Add support for JSON logging for audit events

This will add audit_json.log that writes one line per audit event. For
example:

{
       "severity":"INFO",
       "time":"2018-10-17T17:38:22.523Z",
       "author_id":3,
       "entity_id":2,
       "entity_type":"Project",
       "change":"visibility",
       "from":"Private",
       "to":"Public",
       "author_name":"John Doe4",
       "target_id":2,
       "target_type":"Project",
       "target_details":"namespace2/project2"
}

app/services/audit_event_service.rb

@@ -17,11 +17,29 @@ class AuditEventService
   end
   def security_event
-    SecurityEvent.create(
+    log_security_event_to_file
+    log_security_event_to_database
+  end
+
+  private
+
+  def base_payload
+    {
       author_id: @author.id,
       entity_id: @entity.id,
-      entity_type: @entity.class.name,
-      details: @details
-    )
+      entity_type: @entity.class.name
+    }
+  end
+
+  def file_logger
+    @file_logger ||= Gitlab::AuditJsonLogger.build
+  end
+
+  def log_security_event_to_file
+    file_logger.info(base_payload.merge(@details))
+  end
+
+  def log_security_event_to_database
+    SecurityEvent.create(base_payload.merge(details: @details))
   end
 end

changelogs/unreleased/sh-add-audit-logging-json-ce.yml

+---
+title: Add support for JSON logging for audit events
+merge_request: 22471
+author:
+type: added

doc/administration/logs.md

@@ -144,6 +144,20 @@ December 03, 2014 13:20 -> ERROR -> Command failed [1]: /usr/bin/git --git-dir=/
 error: failed to push some refs to '/Users/vsizov/gitlab-development-kit/repositories/gitlabhq/gitlab_git.git'
 ```
+## `audit_json.log`
+
+This file lives in `/var/log/gitlab/gitlab-rails/audit_json.log` for
+Omnibus GitLab packages or in `/home/git/gitlab/log/audit_json.log` for
+installations from source.
+
+Changes to group or project settings are logged to this file. For example:
+
+```json
+{"severity":"INFO","time":"2018-10-17T17:38:22.523Z","author_id":3,"entity_id":2,"entity_type":"Project","change":"visibility","from":"Private","to":"Public","author_name":"John Doe4","target_id":2,"target_type":"Project","target_details":"namespace2/project2"}
+{"severity":"INFO","time":"2018-10-17T17:38:22.830Z","author_id":5,"entity_id":3,"entity_type":"Project","change":"name","from":"John Doe7 / project3","to":"John Doe7 / new name","author_name":"John Doe6","target_id":3,"target_type":"Project","target_details":"namespace3/project3"}
+{"severity":"INFO","time":"2018-10-17T17:38:23.175Z","author_id":7,"entity_id":4,"entity_type":"Project","change":"path","from":"","to":"namespace4/newpath","author_name":"John Doe8","target_id":4,"target_type":"Project","target_details":"namespace4/newpath"}
+```
+
 ## `sidekiq.log`
 This file lives in `/var/log/gitlab/gitlab-rails/sidekiq.log` for

lib/gitlab/audit_json_logger.rb

+# frozen_string_literal: true
+
+module Gitlab
+  class AuditJsonLogger < Gitlab::JsonLogger
+    def self.file_name_noext
+      'audit_json'
+    end
+  end
+end

spec/services/audit_event_service_spec.rb

+# frozen_string_literal: true
+
+require 'spec_helper'
+
+describe AuditEventService do
+  let(:project) { create(:project) }
+  let(:user) { create(:user) }
+  let(:project_member) { create(:project_member, user: user) }
+  let(:service) { described_class.new(user, project, { action: :destroy }) }
+  let(:logger) { instance_double(Gitlab::AuditJsonLogger) }
+
+  describe '#security_event' do
+    before do
+      expect(service).to receive(:file_logger).and_return(logger)
+    end
+
+    it 'creates an event and logs to a file' do
+      expect(logger).to receive(:info).with(author_id: user.id,
+                                            entity_id: project.id,
+                                            entity_type: "Project",
+                                            action: :destroy)
+
+      expect { service.security_event }.to change(SecurityEvent, :count).by(1)
+    end
+  end
+end