Gracefully recover from deleted LFS file

If an LFS object exists in the database but the underlying file has been
deleted, pushing that same object from another project may result in a
confusing "403 Access Forbidden" error.

Now, if the file no longer exists, we replace the file entry with the
newly-uploaded file so that we can gracefully recover from this case.

Closes https://gitlab.com/gitlab-org/gitlab/-/issues/25852

app/controllers/repositories/lfs_storage_controller.rb

@@ -62,7 +62,10 @@ module Repositories
     # rubocop: disable CodeReuse/ActiveRecord
     def store_file!(oid, size)
       object = LfsObject.find_by(oid: oid, size: size)
-      unless object&.file&.exists?
+
+      if object
+        replace_file!(object) unless object.file&.exists?
+      else
         object = create_file!(oid, size)
       end
 
@@ -79,6 +82,16 @@ module Repositories
       LfsObject.create!(oid: oid, size: size, file: uploaded_file)
     end
 
+    def replace_file!(lfs_object)
+      uploaded_file = params[:file]
+
+      raise UploadedFile::InvalidPathError unless uploaded_file.is_a?(UploadedFile)
+
+      Gitlab::AppJsonLogger.info(message: "LFS file replaced because it did not exist", oid: oid, size: size)
+      lfs_object.file = uploaded_file
+      lfs_object.save!
+    end
+
     def link_to_project!(object)
       return unless object
 

changelogs/unreleased/sh-lfs-replace-deleted-file.yml

+---
+title: Gracefully recover from deleted LFS file
+merge_request: 45459
+author:
+type: fixed

spec/controllers/repositories/lfs_storage_controller_spec.rb

@@ -127,6 +127,41 @@ RSpec.describe Repositories::LfsStorageController do
           end
         end
 
+        context 'when existing file has been deleted' do
+          let(:lfs_object) { create(:lfs_object, :with_file) }
+
+          before do
+            FileUtils.rm(lfs_object.file.path)
+            params[:oid] = lfs_object.oid
+            params[:size] = lfs_object.size
+          end
+
+          it 'replaces the file' do
+            expect(Gitlab::AppJsonLogger).to receive(:info).with(message: "LFS file replaced because it did not exist", oid: lfs_object.oid, size: lfs_object.size)
+
+            subject
+
+            expect(response).to have_gitlab_http_status(:ok)
+            expect(lfs_object.reload.file).to exist
+          end
+
+          context 'with invalid file' do
+            before do
+              allow_next_instance_of(ActionController::Parameters) do |params|
+                allow(params).to receive(:[]).and_call_original
+                allow(params).to receive(:[]).with(:file).and_return({})
+              end
+            end
+
+            it 'renders LFS forbidden' do
+              subject
+
+              expect(response).to have_gitlab_http_status(:forbidden)
+              expect(lfs_object.reload.file).not_to exist
+            end
+          end
+        end
+
         context 'when file is not stored' do
           it 'renders unprocessable entity' do
             expect(controller).to receive(:store_file!).and_return(nil)