Use nil instead of raising AmbiguousRef

app/models/project.rb

@@ -36,7 +36,6 @@ class Project < ActiveRecord::Base
   extend Gitlab::ConfigHelper
 
   BoardLimitExceeded = Class.new(StandardError)
-  AmbiguousRef = Class.new(StandardError)
 
   STATISTICS_ATTRIBUTE = 'repositories_count'.freeze
   NUMBER_OF_PERMITTED_BOARDS = 1
@@ -1166,7 +1165,7 @@ class Project < ActiveRecord::Base
     branch_exists = repository.branch_exists?(ref)
 
     if tag_exists && branch_exists
-      raise AmbiguousRef
+      nil
     elsif tag_exists
       repository.find_tag(ref)
     elsif branch_exists
@@ -1754,6 +1753,8 @@ class Project < ActiveRecord::Base
 
   def protected_for?(ref)
     resolved_ref = resolve_ref(ref)
+    return false unless resolved_ref
+
     full_ref = resolved_ref.full_ref
     ref_name = resolved_ref.name
 

lib/gitlab/ci/pipeline/chain/validate/repository.rb

@@ -17,9 +17,7 @@ module Gitlab
                 return error('Commit not found')
               end
 
-              begin
-                @command.project.resolve_ref(@command.origin_ref)
-              rescue Project::AmbiguousRef
+              unless @command.project.resolve_ref(@command.origin_ref)
                 return error('Ref is ambiguous')
               end
             end

spec/models/project_spec.rb

@@ -2571,6 +2571,14 @@ describe Project do
       allow(project).to receive(:resolve_ref).and_return(ref)
     end
 
+    context 'when ref is ambiguous' do
+      let(:ref) { nil }
+
+      it 'returns false' do
+        is_expected.to be_falsey
+      end
+    end
+
     context 'when the ref is not protected' do
       let(:ref) { project.repository.find_branch('master') }
 
@@ -2820,8 +2828,8 @@ describe Project do
           project.repository.add_branch(project.creator, ref, 'master')
         end
 
-        it 'raises an error' do
-          expect { subject }.to raise_error(described_class::AmbiguousRef)
+        it 'returns nil' do
+          is_expected.to eq(nil)
         end
       end