FIX: in redis readonly raise an exception from DistributedMutex

If we detect redis is in readonly we can not correctly get a mutex
raise an exception to notify caller

When getting optimized images avoid the distributed mutex unless
for some reason it is the first call and we need to generate a thumb

In redis readonly no thumbnails will be generated

lib/distributed_mutex.rb

@@ -7,15 +7,26 @@ class DistributedMutex
 
   def initialize(key, redis = nil)
     @key = key
+    @using_global_redis = true if !redis
     @redis = redis || $redis
     @mutex = Mutex.new
   end
 
+  CHECK_READONLY_ATTEMPT ||= 10
+
   # NOTE wrapped in mutex to maintain its semantics
   def synchronize
+
     @mutex.lock
+    attempts = 0
+
     while !try_to_get_lock
       sleep 0.001
+      attempts += 1
+      # in readonly we will never be able to get a lock
+      if @using_global_redis && attempts > CHECK_READONLY_ATTEMPT
+        raise Discourse::ReadOnly
+      end
     end
 
     yield