Add Rails 6 backport for ActiveRecord

This fixes the "can't create Thread: Resource temporarily unavailable" error.

config/initializers/rails6_backports.rb

@@ -0,0 +1,98 @@
+raise "Remove no-longer-needed #{__FILE__}!" if Rails::VERSION::MAJOR >= 6
+
+require "weakref"
+
+module ActiveRecord
+  # Backport https://github.com/rails/rails/pull/36998 and https://github.com/rails/rails/pull/36999
+  # to avoid `ThreadError: can't create Thread: Resource temporarily unavailable` issues
+  module ConnectionAdapters
+    class ConnectionPool
+      class Reaper
+        @mutex = Mutex.new
+        @pools = {}
+        @threads = {}
+
+        class << self
+          def register_pool(pool, frequency) # :nodoc:
+            @mutex.synchronize do
+              unless @threads[frequency]&.alive?
+                @threads[frequency] = spawn_thread(frequency)
+              end
+              @pools[frequency] ||= []
+              @pools[frequency] << WeakRef.new(pool)
+            end
+          end
+
+          private
+            def spawn_thread(frequency)
+              Thread.new(frequency) do |t|
+                running = true
+                while running
+                  sleep t
+                  @mutex.synchronize do
+                    @pools[frequency].select!(&:weakref_alive?)
+                    @pools[frequency].each do |p|
+                      p.reap
+                      p.flush
+                    rescue WeakRef::RefError
+                    end
+
+                    if @pools[frequency].empty?
+                      @pools.delete(frequency)
+                      @threads.delete(frequency)
+                      running = false
+                    end
+                  end
+                end
+              end
+            end
+        end
+
+        def run
+          return unless frequency && frequency > 0
+          self.class.register_pool(pool, frequency)
+        end
+      end
+
+      def reap
+        stale_connections = synchronize do
+          return unless @connections
+          @connections.select do |conn|
+            conn.in_use? && !conn.owner.alive?
+          end.each do |conn|
+            conn.steal!
+          end
+        end
+
+        stale_connections.each do |conn|
+          if conn.active?
+            conn.reset!
+            checkin conn
+          else
+            remove conn
+          end
+        end
+      end
+
+      def flush(minimum_idle = @idle_timeout)
+        return if minimum_idle.nil?
+
+        idle_connections = synchronize do
+          return unless @connections
+          @connections.select do |conn|
+            !conn.in_use? && conn.seconds_idle >= minimum_idle
+          end.each do |conn|
+            conn.lease
+
+            @available.delete conn
+            @connections.delete conn
+          end
+        end
+
+        idle_connections.each do |conn|
+          conn.disconnect!
+        end
+      end
+    end
+  end
+end