Improve error message when request body size is unknown

Closes #689.

Author: sferik

CHANGELOG.md

@@ -18,6 +18,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- Improved error message when request body size cannot be determined to suggest
+  setting `Content-Length` explicitly or using chunked `Transfer-Encoding` (#560)
 - **BREAKING** `Connection#readpartial` now raises `EOFError` instead of
   returning `nil` at end-of-stream, and supports an `outbuf` parameter,
   conforming to the `IO#readpartial` API. `Body#readpartial` and

lib/http/request/body.rb

@@ -54,7 +54,9 @@ def size
         elsif @source.nil?
           0
         else
-          raise RequestError, "cannot determine size of body: #{@source}"
+          raise RequestError,
+                "cannot determine size of body: #{@source.inspect}; " \
+                "set the Content-Length header explicitly or use chunked Transfer-Encoding"
         end
       end
 

test/http/request/body_test.rb

@@ -291,6 +291,8 @@
         err = assert_raises(HTTP::RequestError) { subject_under_test.size }
         assert_match(/cannot determine size of body/, err.message)
         assert_includes err.message, body.inspect
+        assert_match(/Content-Length/, err.message)
+        assert_match(/chunked Transfer-Encoding/, err.message)
       end
     end
   end