RUBY-3669 Fix memory leak caused by cursors (#2995)

Author: comandeo-mongo

lib/mongo/cluster/reapers/cursor_reaper.rb

@@ -116,6 +116,12 @@ def read_scheduled_kill_specs
           if @active_cursor_ids.include?(kill_spec.cursor_id)
             @to_kill[kill_spec.server_address] ||= Set.new
             @to_kill[kill_spec.server_address] << kill_spec
+          else
+            # Cursor was already closed; end the session immediately to release
+            # references rather than waiting for the kill_spec to go out of scope.
+            if (session = kill_spec.session) && session.implicit?
+              session.end_session
+            end
           end
         end
       rescue ThreadError

lib/mongo/session.rb

@@ -97,7 +97,11 @@ def initialize(server_session, client, options = {})
       @server_session = server_session
       options = options.dup
 
-      @client = client.use(:admin)
+      # Implicit sessions only need the cluster and client options (never run
+      # transactions), so avoid creating a Mongo::Client clone to prevent
+      # memory leaks: use the original client directly instead.
+      @client = options[:implicit] ? client : client.use(:admin)
+      @cluster = @client.cluster
       @options = options.dup.freeze
       @cluster_time = nil
       @state = NO_TRANSACTION_STATE
@@ -115,7 +119,7 @@ def initialize(server_session, client, options = {})
     attr_reader :client
 
     def cluster
-      @client.cluster
+      @cluster
     end
 
     # @return [ true | false ] Whether the session is configured for snapshot
@@ -384,12 +388,13 @@ def end_session
           end
         end
         if @server_session
-          @client.cluster.session_pool.checkin(@server_session)
+          cluster.session_pool.checkin(@server_session)
         end
       end
     ensure
       @server_session = nil
       @ended = true
+      @client = nil
     end
 
     # Executes the provided block in a transaction, retrying as necessary.
@@ -1099,8 +1104,8 @@ def update_state!
     # @since 2.5.0
     # @api private
     def validate!(client)
-      check_matching_cluster!(client)
       check_if_ended!
+      check_matching_cluster!(client)
       self
     end
 
@@ -1280,7 +1285,7 @@ def check_if_ended!
     end
 
     def check_matching_cluster!(client)
-      if @client.cluster != client.cluster
+      if cluster != client.cluster
         raise Mongo::Error::InvalidSession.new(MISMATCHED_CLUSTER_ERROR_MSG)
       end
     end

spec/integration/cursor_memory_leak_spec.rb

@@ -0,0 +1,51 @@
+# frozen_string_literal: true
+# rubocop:todo all
+
+require 'spec_helper'
+
+describe 'Cursor memory leak - RUBY-3669' do
+  # Regression test: when batch_size < limit, each find/each iteration
+  # used to leak a Mongo::Client via the GC finalizer -> KillSpec -> Session
+  # -> Session#@client chain. The fix clears @client in Session#end_session
+  # and ensures CursorReaper discards stale KillSpecs cleanly.
+
+  # ObjectSpace is MRI-specific; on JRuby GC.start is not deterministic
+  require_mri
+
+  let(:collection_name) { 'cursor_memory_leak_spec' }
+  let(:collection) { authorized_client[collection_name] }
+
+  before do
+    collection.delete_many
+    collection.insert_many([{ a: 1 }, { a: 2 }, { a: 3 }])
+  end
+
+  it 'does not leak Mongo::Client objects when batch_size < limit' do
+    # Warm up: run once so any one-time initialization clients are created
+    # before we start counting.
+    collection.find(nil, batch_size: 2, limit: 3).each {}
+
+    GC.start
+    GC.start
+    GC.start
+    sleep Mongo::Cluster::CursorReaper::FREQUENCY * 2 + 1
+
+    client_count_before = ObjectSpace.each_object(Mongo::Client).count
+
+    10.times do
+      collection.find(nil, batch_size: 2, limit: 3).each {}
+    end
+
+    # Give the GC and the periodic cursor reaper time to process finalizers
+    # and discard stale KillSpecs.
+    GC.start
+    GC.start
+    GC.start
+    sleep Mongo::Cluster::CursorReaper::FREQUENCY * 2 + 1
+
+    client_count_after = ObjectSpace.each_object(Mongo::Client).count
+
+    expect(client_count_after).to be <= client_count_before,
+      "Expected Mongo::Client count to stay at #{client_count_before} but got #{client_count_after} — possible memory leak"
+  end
+end