Ignore Range header if served file is 0 bytes

Normally Rack::Files truncates a byte range to fit a file's contents as long as
at least some bytes are within range. However, an empty (0 byte) file is a
special case. Previously, empty files requested with a Range header always
resulted in a 416 error. This changes 0 byte files to ignore the Range header
and return the empty file.

This improves compatibility with clients that speculatively request files with
byte ranges.

lib/rack/utils.rb

@@ -388,6 +388,7 @@ def byte_ranges(env, size)
 
     def get_byte_ranges(http_range, size)
       # See <http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.35>
+      return nil if size.zero?
       return nil unless http_range && http_range =~ /bytes=([^;]+)/
       ranges = []
       $1.split(/,\s*/).each do |range_spec|

test/spec_files.rb

@@ -241,6 +241,15 @@ def filesize(_); 10000 end
     res["content-range"].must_equal "bytes */209"
   end
 
+  it "ignores range when file is 0 bytes" do
+    env = Rack::MockRequest.env_for("/cgi/assets/images/favicon.ico")
+    env["HTTP_RANGE"] = "bytes=0-1234"
+    res = Rack::MockResponse.new(*files(DOCROOT).call(env))
+
+    res.status.must_equal 200
+    res["content-range"].must_be_nil
+  end
+
   it "support custom http headers" do
     env = Rack::MockRequest.env_for("/cgi/test")
     status, heads, _ = files(DOCROOT, 'cache-control' => 'public, max-age=38',

test/spec_utils.rb

@@ -733,11 +733,11 @@ def initialize(*)
   end
 
   it "handle byte ranges of empty files" do
-    Rack::Utils.get_byte_ranges("bytes=123-456", 0).must_equal []
-    Rack::Utils.get_byte_ranges("bytes=0-", 0).must_equal []
-    Rack::Utils.get_byte_ranges("bytes=-100", 0).must_equal []
-    Rack::Utils.get_byte_ranges("bytes=0-0", 0).must_equal []
-    Rack::Utils.get_byte_ranges("bytes=-0", 0).must_equal []
+    Rack::Utils.get_byte_ranges("bytes=123-456", 0).must_be_nil
+    Rack::Utils.get_byte_ranges("bytes=0-", 0).must_be_nil
+    Rack::Utils.get_byte_ranges("bytes=-100", 0).must_be_nil
+    Rack::Utils.get_byte_ranges("bytes=0-0", 0).must_be_nil
+    Rack::Utils.get_byte_ranges("bytes=-0", 0).must_be_nil
   end
 end