FIX: Treat corrupt cache as cache miss

Currently when a cache entry is corrupt, we log the event without doing
anything else. It means the cache is still corrupt, and the proper value
isn’t computed again.

Normally, it’s very rare the cache becomes corrupt, but it can happen
when upgrading Rails for example and the cache format changes. This is
normally handled automatically by Rails but since we’re using a custom
cache class, we have to do it ourselves.

This patch takes the same approach the Rails team did, when a cache
entry is corrupt, we treat it as a miss, recomputing the proper value
and caching it in the new format.

spec/lib/cache_spec.rb

@@ -3,9 +3,7 @@
 require "cache"
 
 RSpec.describe Cache do
-  let :cache do
-    Cache.new
-  end
+  subject(:cache) { Cache.new }
 
   it "supports exist?" do
     cache.write("testing", 1.1)
@@ -104,4 +102,24 @@ RSpec.describe Cache do
 
     expect(cache.redis.ttl("#{cache.namespace}:foo:bar")).to eq(180)
   end
+
+  describe ".fetch" do
+    subject(:fetch_value) { cache.fetch("my_key") { "bob" } }
+
+    context "when the cache is corrupt" do
+      before do
+        cache.delete("my_key")
+        Discourse.redis.setex(cache.normalize_key("my_key"), described_class::MAX_CACHE_AGE, "")
+      end
+
+      it "runs and return the provided block" do
+        expect(fetch_value).to eq("bob")
+      end
+
+      it "generates a new cache entry" do
+        fetch_value
+        expect(cache.read("my_key")).to eq("bob")
+      end
+    end
+  end
 end