Instrument adapter queries for external programs.

When executing queries via the adapter, we may not recieve the
instrumentation we expect. The ActiveRecord adapter uses a raw
connection which bypasses the ActiveRecord instrumentation layer and
instead talks directly to PG, making it impossible to know what queries
are being executed.

This change cribs heavily from the implementation of the `log` method on
the `AbstractAdapter` class in ActiveRecord to make it easy to add
instrumentation to an adapter. By default the instrumenter is set to
`nil`, and instrumentation will be a no-op; otherwise the query will be
instrumented via the `instrument` method and passed the relevant payload
to handle. The structure of this is deliberately similar to that of the
event emitted by ActiveRecord's `sql.active_record` notification.

lib/que/adapters/active_record.rb

@@ -13,6 +13,11 @@ class ActiveRecord < Base
         ::PG::UnableToSend,
       ].freeze
 
+      def initialize(_thing = nil)
+        super
+        @instrumenter = ActiveSupport::Notifications.instrumenter
+      end
+
       def checkout
         checkout_activerecord_adapter { |conn| yield conn.raw_connection }
       rescue *AR_UNAVAILABLE_CONNECTION_ERRORS => e

lib/que/adapters/base.rb

@@ -13,8 +13,11 @@ module Adapters
     class UnavailableConnection < StandardError; end
 
     class Base
+      attr_reader :instrumenter
+
       def initialize(_thing = nil)
         @prepared_statements = {}
+        @instrumenter = nil
       end
 
       # The only method that adapters really need to implement. Should lock a
@@ -62,7 +65,14 @@ def in_transaction?
 
       def execute_sql(sql, params)
         args = params.empty? ? [sql] : [sql, params]
-        checkout { |conn| conn.async_exec(*args) }
+
+        checkout do |conn|
+          log(sql, params, conn, async: true) do |notification_payload|
+            conn.async_exec(*args).tap do |result|
+              notification_payload[:row_count] = result.count
+            end
+          end
+        end
       end
 
       def execute_prepared(name, params)
@@ -81,7 +91,11 @@ def execute_prepared(name, params)
               prepared_just_now = statements[name] = true
             end
 
-            conn.exec_prepared("que_#{name}", params)
+            log(SQL[name], params, conn, async: false) do |notification_payload|
+              conn.exec_prepared("que_#{name}", params).tap do |result|
+                notification_payload[:row_count] = result.count
+              end
+            end
           rescue ::PG::InvalidSqlStatementName => error
             # Reconnections on ActiveRecord can cause the same connection
             # objects to refer to new backends, so recover as well as we can.
@@ -97,6 +111,21 @@ def execute_prepared(name, params)
         end
       end
 
+      def log(sql, binds, conn, statement_name: nil, async: false, &block)
+        return yield({}) if instrumenter.nil?
+
+        instrumenter.instrument(
+          "que.active_record",
+          sql: sql,
+          binds: binds,
+          type_casted_binds: [],
+          async: async,
+          statement_name: statement_name,
+          connection: conn,
+          &block
+        )
+      end
+
       CAST_PROCS = {
         # booleans
         16 => ->(value) {

spec/lib/que/job_spec.rb

@@ -5,6 +5,11 @@
 RSpec.describe Que::Job do
   describe ".enqueue" do
     let(:run_at) { postgres_now }
+    let(:job_class) do
+      Class.new(described_class).tap do |klass|
+        stub_const("FooBarJob", klass)
+      end
+    end
 
     it "adds job to que_jobs table" do
       expect { described_class.enqueue(:hello, run_at: run_at) }.
@@ -41,15 +46,14 @@
         que_job_id: an_instance_of(Integer),
         queue: "default",
         priority: 100,
-        job_class: a_string_including("Class"),
+        job_class: "FooBarJob",
         retryable: true,
         run_at: run_at,
         args: [500, "gbp", "testing"],
         custom_log_1: 500,
         custom_log_2: "test-log",
       )
 
-      job_class = Class.new(described_class)
       job_class.custom_log_context ->(attrs) {
         {
           custom_log_1: attrs[:args][0],
@@ -59,6 +63,31 @@
       job_class.enqueue(500, :gbp, :testing,  run_at: run_at)
     end
 
+    it "instruments queries using ActiveSupport::Notifications" do
+      events = []
+
+      ActiveSupport::Notifications.subscribe("que.active_record") do |event|
+        events << event
+      end
+
+      job_class.enqueue("foobar")
+
+      expect(events).to include(
+        having_attributes(
+          name: "que.active_record",
+          payload: hash_including(
+            sql: /INSERT INTO que_jobs/,
+            binds: [nil, nil, nil, "FooBarJob", true, "[\"foobar\"]"],
+            type_casted_binds: [],
+            async: false,
+            statement_name: nil,
+            connection: instance_of(PG::Connection),
+            row_count: 1,
+          ),
+        ),
+      )
+    end
+
     context "with a custom adapter specified" do
       let(:custom_adapter) { Que.adapter.dup }
       let(:job_with_adapter) { Class.new(described_class) }