Fix header casing compatibility with Rails 7

To support Rack 3, response headers in `Sprockets::Server` were all
downcased. However, this has led to issues with Rack 2 applications (ex.
Rails 7) since they still expect mixed case (ex. `Content-Type`)
headers.

To ensure compatibility with both Rack 2 and Rack 3 applications, this
commit makes the casing of the headers conditional on the Rack version.
Rack itself provides constants to do this easily for most of the headers
used (`Content-Type`, `Content-Length`, `Cache-Control`, and `ETag`) and
the rest are added as constants under `Rack::Server`.

As an alternative to this, the responses could instead be wrapped using
`Rack::Headers` (and `Rack::Utils::HeaderHash` in Rack 2), but making
the header casing conditional seems better to me because it is
relatively easier to implement and there will be less churn if/when Rack
2 support is eventually removed.

CHANGELOG.md

@@ -3,6 +3,7 @@
 Get upgrade notes from Sprockets 3.x to 4.x at https://github.com/rails/sprockets/blob/master/UPGRADING.md
 
 - Fix for precompile issues when multiple extensions map to the same MIME type (eg. `.jpeg` / `.jpg`). [#781](https://github.com/rails/sprockets/pull/781)
+- Fix compatibility with Rack 2 applications. [#790](https://github.com/rails/sprockets/pull/790)
 
 ## 4.2.0
 

lib/sprockets/server.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 require 'set'
 require 'time'
-require 'rack/utils'
+require 'rack'
 
 module Sprockets
   # `Server` is a concern mixed into `Environment` and
@@ -11,6 +11,16 @@ module Server
     # Supported HTTP request methods.
     ALLOWED_REQUEST_METHODS = ['GET', 'HEAD'].to_set.freeze
 
+    # :stopdoc:
+    if Gem::Version.new(Rack::RELEASE) < Gem::Version.new("3")
+      X_CASCADE = "X-Cascade"
+      VARY = "Vary"
+    else
+      X_CASCADE = "x-cascade"
+      VARY = "vary"
+    end
+    # :startdoc:
+
     # `call` implements the Rack 1.x specification which accepts an
     # `env` Hash and returns a three item tuple with the status code,
     # headers, and body.
@@ -148,39 +158,39 @@ def not_modified_response(env, etag)
       # Returns a 400 Forbidden response tuple
       def bad_request_response(env)
         if head_request?(env)
-          [ 400, { "content-type" => "text/plain", "content-length" => "0" }, [] ]
+          [ 400, { Rack::CONTENT_TYPE => "text/plain", Rack::CONTENT_LENGTH => "0" }, [] ]
         else
-          [ 400, { "content-type" => "text/plain", "content-length" => "11" }, [ "Bad Request" ] ]
+          [ 400, { Rack::CONTENT_TYPE => "text/plain", Rack::CONTENT_LENGTH => "11" }, [ "Bad Request" ] ]
         end
       end
 
       # Returns a 403 Forbidden response tuple
       def forbidden_response(env)
         if head_request?(env)
-          [ 403, { "content-type" => "text/plain", "content-length" => "0" }, [] ]
+          [ 403, { Rack::CONTENT_TYPE => "text/plain", Rack::CONTENT_LENGTH => "0" }, [] ]
         else
-          [ 403, { "content-type" => "text/plain", "content-length" => "9" }, [ "Forbidden" ] ]
+          [ 403, { Rack::CONTENT_TYPE => "text/plain", Rack::CONTENT_LENGTH => "9" }, [ "Forbidden" ] ]
         end
       end
 
