fix(graphql): handle nil value for locations in unified_trace

lib/datadog/tracing/contrib/graphql/unified_trace.rb

@@ -176,6 +176,26 @@ def platform_resolve_type_key(type, *args, **kwargs)
             "#{type.graphql_name}.resolve_type"
           end
 
+          # Serialize error's `locations` array as an array of Strings, given
+          # Span Events do not support hashes nested inside arrays.
+          #
+          # Here's an example in which `locations`:
+          #   [
+          #    {"line" => 3, "column" => 10},
+          #    {"line" => 7, "column" => 8},
+          #   ]
+          # is serialized as:
+          #   ["3:10", "7:8"]
+          def self.serialize_error_locations(locations)
+            # locations are only provided by the `graphql` library when the error can
+            # be associated to a particular point in the query.
+            return [] if locations.nil?
+
+            locations.map do |location|
+              "#{location["line"]}:#{location["column"]}"
+            end
+          end
+
           private
 
           # Traces the given callable with the given trace key, resource, and kwargs.
@@ -268,28 +288,13 @@ def add_query_error_events(span, errors)
                   @type_key => parsed_error.type,
                   @stacktrace_key => parsed_error.backtrace,
                   @message_key => graphql_error['message'],
-                  @locations_key => serialize_error_locations(graphql_error['locations']),
+                  @locations_key =>
+                    Datadog::Tracing::Contrib::GraphQL::UnifiedTrace.serialize_error_locations(graphql_error['locations']),
                   @path_key => graphql_error['path'],
                 )
               )
             end
           end
-
-          # Serialize error's `locations` array as an array of Strings, given
-          # Span Events do not support hashes nested inside arrays.
-          #
-          # Here's an example in which `locations`:
-          #   [
-          #    {"line" => 3, "column" => 10},
-          #    {"line" => 7, "column" => 8},
-          #   ]
-          # is serialized as:
-          #   ["3:10", "7:8"]
-          def serialize_error_locations(locations)
-            locations.map do |location|
-              "#{location["line"]}:#{location["column"]}"
-            end
-          end
         end
       end
     end

sig/datadog/tracing/contrib/graphql/unified_trace.rbs

@@ -99,7 +99,7 @@ module Datadog
 
           def add_query_error_events: (SpanOperation span, Array[GraphQL::Error] errors) -> void
 
-          def serialize_error_locations: (Array[Hash[String, Integer]] locations) -> Array[String]
+          def self.serialize_error_locations: (::Array[::Hash[::String, ::Integer]]? locations) -> ::Array[::String]
         end
       end
     end

spec/datadog/tracing/contrib/graphql/unified_trace_spec.rb

@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+require 'datadog/tracing/contrib/support/spec_helper'
+require 'graphql'
+
+RSpec.describe "Datadog::Tracing::Contrib::GraphQL::UnifiedTrace" do
+  before do
+    skip 'UnifiedTrace is only supported in GraphQL 2.0.19 and above' if Gem::Version.new(::GraphQL::VERSION) < Gem::Version.new('2.0.19')
+    require 'datadog/tracing/contrib/graphql/unified_trace'
+  end
+
+  let(:described_class) { Datadog::Tracing::Contrib::GraphQL::UnifiedTrace }
+
+  describe '.serialize_error_locations' do
+    subject(:result) { described_class.serialize_error_locations(locations) }
+
+    context 'when locations is nil' do
+      let(:locations) { nil }
+
+      it 'returns an empty array' do
+        expect(result).to eq([])
+      end
+    end
+
+    context 'when locations is an array' do
+      let(:locations) do
+        [
+          {'line' => 3, 'column' => 10},
+          {'line' => 7, 'column' => 8}
+        ]
+      end
+
+      it 'maps locations to formatted strings' do
+        expect(result).to eq(['3:10', '7:8'])
+      end
+    end
+
+    context 'when locations is an empty array' do
+      let(:locations) { [] }
+
+      it 'returns an empty array' do
+        expect(result).to eq([])
+      end
+    end
+  end
+end