Fix ResultsStore#read to return a CheckResult

`MergeRequests::Mergeability::RunChecksService#run_check` expects
that `MergeRequests::Mergeability::ResultsStore#read` to return
a `MergeRequests::Mergeability::CheckResult` object.

This fixes the `ResultsStore#read` to return a `CheckResult` object
once it received a result from cache (e.g. Redis).

This is behind the `mergeability_caching` feature flag.

lib/gitlab/merge_requests/mergeability/check_result.rb

@@ -22,8 +22,8 @@ module Gitlab
 
         def self.from_hash(data)
           new(
-            status: data.fetch(:status),
-            payload: data.fetch(:payload))
+            status: data.fetch('status').to_sym,
+            payload: data.fetch('payload'))
         end
 
         def initialize(status:, payload: {})

lib/gitlab/merge_requests/mergeability/results_store.rb

@@ -9,7 +9,11 @@ module Gitlab
         end
 
         def read(merge_check:)
-          interface.retrieve_check(merge_check: merge_check)
+          result_hash = interface.retrieve_check(merge_check: merge_check)
+
+          return if result_hash.blank?
+
+          CheckResult.from_hash(result_hash)
         end
 
         def write(merge_check:, result_hash:)

spec/lib/gitlab/merge_requests/mergeability/check_result_spec.rb

@@ -70,8 +70,8 @@ RSpec.describe Gitlab::MergeRequests::Mergeability::CheckResult do
     let(:payload) { { test: 'test' } }
     let(:hash) do
       {
-        status: status,
-        payload: payload
+        'status' => status,
+        'payload' => payload
       }
     end
 

spec/lib/gitlab/merge_requests/mergeability/results_store_spec.rb

@@ -10,10 +10,22 @@ RSpec.describe Gitlab::MergeRequests::Mergeability::ResultsStore do
   let(:merge_request) { double }
 
   describe '#read' do
-    it 'calls #retrieve on the interface' do
-      expect(interface).to receive(:retrieve_check).with(merge_check: merge_check)
+    let(:result_hash) { { 'status' => 'success', 'payload' => {} } }
 
-      results_store.read(merge_check: merge_check)
+    it 'calls #retrieve_check on the interface' do
+      expect(interface).to receive(:retrieve_check).with(merge_check: merge_check).and_return(result_hash)
+
+      cached_result = results_store.read(merge_check: merge_check)
+
+      expect(cached_result.status).to eq(result_hash['status'].to_sym)
+      expect(cached_result.payload).to eq(result_hash['payload'])
+    end
+
+    context 'when #retrieve_check returns nil' do
+      it 'returns nil' do
+        expect(interface).to receive(:retrieve_check).with(merge_check: merge_check).and_return(nil)
+        expect(results_store.read(merge_check: merge_check)).to be_nil
+      end
     end
   end