Record logs in query limiting middleware

This test support system helps us detect expensive endpoints. This
change adds sufficient information to assess what the cause of the
excessive queries is. To avoid storing unbounded numbers of queries,
only the first 1.5x the threshold queries are recorded.

lib/gitlab/query_limiting/active_support_subscriber.rb

@@ -6,9 +6,10 @@ module Gitlab
       attach_to :active_record
 
       def sql(event)
-        unless event.payload.fetch(:cached, event.payload[:name] == 'CACHE')
-          Transaction.current&.increment
-        end
+        return if !Transaction.current || event.payload.fetch(:cached, event.payload[:name] == 'CACHE')
+
+        Transaction.current.increment
+        Transaction.current.executed_sql(event.payload[:sql])
       end
     end
   end

lib/gitlab/query_limiting/transaction.rb

@@ -15,6 +15,7 @@ module Gitlab
       # the sake of keeping things simple we hardcode this value here, it's not
       # supposed to be changed very often anyway.
       THRESHOLD = 100
+      LOG_THRESHOLD = THRESHOLD * 1.5
 
       # Error that is raised whenever exceeding the maximum number of queries.
       ThresholdExceededError = Class.new(StandardError)
@@ -45,6 +46,7 @@ module Gitlab
         @action = nil
         @count = 0
         @whitelisted = false
+        @sql_executed = []
       end
 
       # Sends a notification based on the number of executed SQL queries.
@@ -60,6 +62,10 @@ module Gitlab
         @count += 1 unless whitelisted
       end
 
+      def executed_sql(sql)
+        @sql_executed << sql unless @count > LOG_THRESHOLD
+      end
+
       def raise_error?
         Rails.env.test?
       end
@@ -71,8 +77,11 @@ module Gitlab
       def error_message
         header = 'Too many SQL queries were executed'
         header = "#{header} in #{action}" if action
+        msg = "a maximum of #{THRESHOLD} is allowed but #{count} SQL queries were executed"
+        log = @sql_executed.each_with_index.map { |sql, i| "#{i}: #{sql}" }.join("\n").presence
+        ellipsis = '...' if @count > LOG_THRESHOLD
 
-        "#{header}: a maximum of #{THRESHOLD} is allowed but #{count} SQL queries were executed"
+        ["#{header}: #{msg}", log, ellipsis].compact.join("\n")
       end
     end
   end

spec/lib/gitlab/query_limiting/active_support_subscriber_spec.rb

@@ -3,7 +3,7 @@
 require 'spec_helper'
 
 RSpec.describe Gitlab::QueryLimiting::ActiveSupportSubscriber do
-  let(:transaction) { instance_double(Gitlab::QueryLimiting::Transaction, increment: true) }
+  let(:transaction) { instance_double(Gitlab::QueryLimiting::Transaction, executed_sql: true, increment: true) }
 
   before do
     allow(Gitlab::QueryLimiting::Transaction)
@@ -18,6 +18,11 @@ RSpec.describe Gitlab::QueryLimiting::ActiveSupportSubscriber do
       expect(transaction)
         .to have_received(:increment)
         .once
+
+      expect(transaction)
+        .to have_received(:executed_sql)
+        .once
+        .with(String)
     end
 
     context 'when the query is actually a rails cache hit' do
@@ -30,6 +35,11 @@ RSpec.describe Gitlab::QueryLimiting::ActiveSupportSubscriber do
         expect(transaction)
           .to have_received(:increment)
           .once
+
+        expect(transaction)
+          .to have_received(:executed_sql)
+          .once
+          .with(String)
       end
     end
   end

spec/lib/gitlab/query_limiting/transaction_spec.rb

@@ -118,6 +118,30 @@ RSpec.describe Gitlab::QueryLimiting::Transaction do
       )
     end
 
+    it 'includes a list of executed queries' do
+      transaction = described_class.new
+      transaction.count = max = described_class::THRESHOLD
+      %w[foo bar baz].each { |sql| transaction.executed_sql(sql) }
+
+      message = transaction.error_message
+
+      expect(message).to start_with(
+        "Too many SQL queries were executed: a maximum of #{max} " \
+        "is allowed but #{max} SQL queries were executed"
+      )
+
+      expect(message).to include("0: foo", "1: bar", "2: baz")
+    end
+
+    it 'indicates if the log is truncated' do
+      transaction = described_class.new
+      transaction.count = described_class::THRESHOLD * 2
+
+      message = transaction.error_message
+
+      expect(message).to end_with('...')
+    end
+
     it 'includes the action name in the error message when present' do
       transaction = described_class.new
       transaction.count = max = described_class::THRESHOLD