Only allow specific trailers

Only allow trailers listed in the `Trailer` header. Even if listed
there, disallow the following names, based on Mozilla
recommendations:

```
content-encoding content-type content-range trailer authorization
set-cookie transfer-encoding content-length host cache-control
max-forwards te
```

There are probably additional ones we should disallow, but this
is a decent start.

Do not merge the header and trailer data. Parse the trailers into
a separate hash, and for allowed names, copy the value into the
headers hash.

This ignores invalid trailers instead of raising an exception,
which is preferable for backwards compatibility.

In order to get the new test to pass, make content_length return
nil instead of of raising a TypeError if no content length was
provided. Also, parse the content length as decimal instead of
trying to autodetect the radix.

Fixes #198

Author: jeremyevans

lib/webrick/httprequest.rb

@@ -308,7 +308,9 @@ def query
     # The content-length header
 
     def content_length
-      return Integer(self['content-length'])
+      if length = self['content-length']
+        Integer(length, 10)
+      end
     end
 
     ##
@@ -474,10 +476,21 @@ def read_request_line(socket)
       end
     end
 
+    DISALLOWED_TRAILERS = %w[content-encoding content-type content-range
+      trailer authorization set-cookie transfer-encoding content-length
+      host cache-control max-forwards te].freeze
+    private_constant :DISALLOWED_TRAILERS
+
     def read_header(socket)
       if socket
         end_of_headers = false
 
+        raw = if @header
+          trailer_lines = []
+        else
+          @raw_header
+        end
+
         while line = read_line(socket)
           if line == CRLF
             end_of_headers = true
@@ -489,19 +502,35 @@ def read_header(socket)
           if line.include?("\x00")
             raise HTTPStatus::BadRequest, 'null byte in header'
           end
-          @raw_header << line
+          raw << line
         end
 
         # Allow if @header already set to support chunked trailers
-        raise HTTPStatus::EOFError unless end_of_headers || @header
+        raise HTTPStatus::EOFError unless end_of_headers || trailer_lines
       end
-      @header = HTTPUtils::parse_header(@raw_header.join)
 
-      if (content_length = @header['content-length']) && content_length.length != 0
-        if content_length.length > 1
-          raise HTTPStatus::BadRequest, "multiple content-length request headers"
-        elsif !/\A\d+\z/.match?(content_length[0])
-          raise HTTPStatus::BadRequest, "invalid content-length request header"
+      if trailer_lines
+        if requested_trailers = self["trailer"]
+          requested_trailers = requested_trailers.downcase.split
+          requested_trailers -= DISALLOWED_TRAILERS
+          unless requested_trailers.empty?
+            trailers = HTTPUtils::parse_header(trailer_lines.join)
+            requested_trailers.each do |key|
+              if value = trailers[key]
+                @header[key] = value
+              end
+            end
+          end
+        end
+      else
+        @header = HTTPUtils::parse_header(@raw_header.join)
+
+        if (content_length = @header['content-length']) && content_length.length != 0
+          if content_length.length > 1
+            raise HTTPStatus::BadRequest, "multiple content-length request headers"
+          elsif !/\A\d+\z/.match?(content_length[0])
+            raise HTTPStatus::BadRequest, "invalid content-length request header"
+          end
         end
       end
     end

test/webrick/test_httprequest.rb

@@ -441,6 +441,28 @@ def test_bad_chunked
     end
   end
 
+  def test_chunked_trailers
+    req = WEBrick::HTTPRequest.new(WEBrick::Config::HTTP)
+    req.parse(StringIO.new(<<~HTTP))
+      POST /path HTTP/1.1\r
+      host: foo.example.com\r
+      Transfer-Encoding: chunked\r
+      Trailer: host Foo\r
+      \r
+      1\r
+      2\r
+      0\r
+      Content-Length: 2\r
+      Foo: a\r
+      Host: bar.example.com\r
+      \r
+    HTTP
+    assert_nil(req.content_length)
+    assert_equal("2", req.body)
+    assert_equal("a", req["foo"])
+    assert_equal("foo.example.com", req["host"])
+  end
+
   def test_bad_chunked_extra_data
     msg = <<~HTTP
       POST /path HTTP/1.1\r