Explicit cache key.

This cache key defaults to `:rack`. Sub-classes need to explicitly change
this if they alter the way cached values are generated.

lib/rack/request.rb

@@ -501,11 +501,22 @@ def parseable_data?
         raise
       end
 
+      # This cache key is used by cached values generated by class_cache_for,
+      # specfically GET and POST. This is to ensure that the cache is not
+      # shared between instances of different classes which have different
+      # behaviour. This includes sub-classes that override query_parser or
+      # expand_params.
+      def cache_key
+        :rack
+      end
+
       private def class_cache_for(key, validity_key, current_value)
+        cache_key = self.cache_key
+
         if get_header(validity_key).equal?(current_value)
           if cache = get_header(key)
-            if cache.key?(self.class)
-              value = cache[self.class]
+            if cache.key?(cache_key)
+              value = cache[cache_key]
               if value.is_a?(Exception)
                 raise value.class, value.message, cause: value.cause
               else
@@ -519,15 +530,15 @@ def parseable_data?
           set_header(key, cache = {})
         end
 
-        cache.fetch(self.class) do
+        cache.fetch(cache_key) do
           value = yield(current_value)
 
           # Only set this after generating the value, so that if an error or other cache depending on the same key, it will be invalidated correctly:
           set_header(validity_key, current_value)
 
-          return cache[self.class] = value
+          return cache[cache_key] = value
         rescue => error
-          cache[self.class] = error
+          cache[cache_key] = error
           raise
         end
       end

test/spec_request.rb

@@ -1564,7 +1564,7 @@ def initialize(*)
     )
 
     form_hash = {'foo' => 'bar'}.freeze
-    form_hash_cache[req.class] = form_hash
+    form_hash_cache[req.cache_key] = form_hash
 
     req.POST.must_equal form_hash
   end