Improve error tracking stacktrace method

Make sure stacktrace is generated even when context is missing.
For that we add another fixture from browser error when there is no
context around. It will also work when part of context is missing like
pre context or post context.

Changelog: changed
Signed-off-by: Dmytro Zaporozhets <dzaporozhets@gitlab.com>

app/models/error_tracking/error_event.rb

@@ -61,9 +61,9 @@ class ErrorTracking::ErrorEvent < ApplicationRecord
     pre_context = entry['pre_context']
     post_context = entry['post_context']
 
-    context += lines_with_position(pre_context, error_line_no - pre_context.size)
+    context += lines_with_position(pre_context, error_line_no - pre_context.size) if pre_context
     context += lines_with_position([error_line], error_line_no)
-    context += lines_with_position(post_context, error_line_no + 1)
+    context += lines_with_position(post_context, error_line_no + 1) if post_context
 
     context.reject(&:blank?)
   end

spec/factories/error_tracking/error_event.rb

@@ -63,5 +63,9 @@ FactoryBot.define do
     level { 'error' }
     occurred_at { Time.now.iso8601 }
     payload { Gitlab::Json.parse(File.read(Rails.root.join('spec/fixtures/', 'error_tracking/parsed_event.json'))) }
+
+    trait :browser do
+      payload { Gitlab::Json.parse(File.read(Rails.root.join('spec/fixtures/', 'error_tracking/browser_event.json'))) }
+    end
   end
 end

spec/fixtures/error_tracking/browser_event.json

+{"sdk":{"name":"sentry.javascript.browser","version":"5.7.1","packages":[{"name":"npm:@sentry/browser","version":"5.7.1"}],"integrations":["InboundFilters","FunctionToString","TryCatch","Breadcrumbs","GlobalHandlers","LinkedErrors","UserAgent","Dedupe","ExtraErrorData","ReportingObserver","RewriteFrames","Vue"]},"level":"error","request":{"url":"http://localhost:5444/","headers":{"User-Agent":"Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:91.0) Gecko/20100101 Firefox/91.0"}},"event_id":"6a32dc45cd924196930e06aa21b48c8d","platform":"javascript","exception":{"values":[{"type":"TypeError","value":"Cannot read property 'filter' of undefined","mechanism":{"type":"generic","handled":true},"stacktrace":{"frames":[{"colno":34,"in_app":true,"lineno":6395,"filename":"webpack-internal:///./node_modules/vue/dist/vue.runtime.esm.js","function":"hydrate"},{"colno":57,"in_app":true,"lineno":6362,"filename":"webpack-internal:///./node_modules/vue/dist/vue.runtime.esm.js","function":"hydrate"},{"colno":13,"in_app":true,"lineno":3115,"filename":"webpack-internal:///./node_modules/vue/dist/vue.runtime.esm.js","function":"init"},{"colno":10,"in_app":true,"lineno":8399,"filename":"webpack-internal:///./node_modules/vue/dist/vue.runtime.esm.js","function":"Vue.prototype.$mount"},{"colno":3,"in_app":true,"lineno":4061,"filename":"webpack-internal:///./node_modules/vue/dist/vue.runtime.esm.js","function":"mountComponent"},{"colno":12,"in_app":true,"lineno":4456,"filename":"webpack-internal:///./node_modules/vue/dist/vue.runtime.esm.js","function":"Watcher"},{"colno":25,"in_app":true,"lineno":4467,"filename":"webpack-internal:///./node_modules/vue/dist/vue.runtime.esm.js","function":"get"},{"colno":10,"in_app":true,"lineno":4048,"filename":"webpack-internal:///./node_modules/vue/dist/vue.runtime.esm.js","function":"updateComponent"},{"colno":19,"in_app":true,"lineno":3933,"filename":"webpack-internal:///./node_modules/vue/dist/vue.runtime.esm.js","function":"lifecycleMixin/Vue.prototype._update"},{"colno":24,"in_app":true,"lineno":6477,"filename":"webpack-internal:///./node_modules/vue/dist/vue.runtime.esm.js","function":"patch"},{"colno":34,"in_app":true,"lineno":6395,"filename":"webpack-internal:///./node_modules/vue/dist/vue.runtime.esm.js","function":"hydrate"},{"colno":64,"in_app":true,"lineno":78,"filename":"webpack-internal:///./node_modules/babel-loader/lib/index.js?!./node_modules/vue-loader/lib/index.js?!./pages/index.vue?vue&type=script&lang=js&","function":"data"}]}}]},"environment":"development"}
\ No newline at end of file

spec/models/error_tracking/error_event_spec.rb

@@ -37,6 +37,23 @@ RSpec.describe ErrorTracking::ErrorEvent, type: :model do
       expect(event.stacktrace).to be_kind_of(Array)
       expect(event.stacktrace.first).to eq(expected_entry)
     end
+
+    context 'error context is missing' do
+      let(:event) { create(:error_tracking_error_event, :browser) }
+
+      it 'generates a stacktrace without context' do
+        expected_entry = {
+          'lineNo' => 6395,
+          'context' => [],
+          'filename' => 'webpack-internal:///./node_modules/vue/dist/vue.runtime.esm.js',
+          'function' => 'hydrate',
+          'colNo' => 0
+        }
+
+        expect(event.stacktrace).to be_kind_of(Array)
+        expect(event.stacktrace.first).to eq(expected_entry)
+      end
+    end
   end
 
   describe '#to_sentry_error_event' do