project_from_ref returns nil when reference can't be accessed

Prior it would return the project from the current context, which wasn't
the intended behavior.

lib/gitlab/markdown/commit_range_reference_filter.rb

@@ -91,7 +91,7 @@ module Gitlab
       end
 
       def valid_range?(project, from_id, to_id)
-        project.valid_repo? && commit(from_id) && commit(to_id)
+        project && project.valid_repo? && commit(from_id) && commit(to_id)
       end
 
       def url_for_commit_range(project, from_id, to_id)

lib/gitlab/markdown/commit_reference_filter.rb

@@ -47,7 +47,7 @@ module Gitlab
         self.class.references_in(text) do |match, commit_ref, project_ref|
           project = self.project_from_ref(project_ref)
 
-          if project.valid_repo? && commit = project.repository.commit(commit_ref)
+          if commit = commit_from_ref(project, commit_ref)
             url = url_for_commit(project, commit)
 
             title = escape_once(commit.link_title)
@@ -64,6 +64,12 @@ module Gitlab
         end
       end
 
+      def commit_from_ref(project, commit_ref)
+        if project && project.valid_repo?
+          project.repository.commit(commit_ref)
+        end
+      end
+
       def url_for_commit(project, commit)
         h = Rails.application.routes.url_helpers
         h.namespace_project_commit_url(project.namespace, project, commit,

lib/gitlab/markdown/cross_project_reference.rb

@@ -10,18 +10,17 @@ module Gitlab
       #
       # Defaults to value of `context[:project]` if:
       # * No reference is given OR
-      # * Reference given doesn't exist OR
-      # * Reference given can't be read by the current user
+      # * Reference given doesn't exist
       #
       # ref - String reference.
       #
-      # Returns a Project
+      # Returns a Project, or nil if the reference can't be accessed
       def project_from_ref(ref)
         if ref && other = Project.find_with_namespace(ref)
           if user_can_reference_project?(other)
             other
           else
-            context[:project]
+            nil
           end
         else
           context[:project]

lib/gitlab/markdown/issue_reference_filter.rb

@@ -47,7 +47,7 @@ module Gitlab
         self.class.references_in(text) do |match, issue, project_ref|
           project = self.project_from_ref(project_ref)
 
-          if project.issue_exists?(issue)
+          if project && project.issue_exists?(issue)
             url = url_for_issue(issue, project, only_path: context[:only_path])
 
             title = escape_once("Issue: #{title_for_issue(issue, project)}")

lib/gitlab/markdown/merge_request_reference_filter.rb

@@ -47,7 +47,7 @@ module Gitlab
         self.class.references_in(text) do |match, id, project_ref|
           project = self.project_from_ref(project_ref)
 
-          if merge_request = project.merge_requests.find_by(iid: id)
+          if project && merge_request = project.merge_requests.find_by(iid: id)
             title = escape_once("Merge Request: #{merge_request.title}")
             klass = reference_class(:merge_request)
 

lib/gitlab/markdown/snippet_reference_filter.rb

@@ -47,7 +47,7 @@ module Gitlab
         self.class.references_in(text) do |match, id, project_ref|
           project = self.project_from_ref(project_ref)
 
-          if snippet = project.snippets.find_by(id: id)
+          if project && snippet = project.snippets.find_by(id: id)
             title = escape_once("Snippet: #{snippet.title}")
             klass = reference_class(:snippet)
 

spec/lib/gitlab/markdown/cross_project_reference_spec.rb

@@ -37,11 +37,11 @@ module Gitlab::Markdown
         end
 
         context 'and the user does not have permission to read it' do
-          it 'returns the project from context' do
+          it 'returns nil' do
             expect(self).to receive(:user_can_reference_project?).
               with(project2).and_return(false)
 
-            expect(project_from_ref('cross/reference')).to eq context[:project]
+            expect(project_from_ref('cross/reference')).to be_nil
           end
         end
       end