Fix hanging staging on cc-uploader kill

add idempotent droplet creation via build lock + find-or-create:
- lock build:
  SELECT * FROM builds WHERE id = $1 FOR UPDATE LIMIT 1;
- if there exists already a droplet, take that one and ensure
  buildpack_lifecycle_data is attached (but don’t overwrite)

Author: kathap

app/actions/droplet_create.rb

@@ -49,22 +49,30 @@ def create_docker_droplet(build)
       droplet
     end
 
-    def create_buildpack_droplet(build)
-      droplet = droplet_from_build(build)
-
+    def find_or_create_buildpack_droplet(build)
       DropletModel.db.transaction do
+        BuildModel.where(id: build.id).for_update.first or
+          raise "Build #{build.id} not found for locking"
+
+        existing = DropletModel.where(build_guid: build.guid).first
+        return existing if existing
+
+        droplet = droplet_from_build(build)
         droplet.save
+
         if build.cnb_lifecycle?
           droplet.cnb_lifecycle_data = build.cnb_lifecycle_data
         else
           droplet.buildpack_lifecycle_data = build.buildpack_lifecycle_data
         end
-      end
 
-      droplet.reload
-      Steno.logger('build_completed').info("droplet created: #{droplet.guid}")
-      record_audit_event(droplet, build.package, user_audit_info_from_build(build))
-      droplet
+        droplet.reload
+        Steno.logger('build_completed').info("droplet created: #{droplet.guid}")
+        record_audit_event(droplet, build.package, user_audit_info_from_build(build))
+        droplet
+      end
+    rescue Sequel::UniqueConstraintViolation
+      DropletModel.where(build_guid: build.guid).first
     end
 
     private

app/controllers/runtime/stagings_controller.rb

@@ -134,7 +134,7 @@ def droplet_from_build(build, guid)
     end
 
     def create_droplet_from_build(build)
-      DropletCreate.new.create_buildpack_droplet(build)
+      DropletCreate.new.find_or_create_buildpack_droplet(build)
     end
 
     def upload_path

spec/unit/actions/droplet_create_spec.rb

@@ -158,13 +158,13 @@ module VCAP::CloudController
       end
     end
 
-    describe '#create_buildpack_droplet' do
+    describe '#find_or_create_buildpack_droplet' do
       context 'buildpack lifecycle' do
         let!(:buildpack_lifecycle_data) { BuildpackLifecycleDataModel.make(build:) }
 
         it 'sets it on the droplet' do
           expect do
-            droplet_create.create_buildpack_droplet(build)
+            droplet_create.find_or_create_buildpack_droplet(build)
           end.to change { [DropletModel.count, Event.count] }.by([1, 1])
 
           droplet = DropletModel.last
@@ -195,6 +195,55 @@ module VCAP::CloudController
                                        })
         end
 
+        it 'creates the droplet once and attaches lifecycle data' do
+          expect {
+            droplet_create.find_or_create_buildpack_droplet(build)
+          }.to change { [DropletModel.count, Event.count] }.by([1, 1])
+
+          droplet = DropletModel.last
+          expect(droplet.state).to eq(DropletModel::STAGING_STATE)
+          expect(droplet.app).to eq(app)
+          expect(droplet.package).to eq(package)
+          expect(droplet.build).to eq(build)
+
+          buildpack_lifecycle_data.reload
+          expect(buildpack_lifecycle_data.droplet).to eq(droplet)
+        end
+
+        it 'is idempotent on repeated calls (no duplicate droplet, same GUID, no extra event)' do
+          first = droplet_create.find_or_create_buildpack_droplet(build)
+
+          expect {
+            second = droplet_create.find_or_create_buildpack_droplet(build)
+            expect(second.guid).to eq(first.guid)
+          }.to change(DropletModel, :count).by(0)
+                                           .and change(Event, :count).by(0)
+
+          # Ensure still only one droplet for this build
+          expect(DropletModel.where(build_guid: build.guid).count).to eq(1)
+
+          buildpack_lifecycle_data.reload
+          expect(buildpack_lifecycle_data.droplet).to eq(first)
+        end
+
+        it 'returns the pre-existing droplet when one already exists for the build' do
+          existing = DropletModel.make(
+            app: app,
+            package: package,
+            build: build,
+            state: DropletModel::STAGING_STATE
+          )
+          buildpack_lifecycle_data.update(droplet: existing)
+
+          expect {
+            returned = droplet_create.find_or_create_buildpack_droplet(build)
+            expect(returned.guid).to eq(existing.guid)
+          }.to change(DropletModel, :count).by(0)
+                                           .and change(Event, :count).by(0)
+
+          expect(DropletModel.where(build_guid: build.guid).first.guid).to eq(existing.guid)
+        end
+
         context 'when the build does not contain created_by fields' do
           let(:build) do
             BuildModel.make(
@@ -205,7 +254,7 @@ module VCAP::CloudController
 
           it 'sets the actor to UNKNOWN' do
             expect do
-              droplet_create.create_buildpack_droplet(build)
+              droplet_create.find_or_create_buildpack_droplet(build)
             end.to change { [DropletModel.count, Event.count] }.by([1, 1])
 
             droplet = DropletModel.last
@@ -230,7 +279,7 @@ module VCAP::CloudController
 
         it 'sets it on the droplet' do
           expect do
-            droplet_create.create_buildpack_droplet(build)
+            droplet_create.find_or_create_buildpack_droplet(build)
           end.to change { [DropletModel.count, Event.count] }.by([1, 1])
 
           droplet = DropletModel.last
@@ -271,7 +320,7 @@ module VCAP::CloudController
 
           it 'sets the actor to UNKNOWN' do
             expect do
-              droplet_create.create_buildpack_droplet(build)
+              droplet_create.find_or_create_buildpack_droplet(build)
             end.to change { [DropletModel.count, Event.count] }.by([1, 1])
 
             droplet = DropletModel.last