Speed up project exports by moving the archive to the cache dir

As described in
https://github.com/carrierwaveuploader/carrierwave#large-files,
CarrierWave will first copy a file into the cache and then copy the
file to its final store.

Previously the `move_to_cache` value was always set to `false`, which
meant that CarrierWave would always copy the generated archive file
(e.g. `/path/tmp/work`) to the cache dir
(e.g. `/path/tmp/cache`). Since both paths are on the same filesystem,
this copy is unnecessary and slows down the generation of project
exports.

To ensure files are moved instead of copied, we can just inherit from
the GitlabUploader implementation of `move_to_cache`, which returns
`true` if it's a local file, `false` otherwise. We have to be careful
to only allow this optimization for project/group exports because
imports might be importing a static template.

For my test with the Linux kernel, this changed saved 47 seconds of
unnecessary I/O with a 3.4 GB archive.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/349425

Changelog: performance

app/uploaders/import_export_uploader.rb

@@ -12,6 +12,10 @@ class ImportExportUploader < AttachmentUploader
   end
 
   def move_to_cache
+    # Exports create temporary files that we can safely move.
+    # Imports may be from project templates that we want to copy.
+    return super if mounted_as == :export_file
+
     false
   end
 

spec/uploaders/import_export_uploader_spec.rb

@@ -10,6 +10,20 @@ RSpec.describe ImportExportUploader do
   subject { described_class.new(model, :import_file) }
 
   context 'local store' do
+    describe '#move_to_cache' do
+      it 'returns false' do
+        expect(subject.move_to_cache).to be false
+      end
+
+      context 'with project export' do
+        subject { described_class.new(model, :export_file) }
+
+        it 'returns true' do
+          expect(subject.move_to_cache).to be true
+        end
+      end
+    end
+
     describe '#move_to_store' do
       it 'returns true' do
         expect(subject.move_to_store).to be true
@@ -33,6 +47,20 @@ RSpec.describe ImportExportUploader do
       let(:fixture) { File.join('spec', 'fixtures', 'group_export.tar.gz') }
     end
 
+    describe '#move_to_cache' do
+      it 'returns false' do
+        expect(subject.move_to_cache).to be false
+      end
+
+      context 'with project export' do
+        subject { described_class.new(model, :export_file) }
+
+        it 'returns true' do
+          expect(subject.move_to_cache).to be false
+        end
+      end
+    end
+
     describe '#move_to_store' do
       it 'returns false' do
         expect(subject.move_to_store).to be false