Fix Docker repository sync for direct link case

When local storage is used we get a direct link without redirect
it used to work but it was broken by
https://gitlab.com/gitlab-org/gitlab/merge_requests/21068.
It wasn't noticed because this bug was disguised by another small bug -
if primaty_api_url was specified with trailing slash it worked well.

changelogs/unreleased/196158-fix-no-redirect-case-for-docker-blob-replication.yml

+---
+title: 'Geo: Fix Docker repository synchronization for local storage'
+merge_request: 22981
+author:
+type: fixed

ee/lib/ee/container_registry/client.rb

@@ -42,13 +42,16 @@ module EE
       def pull_blob(name, digest)
         file = Tempfile.new("blob-#{digest}")
 
+        blob_url = "/v2/#{name}/blobs/#{digest}"
+
         response = HTTP
           .headers({ "Authorization" => "Bearer #{@options[:token]}" }) # rubocop:disable Gitlab/ModuleWithInstanceVariables
-          .get("#{@base_uri}/v2/#{name}/blobs/#{digest}") # rubocop:disable Gitlab/ModuleWithInstanceVariables
-
-        raise Error.new("Pull Blob error: #{response.body}") unless response.status.redirect?
+          .get(::Gitlab::Utils.append_path(@base_uri, blob_url)) # rubocop:disable Gitlab/ModuleWithInstanceVariables
 
+        if response.status.redirect?
           response = HTTP.get(response['Location'])
+        end
+
         response.body.each do |chunk|
           file.binmode
           file.write(chunk)

ee/spec/lib/container_registry/client_spec.rb

@@ -146,7 +146,7 @@ describe ContainerRegistry::Client do
           .to_return(status: 302, headers: { "Location" => 'http://download-link.com' })
     end
 
-    it 'GET "/v2/:name/blobs/:reference' do
+    it 'downloads file successfully when' do
       stub_request(:get, "http://download-link.com/")
         .to_return(status: 200)
 
@@ -173,5 +173,30 @@ describe ContainerRegistry::Client do
 
       expect { client.pull_blob('group/test', 'e2312abc') }.to raise_error(EE::ContainerRegistry::Client::Error)
     end
+
+    context 'when primary_api_url is specified with trailing slash' do
+      let(:client) { described_class.new("http://registry/", options) }
+
+      it 'builds correct URL' do
+        stub_request(:get, "http://registry//v2/group/test/blobs/e2312abc")
+          .with(headers: auth_headers)
+          .to_return(status: 500)
+
+        stub_request(:get, "http://download-link.com/")
+          .to_return(status: 200)
+
+        expect(client.pull_blob('group/test', 'e2312abc')).to be_a_kind_of(Tempfile)
+      end
+    end
+
+    context 'direct link to download, no redirect' do
+      it 'downloads blob successfully' do
+        stub_request(:get, "http://registry/v2/group/test/blobs/e2312abc")
+            .with(headers: auth_headers)
+            .to_return(status: 200)
+
+        expect(client.pull_blob('group/test', 'e2312abc')).to be_a_kind_of(Tempfile)
+      end
+    end
   end
 end