,7 @@ module ActiveSupport #:nodoc:
       # concurrent activity, return immediately (without executing the
       # block) instead of waiting.
       def attempt_unloading
-        @lock.exclusive(purpose: :unload, compatible: [:load, :unload], no_wait: true) do
+        @lock.exclusive(purpose: :unload, compatible: [:load, :unload], after_compatible: [:load, :unload], no_wait: true) do
           yield
         end
       end
diff --git a/activesupport/test/share_lock_test.rb b/activesupport/test/share_lock_test.rb
index 465a657308..12953d99a6 100644
--- a/activesupport/test/share_lock_test.rb
+++ b/activesupport/test/share_lock_test.rb
@@ -114,14 +114,17 @@ class ShareLockTest < ActiveSupport::TestCase
     [true, false].each do |use_upgrading|
       with_thread_waiting_in_lock_section(:sharing) do |sharing_thread_release_latch|
         begin
+          together = Concurrent::CyclicBarrier.new(2)
           conflicting_exclusive_threads = [
             Thread.new do
               @lock.send(use_upgrading ? :sharing : :tap) do
+                together.wait
                 @lock.exclusive(purpose: :red, compatible: [:green, :purple]) {}
               end
             end,
             Thread.new do
               @lock.send(use_upgrading ? :sharing : :tap) do
+                together.wait
                 @lock.exclusive(purpose: :blue, compatible: [:green]) {}
               end
             end
@@ -183,11 +186,14 @@ class ShareLockTest < ActiveSupport::TestCase
     load_params   = [:load,   [:load]]
     unload_params = [:unload, [:unload, :load]]
 
+    all_sharing = Concurrent::CyclicBarrier.new(4)
+
     [load_params, load_params, unload_params, unload_params].permutation do |thread_params|
       with_thread_waiting_in_lock_section(:sharing) do |sharing_thread_release_latch|
         threads = thread_params.map do |purpose, compatible|
           Thread.new do
             @lock.sharing do
+              all_sharing.wait
               @lock.exclusive(purpose: purpose, compatible: compatible) do
                 scratch_pad_mutex.synchronize { scratch_pad << purpose }
               end
@@ -209,6 +215,78 @@ class ShareLockTest < ActiveSupport::TestCase
     end
   end
 
+  def test_new_share_attempts_block_on_waiting_exclusive
+    with_thread_waiting_in_lock_section(:sharing) do |sharing_thread_release_latch|
+      release_exclusive = Concurrent::CountDownLatch.new
+
+      waiting_exclusive = Thread.new do
+        @lock.sharing do
+          @lock.exclusive do
+            release_exclusive.wait
+          end
+        end
+      end
+      assert_threads_stuck waiting_exclusive
+
+      late_share_attempt = Thread.new do
+        @lock.sharing {}
+      end
+      assert_threads_stuck late_share_attempt
+
+      sharing_thread_release_latch.count_down
+      assert_threads_stuck late_share_attempt
+
+      release_exclusive.count_down
+      assert_threads_not_stuck late_share_attempt
+    end
+  end
+
+  def test_share_remains_reentrant_ignoring_a_waiting_exclusive
+    with_thread_waiting_in_lock_section(:sharing) do |sharing_thread_release_latch|
+      ready = Concurrent::CyclicBarrier.new(2)
+      attempt_reentrancy = Concurrent::CountDownLatch.new
+
+      sharer = Thread.new do
+        @lock.sharing do
+          ready.wait
+          attempt_reentrancy.wait
+          @lock.sharing {}
+        end
+      end
+
+      exclusive = Thread.new do
+        @lock.sharing do
+          ready.wait
+          @lock.exclusive {}
+        end
+      end
+
+      assert_threads_stuck exclusive
+
+      attempt_reentrancy.count_down
+
+      assert_threads_not_stuck sharer
+      assert_threads_stuck exclusive
+    end
+  end
+
+  def test_compatible_exclusives_cooperate_to_both_proceed
+    ready = Concurrent::CyclicBarrier.new(2)
+    done = Concurrent::CyclicBarrier.new(2)
+
+    threads = 2.times.map do
+      Thread.new do
+        @lock.sharing do
+          ready.wait
+          @lock.exclusive(purpose: :x, compatible: [:x], after_compatible: [:x]) {}
+          done.wait
+        end
+      end
+    end
+
+    assert_threads_not_stuck threads
+  end
+
   def test_in_shared_section_incompatible_non_upgrading_threads_cannot_preempt_upgrading_threads
     scratch_pad       = []
     scratch_pad_mutex = Mutex.new
diff --git a/railties/lib/rails/application/finisher.rb b/railties/lib/rails/application/finisher.rb
index 404e3c3e23..411cdbad19 100644
--- a/railties/lib/rails/application/finisher.rb
+++ b/railties/lib/rails/application/finisher.rb
@@ -86,7 +86,7 @@ module Rails
       # added in the hook are taken into account.
       initializer :set_clear_dependencies_hook, group: :all do
         callback = lambda do
-          ActiveSupport::Dependencies.interlock.attempt_unloading do
+          ActiveSupport::Dependencies.interlock.unloading do
             ActiveSupport::DescendantsTracker.clear
             ActiveSupport::Dependencies.clear
           end