Skip or retain project while deleting the project:

* Skip Ci::Build#update_project_statistics whenever there's no
  project (i.e. we're deleting the project!)
* Retain the unscoped_project before deleting the build,
  so that we could use the data to delete the artifacts.
  Note that carrierwave uses `after_commit` for this,
  so we need to retain it in the memory.

Closes #15005

app/models/ci/build.rb

@@ -41,7 +41,7 @@ module Ci
 
     before_save :update_artifacts_size, if: :artifacts_file_changed?
     before_save :ensure_token
-    before_destroy { project }
+    before_destroy { unscoped_project }
 
     after_create :execute_hooks
     after_save :update_project_statistics, if: :artifacts_size_changed?
@@ -416,16 +416,23 @@ module Ci
     # This method returns old path to artifacts only if it already exists.
     #
     def artifacts_path
+      # We need the project even if it's soft deleted, because whenever
+      # we're really deleting the project, we'll also delete the builds,
+      # and in order to delete the builds, we need to know where to find
+      # the artifacts, which is depending on the data of the project.
+      # We need to retain the project in this case.
+      the_project = project || unscoped_project
+
       old = File.join(created_at.utc.strftime('%Y_%m'),
-                      project.ci_id.to_s,
+                      the_project.ci_id.to_s,
                       id.to_s)
 
       old_store = File.join(ArtifactUploader.artifacts_path, old)
-      return old if project.ci_id && File.directory?(old_store)
+      return old if the_project.ci_id && File.directory?(old_store)
 
       File.join(
         created_at.utc.strftime('%Y_%m'),
-        project.id.to_s,
+        the_project.id.to_s,
         id.to_s
       )
     end
@@ -559,6 +566,10 @@ module Ci
       self.update(erased_by: user, erased_at: Time.now, artifacts_expire_at: nil)
     end
 
+    def unscoped_project
+      @unscoped_project ||= Project.unscoped.find_by(id: gl_project_id)
+    end
+
     def predefined_variables
       variables = [
         { key: 'CI', value: 'true', public: true },
@@ -597,6 +608,8 @@ module Ci
     end
 
     def update_project_statistics
+      return unless project
+
       ProjectCacheWorker.perform_async(project_id, [], [:build_artifacts_size])
     end
   end

changelogs/unreleased/fix-deleting-project-again.yml

+---
+title: Fix deleting projects with pipelines and builds
+merge_request: 8960
+author:

spec/workers/project_destroy_worker_spec.rb

 require 'spec_helper'
 
 describe ProjectDestroyWorker do
-  let(:project) { create(:project) }
+  let(:project) { create(:project, pending_delete: true) }
   let(:path) { project.repository.path_to_repo }
 
   subject { ProjectDestroyWorker.new }
 
-  describe "#perform" do
-    it "deletes the project" do
+  describe '#perform' do
+    context 'with pipelines and builds' do
+      let!(:pipeline) { create(:ci_pipeline, project: project) }
+      let!(:build) { create(:ci_build, :artifacts, pipeline: pipeline) }
+
+      it 'deletes the project along with pipelines and builds' do
         subject.perform(project.id, project.owner.id, {})
 
         expect(Project.all).not_to include(project)
+        expect(Ci::Pipeline.all).not_to include(pipeline)
+        expect(Ci::Build.all).not_to include(build)
         expect(Dir.exist?(path)).to be_falsey
       end
+    end
 
-    it "deletes the project but skips repo deletion" do
-      subject.perform(project.id, project.owner.id, { "skip_repo" => true })
+    it 'deletes the project but skips repo deletion' do
+      subject.perform(project.id, project.owner.id, { 'skip_repo' => true })
 
       expect(Project.all).not_to include(project)
       expect(Dir.exist?(path)).to be_truthy