       # Returns a 404 Not Found response tuple
       def not_found_response(env)
         if head_request?(env)
-          [ 404, { "content-type" => "text/plain", "content-length" => "0", "x-cascade" => "pass" }, [] ]
+          [ 404, { Rack::CONTENT_TYPE => "text/plain", Rack::CONTENT_LENGTH => "0", X_CASCADE => "pass" }, [] ]
         else
-          [ 404, { "content-type" => "text/plain", "content-length" => "9", "x-cascade" => "pass" }, [ "Not found" ] ]
+          [ 404, { Rack::CONTENT_TYPE => "text/plain", Rack::CONTENT_LENGTH => "9", X_CASCADE => "pass" }, [ "Not found" ] ]
         end
       end
 
       def method_not_allowed_response
-        [ 405, { "content-type" => "text/plain", "content-length" => "18" }, [ "Method Not Allowed" ] ]
+        [ 405, { Rack::CONTENT_TYPE => "text/plain", Rack::CONTENT_LENGTH => "18" }, [ "Method Not Allowed" ] ]
       end
 
       def precondition_failed_response(env)
         if head_request?(env)
-          [ 412, { "content-type" => "text/plain", "content-length" => "0", "x-cascade" => "pass" }, [] ]
+          [ 412, { Rack::CONTENT_TYPE => "text/plain", Rack::CONTENT_LENGTH => "0", X_CASCADE => "pass" }, [] ]
         else
-          [ 412, { "content-type" => "text/plain", "content-length" => "19", "x-cascade" => "pass" }, [ "Precondition Failed" ] ]
+          [ 412, { Rack::CONTENT_TYPE => "text/plain", Rack::CONTENT_LENGTH => "19", X_CASCADE => "pass" }, [ "Precondition Failed" ] ]
         end
       end
 
@@ -189,7 +199,7 @@ def precondition_failed_response(env)
       def javascript_exception_response(exception)
         err  = "#{exception.class.name}: #{exception.message}\n  (in #{exception.backtrace[0]})"
         body = "throw Error(#{err.inspect})"
-        [ 200, { "content-type" => "application/javascript", "content-length" => body.bytesize.to_s }, [ body ] ]
+        [ 200, { Rack::CONTENT_TYPE => "application/javascript", Rack::CONTENT_LENGTH => body.bytesize.to_s }, [ body ] ]
       end
 
       # Returns a CSS response that hides all elements on the page and
@@ -242,7 +252,7 @@ def css_exception_response(exception)
           }
         CSS
 
-        [ 200, { "content-type" => "text/css; charset=utf-8", "content-length" => body.bytesize.to_s }, [ body ] ]
+        [ 200, { Rack::CONTENT_TYPE => "text/css; charset=utf-8", Rack::CONTENT_LENGTH => body.bytesize.to_s }, [ body ] ]
       end
 
       # Escape special characters for use inside a CSS content("...") string
@@ -258,18 +268,18 @@ def cache_headers(env, etag)
         headers = {}
 
         # Set caching headers
-        headers["cache-control"] = +"public"
-        headers["etag"]          = %("#{etag}")
+        headers[Rack::CACHE_CONTROL] = +"public"
+        headers[Rack::ETAG]          = %("#{etag}")
 
         # If the request url contains a fingerprint, set a long
         # expires on the response
         if path_fingerprint(env["PATH_INFO"])
-          headers["cache-control"] << ", max-age=31536000, immutable"
+          headers[Rack::CACHE_CONTROL] << ", max-age=31536000, immutable"
 
         # Otherwise set `must-revalidate` since the asset could be modified.
         else
-          headers["cache-control"] << ", must-revalidate"
-          headers["vary"] = "Accept-Encoding"
+          headers[Rack::CACHE_CONTROL] << ", must-revalidate"
+          headers[VARY] = "Accept-Encoding"
         end
 
         headers
@@ -279,15 +289,15 @@ def headers(env, asset, length)
         headers = {}
 
         # Set content length header
-        headers["content-length"] = length.to_s
+        headers[Rack::CONTENT_LENGTH] = length.to_s
 
         # Set content type header
         if type = asset.content_type
           # Set charset param for text/* mime types
           if type.start_with?("text/") && asset.charset
             type += "; charset=#{asset.charset}"
           end
-          headers["content-type"] = type
+          headers[Rack::CONTENT_TYPE] = type
         end
 
         headers.merge(cache_headers(env, asset.etag))