Avoid replica connection checkout/checkin for every read query

lib/active_record_proxy_adapters/primary_replica_proxy.rb

@@ -173,15 +173,6 @@ def connection_for(role, sql_string)
       update_primary_latest_write_timestamp if !replica_connection?(connection) && write_statement?(sql_string)
 
       result
-    ensure
-      # Check the connection back into its own pool, not whatever pool currently
-      # answers to the :reading role. In Rails system tests,
-      # ActiveRecord::TestFixtures#setup_shared_connection_pool reassigns the
-      # :reading role's pool_config on every transactional-fixture test's
-      # before_setup, so re-resolving `replica_pool` here can return a pool
-      # different from the one the connection was checked out of — which would
-      # check a replica adapter into the primary pool and poison it.
-      connection.pool.checkin(connection) if replica_connection?(connection)
     end
 
     def connected_to(role:, &block)
@@ -195,7 +186,7 @@ def replica_connection?(connection)
     end
 
     def checkout_replica_connection
-      replica_pool.checkout(proxy_checkout_timeout)
+      replica_pool.lease_connection
     # rescue NoDatabaseError to avoid crashing when running db:create rake task
     # rescue ConnectionNotEstablished to handle connectivity issues in the replica
     # (for example, replication delay)

spec/shared_examples/a_sql_statement.rb

@@ -3,22 +3,21 @@
 require "shared_contexts/a_proxied_method_setup"
 
 RSpec.shared_examples_for "a SQL read statement" do
-  it "checks out a connection from the replica pool" do
-    allow(replica_pool).to receive(:checkout).and_call_original
+  it "leases a connection from the replica pool" do
+    allow(replica_pool).to receive(:lease_connection).and_call_original
 
     run_test
 
-    expect(replica_pool).to have_received(:checkout).once
+    expect(replica_pool).to have_received(:lease_connection).once
   end
 
-  it "checks replica connection back in to the pool" do
+  it "uses the leased replica connection" do
     conn = instance_double(adapter_class, method_name => nil, pool: replica_pool)
-    allow(replica_pool).to receive(:checkout).and_return(conn)
-    allow(replica_pool).to receive(:checkin)
+    allow(replica_pool).to receive(:lease_connection).and_return(conn)
 
     run_test
 
-    expect(replica_pool).to have_received(:checkin).with(conn).once
+    expect(conn).to have_received(method_name).once
   end
 
   context "when a transaction is open" do
@@ -31,11 +30,11 @@
     end
 
     it "does not checkout a connection from the replica pool" do
-      allow(replica_pool).to receive(:checkout).and_call_original
+      allow(replica_pool).to receive(:lease_connection).and_call_original
 
       primary_adapter.transaction { run_test }
 
-      expect(replica_pool).not_to have_received(:checkout)
+      expect(replica_pool).not_to have_received(:lease_connection)
     end
   end
 
@@ -49,22 +48,22 @@
     end
 
     it "does not checkout a connection from the replica pool" do
-      allow(replica_pool).to receive(:checkout).and_call_original
+      allow(replica_pool).to receive(:lease_connection).and_call_original
 
       model_class.connected_to(role: TestHelper.writing_role) { run_test }
 
-      expect(replica_pool).not_to have_received(:checkout)
+      expect(replica_pool).not_to have_received(:lease_connection)
     end
   end
 end
 
 RSpec.shared_examples_for "a SQL write statement" do
   it "does not checkout a connection from replica pool" do
-    allow(replica_pool).to receive(:checkout).and_call_original
+    allow(replica_pool).to receive(:lease_connection).and_call_original
 
     run_test
 
-    expect(replica_pool).not_to have_received(:checkout)
+    expect(replica_pool).not_to have_received(:lease_connection)
   end
 
   it "sends query to primary connection" do