Fix TypeError from Tags::Reference

Some missing references were raising a TypeError when using `dig` to
extract the referenced object from a reference, in cases where the
reference included too many symbols and we ended up attempting to
extract a value from an array using a symbol.

This commit fixes the issue by rescuing from TypeError and properly
returning a message stating that the given reference cannot be found in
the given configuration.

Changelog: fixed

lib/gitlab/ci/config/yaml/tags/reference.rb

@@ -27,7 +27,7 @@ module Gitlab
 
             override :_resolve
             def _resolve(resolver)
-              object = resolver.config.dig(*location)
+              object = config_at_location(resolver)
               value = resolver.deep_resolve(object)
 
               raise MissingReferenceError, missing_ref_error_message unless value
@@ -35,6 +35,12 @@ module Gitlab
               value
             end
 
+            def config_at_location(resolver)
+              resolver.config.dig(*location)
+            rescue TypeError
+              raise MissingReferenceError, missing_ref_error_message
+            end
+
             def missing_ref_error_message
               "#{data[:tag]} #{data[:seq].inspect} could not be found"
             end

spec/lib/gitlab/ci/config/yaml/tags/reference_spec.rb

@@ -69,6 +69,23 @@ RSpec.describe Gitlab::Ci::Config::Yaml::Tags::Reference do
       end
     end
 
+    context 'when the references are valid but do not match the config' do
+      let(:yaml) do
+        <<~YML
+        a: [1, 2]
+        b: [3, 4]
+        c: !reference [a, b]
+        YML
+      end
+
+      it 'raises a MissingReferenceError' do
+        expect { subject }.to raise_error(
+          Gitlab::Ci::Config::Yaml::Tags::Reference::MissingReferenceError,
+          '!reference ["a", "b"] could not be found'
+        )
+      end
+    end
+
     context 'with arrays' do
       let(:yaml) do
         <<~YML