Ensure that cache-control headers are merged

There are several aspects to this commit, that don't well fit into broken down
commits, so they are detailed here:

 * When a user uses response.headers['Cache-Control'] = some_value, then the
   documented convention in ConditionalGet is not adhered to, in this case,
   response.cache_control is ignored due to `return if
   self[CACHE_CONTROL].present?`
 * When a middleware sets cache-control headers that would clobber, they're
   converted to symbols directly, without underscores. This would lead to bugs.
 * Items that would live in :extras if set through expires_in, are placed
   directly in the @cache_control hash, and not respected in many cases
   (somewhat adhering to the aforementioned documentation).
 * Although quite useless, any directive named 'extras' would be ignored.

The general convention applied is that expires_* take precedence, but no longer
overwrite everything and expires_* are ALWAYS applied, even if the header is
set.

I am still unhappy about the contents of this commit, and the code in general.
Ideally it should be refactored to no longer use :extras. I'd likely recommend
expanding @cache_control into a class, and giving it the power to handle the
merge in a more efficient fashion. Such a commit would be a larger change that
could have additional semantic changes for other libraries unless they utilize
expires_in in very standard ways.

1 files changed, 32 insertions, 10 deletions

diff --git a/actionpack/lib/action_dispatch/http/cache.rb b/actionpack/lib/action_dispatch/http/cache.rb
index 5ee4c044ea..0b895e7860 100644
--- a/actionpack/lib/action_dispatch/http/cache.rb
+++ b/actionpack/lib/action_dispatch/http/cache.rb
@@ -84,17 +84,29 @@ module ActionDispatch
         LAST_MODIFIED = "Last-Modified".freeze
         ETAG          = "ETag".freeze
         CACHE_CONTROL = "Cache-Control".freeze
+        SPESHUL_KEYS  = %w[extras no-cache max-age public must-revalidate]
+
+        def cache_control_headers
+          cache_control = {}
+          if cc = self[CACHE_CONTROL]
+            cc.delete(' ').split(',').each do |segment|
+              directive, argument = segment.split('=', 2)
+              case directive
+              when *SPESHUL_KEYS
+                key = directive.tr('-', '_')
+                cache_control[key.to_sym] = argument || true
+              else
+                cache_control[:extras] ||= []
+                cache_control[:extras] << segment
+              end
+            end
+          end
+          cache_control
+        end
 
         def prepare_cache_control!
-          @cache_control = {}
+          @cache_control = cache_control_headers
           @etag = self[ETAG]
-
-          if cache_control = self[CACHE_CONTROL]
-            cache_control.split(/,\s*/).each do |segment|
-              first, last = segment.split("=")
-              @cache_control[first.to_sym] = last || true
-            end
-          end
         end
 
         def handle_conditional_get!
@@ -110,14 +122,24 @@ module ActionDispatch
         MUST_REVALIDATE       = "must-revalidate".freeze
 
         def set_conditional_cache_control!
-          return if self[CACHE_CONTROL].present?
+          control = {}
+          cc_headers = cache_control_headers
+          if extras = cc_headers.delete(:extras)
+            @cache_control[:extras] ||= []
+            @cache_control[:extras] += extras
+            @cache_control[:extras].uniq!
+          end
 
-          control = @cache_control
+          control.merge! cc_headers
+          control.merge! @cache_control
 
           if control.empty?
             headers[CACHE_CONTROL] = DEFAULT_CACHE_CONTROL
           elsif control[:no_cache]
             headers[CACHE_CONTROL] = NO_CACHE
+            if control[:extras]
+              headers[CACHE_CONTROL] += ", #{control[:extras].join(', ')}"
+            end
           else
             extras  = control[:extras]
             max_age = control[:max_age]