Fix ConnectionPool::Reaper reaping parent connection pools after fork.

After forking, the connection handler will discard any connection pools that belongs to the parent process. However, it will continue to hold a reference to the connection pools of the parent process which is used for retrieval of the connection pool in the child process. Therefore, the `weakref_alive?` check in the connection pool reaper is insufficient since the connection pools of the parent process is never GCed.

activerecord/lib/active_record/connection_adapters/abstract/connection_pool.rb

@@ -336,7 +336,10 @@ def spawn_thread(frequency)
                 while running
                   sleep t
                   @mutex.synchronize do
-                    @pools[frequency].select!(&:weakref_alive?)
+                    @pools[frequency].select! do |pool|
+                      pool.weakref_alive? && !pool.discarded?
+                    end
+
                     @pools[frequency].each do |p|
                       p.reap
                       p.flush
@@ -532,7 +535,7 @@ def disconnect!
       # See AbstractAdapter#discard!
       def discard! # :nodoc:
         synchronize do
-          return if @connections.nil? # already discarded
+          return if self.discarded?
           @connections.each do |conn|
             conn.discard!
           end
@@ -540,6 +543,10 @@ def discard! # :nodoc:
         end
       end
 
+      def discarded? # :nodoc:
+        @connections.nil?
+      end
+
       # Clears the cache which maps classes and re-connects connections that
       # require reloading.
       #
@@ -648,7 +655,7 @@ def remove(conn)
       # or a thread dies unexpectedly.
       def reap
         stale_connections = synchronize do
-          return unless @connections
+          return if self.discarded?
           @connections.select do |conn|
             conn.in_use? && !conn.owner.alive?
           end.each do |conn|
@@ -673,7 +680,7 @@ def flush(minimum_idle = @idle_timeout)
         return if minimum_idle.nil?
 
         idle_connections = synchronize do
-          return unless @connections
+          return if self.discarded?
           @connections.select do |conn|
             !conn.in_use? && conn.seconds_idle >= minimum_idle
           end.each do |conn|

activerecord/test/cases/reaper_test.rb

@@ -5,23 +5,13 @@
 module ActiveRecord
   module ConnectionAdapters
     class ReaperTest < ActiveRecord::TestCase
-      attr_reader :pool
-
-      def setup
-        super
-        @pool = ConnectionPool.new ActiveRecord::Base.connection_pool.spec
-      end
-
-      teardown do
-        @pool.connections.each(&:close)
-      end
-
       class FakePool
         attr_reader :reaped
         attr_reader :flushed
 
-        def initialize
+        def initialize(discarded: false)
           @reaped = false
+          @discarded = discarded
         end
 
         def reap
@@ -31,6 +21,14 @@ def reap
         def flush
           @flushed = true
         end
+
+        def discard!
+          @discarded = true
+        end
+
+        def discarded?
+          @discarded
+        end
       end
 
       # A reaper with nil time should never reap connections
@@ -40,6 +38,8 @@ def test_nil_time
         reaper = ConnectionPool::Reaper.new(fp, nil)
         reaper.run
         assert_not fp.reaped
+      ensure
+        fp.discard!
       end
 
       def test_some_time
@@ -53,10 +53,17 @@ def test_some_time
         end
         assert fp.reaped
         assert fp.flushed
+      ensure
+        fp.discard!
       end
 
       def test_pool_has_reaper
+        spec = ActiveRecord::Base.connection_pool.spec.dup
+        pool = ConnectionPool.new spec
+
         assert pool.reaper
+      ensure
+        pool.discard!
       end
 
       def test_reaping_frequency_configuration
@@ -64,6 +71,8 @@ def test_reaping_frequency_configuration
         spec.config[:reaping_frequency] = "10.01"
         pool = ConnectionPool.new spec
         assert_equal 10.01, pool.reaper.frequency
+      ensure
+        pool.discard!
       end
 
       def test_connection_pool_starts_reaper
@@ -80,6 +89,8 @@ def test_connection_pool_starts_reaper
 
         wait_for_conn_idle(conn)
         assert_not_predicate conn, :in_use?
+      ensure
+        pool.discard!
       end
 
       def test_reaper_works_after_pool_discard
@@ -136,6 +147,26 @@ def test_connection_pool_starts_reaper_in_fork
 
         Process.waitpid(pid)
         assert $?.success?
+      ensure
+        pool.discard!
+      end
+
+      def test_reaper_does_not_reap_discarded_connection_pools
+        discarded_pool = FakePool.new(discarded: true)
+        pool = FakePool.new
+        frequency = 0.001
+
+        ConnectionPool::Reaper.new(discarded_pool, frequency).run
+        ConnectionPool::Reaper.new(pool, frequency).run
+
+        until pool.flushed
+          Thread.pass
+        end
+
+        assert_not discarded_pool.reaped
+        assert pool.reaped
+      ensure
+        pool.discard!
       end
 
       def new_conn_in_thread(pool)