Fix nil actor errors in Sentry API handler

The Golang SDK often sends up multiple exceptions, with the second one
containing more context with the appropriate stack trace. Previously the
API handler assumed that the stacktrace existed for the first exception.
However, this assumption can be wrong, leading to 500 errors when
clients attempt to report an error to GitLab.

Since the GitLab API can only track one Sentry exception, we now choose
the first exception that has a stacktrace. We also also gracefully
handle an empty stacktrace by reporting no actor.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/348672

Changelog: fixed

app/services/error_tracking/collect_error_service.rb

@@ -2,6 +2,8 @@
 
 module ErrorTracking
   class CollectErrorService < ::BaseService
+    include Gitlab::Utils::StrongMemoize
+
     def execute
       # Error is a way to group events based on common data like name or cause
       # of exception. We need to keep a sane balance here between taking too little
@@ -43,16 +45,29 @@ module ErrorTracking
     end
 
     def exception
-      event['exception']['values'].first
+      strong_memoize(:exception) do
+        # Find the first exception that has a stacktrace since the first
+        # exception may not provide adequate context (e.g. in the Go SDK).
+        entries = event['exception']['values']
+        entries.find { |x| x.key?('stacktrace') } || entries.first
+      end
+    end
+
+    def stacktrace_frames
+      strong_memoize(:stacktrace_frames) do
+        exception.dig('stacktrace', 'frames')
+      end
     end
 
     def actor
       return event['transaction'] if event['transaction']
 
-      # Some SDK do not have transaction attribute.
+      # Some SDKs do not have a transaction attribute.
       # So we build it by combining function name and module name from
       # the last item in stacktrace.
-      last_line = exception.dig('stacktrace', 'frames').last
+      return unless stacktrace_frames.present?
+
+      last_line = stacktrace_frames.last
 
       "#{last_line['function']}(#{last_line['module']})"
     end

spec/fixtures/error_tracking/go_two_exception_event.json

+{"contexts":{"device":{"arch":"amd64","num_cpu":16},"os":{"name":"darwin"},"runtime":{"go_maxprocs":16,"go_numcgocalls":1,"go_numroutines":2,"name":"go","version":"go1.16.10"}},"event_id":"f92492349cda4ceaba1aab9dac55a412","level":"error","platform":"go","release":"v0.12.0-1-g6b72962","sdk":{"name":"sentry.go","version":"0.12.0","integrations":["ContextifyFrames","Environment","IgnoreErrors","Modules"],"packages":[{"name":"sentry-go","version":"0.12.0"}]},"server_name":"jet.fios-router.home","user":{},"modules":{"github.com/getsentry/sentry-go":"(devel)","golang.org/x/sys":"v0.0.0-20211007075335-d3039528d8ac"},"exception":[{"type":"*errors.errorString","value":"unsupported protocol scheme \"\""},{"type":"*url.Error","value":"Get \"foobar\": unsupported protocol scheme \"\"","stacktrace":{"frames":[{"function":"main","module":"main","abs_path":"/Users/stanhu/github/sentry-go/example/basic/main.go","lineno":54,"pre_context":["\t// Set the timeout to the maximum duration the program can afford to wait.","\tdefer sentry.Flush(2 * time.Second)","","\tresp, err := http.Get(os.Args[1])","\tif err != nil {"],"context_line":"\t\tsentry.CaptureException(err)","post_context":["\t\tlog.Printf(\"reported to Sentry: %s\", err)","\t\treturn","\t}","\tdefer resp.Body.Close()",""],"in_app":true}]}}],"timestamp":"2021-12-25T22:32:06.191665-08:00"}

spec/services/error_tracking/collect_error_service_spec.rb

@@ -92,6 +92,25 @@ RSpec.describe ErrorTracking::CollectErrorService do
         expect(event.environment).to eq 'Accumulate'
         expect(event.payload).to eq parsed_event
       end
+
+      context 'with two exceptions' do
+        let(:parsed_event) { Gitlab::Json.parse(fixture_file('error_tracking/go_two_exception_event.json')) }
+
+        it 'reports using second exception', :aggregate_failures do
+          subject.execute
+
+          event = ErrorTracking::ErrorEvent.last
+          error = event.error
+
+          expect(error.name).to eq '*url.Error'
+          expect(error.description).to eq(%(Get \"foobar\": unsupported protocol scheme \"\"))
+          expect(error.platform).to eq 'go'
+          expect(error.actor).to eq('main(main)')
+
+          expect(event.description).to eq(%(Get \"foobar\": unsupported protocol scheme \"\"))
+          expect(event.payload).to eq parsed_event
+        end
+      end
     end
   end
 end