Gracefully handle orphaned write deploy keys in /internal/post_receivesh-handle-orphaned-deploy-keys

On GitLab.com, there are write deploy keys with no associated users.
Pushes with these deploy keys end with an Error 500 since we attempt
to look up redirect message. If there is no user, don't attempt
to display a redirect message.

Closes #41466

3 files changed, 21 insertions, 3 deletions

diff --git a/changelogs/unreleased/sh-handle-orphaned-deploy-keys.yml b/changelogs/unreleased/sh-handle-orphaned-deploy-keys.yml
new file mode 100644
index 00000000000..7d3b622534e
--- /dev/null
+++ b/changelogs/unreleased/sh-handle-orphaned-deploy-keys.yml
@@ -0,0 +1,5 @@
+---
+title: Gracefully handle orphaned write deploy keys in /internal/post_receive
+merge_request:
+author:
+type: fixed
diff --git a/lib/api/internal.rb b/lib/api/internal.rb
index ccaaeca10d4..79b302aae70 100644
--- a/lib/api/internal.rb
+++ b/lib/api/internal.rb
@@ -190,9 +190,12 @@ module API
 
         project = Gitlab::GlRepository.parse(params[:gl_repository]).first
         user = identify(params[:identifier])
-        redirect_message = Gitlab::Checks::ProjectMoved.fetch_redirect_message(user.id, project.id)
-        if redirect_message
-          output[:redirected_message] = redirect_message
+
+        # A user is not guaranteed to be returned; an orphaned write deploy
+        # key could be used
+        if user
+          redirect_message = Gitlab::Checks::ProjectMoved.fetch_redirect_message(user.id, project.id)
+          output[:redirected_message] = redirect_message if redirect_message
         end
 
         output
diff --git a/spec/requests/api/internal_spec.rb b/spec/requests/api/internal_spec.rb
index bbcd1194ae5..7b25047ea8f 100644
--- a/spec/requests/api/internal_spec.rb
+++ b/spec/requests/api/internal_spec.rb
@@ -784,6 +784,16 @@ describe API::Internal do
         expect(json_response["redirected_message"]).to eq(project_moved.redirect_message)
       end
     end
+
+    context 'with an orphaned write deploy key' do
+      it 'does not try to notify that project moved' do
+        allow_any_instance_of(Gitlab::Identifier).to receive(:identify).and_return(nil)
+
+        post api("/internal/post_receive"), valid_params
+
+        expect(response).to have_gitlab_http_status(200)
+      end
+    end
   end
 
   describe 'POST /internal/pre_receive' do