Fix division by error when upload max size is set to 0

When pre-signed multipart URLs are generated and the max size is 0,
previously the code would error out while calculating the URLs.

We now handle this case by assuming the file is the maximum number of
parts.

Relates to https://gitlab.com/gitlab-org/gitlab/-/issues/259801

changelogs/unreleased/sh-handle-zero-maximum-upload-size.yml

+---
+title: Fix division by error when upload max size is set to 0
+merge_request: 49482
+author:
+type: fixed

lib/object_storage/direct_upload.rb

@@ -184,15 +184,20 @@ module ObjectStorage
     private
 
     def rounded_multipart_part_size
-      # round multipart_part_size up to minimum_mulitpart_size
+      # round multipart_part_size up to minimum_multipart_size
       (multipart_part_size + MINIMUM_MULTIPART_SIZE - 1) / MINIMUM_MULTIPART_SIZE * MINIMUM_MULTIPART_SIZE
     end
 
     def multipart_part_size
+      return MINIMUM_MULTIPART_SIZE if maximum_size == 0
+
       maximum_size / number_of_multipart_parts
     end
 
     def number_of_multipart_parts
+      # If we don't have max length, we can only assume the file is as large as possible.
+      return MAXIMUM_MULTIPART_PARTS if maximum_size == 0
+
       [
         # round maximum_size up to minimum_mulitpart_size
         (maximum_size + MINIMUM_MULTIPART_SIZE - 1) / MINIMUM_MULTIPART_SIZE,
@@ -201,7 +206,11 @@ module ObjectStorage
     end
 
     def requires_multipart_upload?
-      config.aws? && !has_length
+      config.aws? && !has_length && can_use_multipart_upload?
+    end
+
+    def can_use_multipart_upload?
+      (maximum_size == 0) || (maximum_size > MINIMUM_MULTIPART_SIZE)
     end
 
     def upload_id

spec/lib/object_storage/direct_upload_spec.rb

@@ -340,6 +340,26 @@ RSpec.describe ObjectStorage::DirectUpload do
             stub_object_storage_multipart_init(storage_url, "myUpload")
           end
 
+          context 'when maximum upload size is 0' do
+            let(:maximum_size) { 0 }
+
+            it 'returns maximum number of parts' do
+              expect(subject[:MultipartUpload][:PartURLs].length).to eq(100)
+            end
+
+            it 'part size is minimum, 5MB' do
+              expect(subject[:MultipartUpload][:PartSize]).to eq(5.megabyte)
+            end
+          end
+
+          context 'when maximum upload size is < 5 MB' do
+            let(:maximum_size) { 1024 }
+
+            it 'does not include multipart URLs' do
+              expect(subject).not_to include(:MultipartUpload)
+            end
+          end
+
           context 'when maximum upload size is 10MB' do
             let(:maximum_size) { 10.megabyte }