Fix on_headers callbacks being called early

lib/ethon/easy/callbacks.rb

@@ -37,10 +37,10 @@ def set_callbacks
       # @return [ Proc ] The callback.
       def body_write_callback
         @body_write_callback ||= proc do |stream, size, num, object|
-          headers
+          headers_user_callback_result = headers
           result = body(chunk = stream.read_string(size * num))
           @response_body << chunk if result == :unyielded
-          result != :abort ? size * num : -1
+          (result != :abort && headers_user_callback_result != :abort) ? size * num : -1
         end
       end
 
@@ -52,9 +52,8 @@ def body_write_callback
       # @return [ Proc ] The callback.
       def header_write_callback
         @header_write_callback ||= proc {|stream, size, num, object|
-          result = headers
           @response_headers << stream.read_string(size * num)
-          result != :abort ? size * num : -1
+          size * num
         }
       end
 

spec/ethon/easy/callbacks_spec.rb

@@ -71,10 +71,12 @@
         easy.on_headers.clear
         easy.on_headers { :abort }
       end
-      let(:header_write_callback) { easy.instance_variable_get(:@header_write_callback) }
-
+      let(:body_write_callback) { easy.instance_variable_get(:@body_write_callback) }
+      # on_headers callbacks are expected to be called exactly one time, once all headers are in.
+      # We can't abort exactly on receiving the headers, as we might need to follow redirects,
+      # so we do the next best thing, which is aborting on the first body chunk.
       it "returns -1 to indicate abort to libcurl" do
-        expect(header_write_callback.call(stream, 1, 1, nil)).to eq(-1)
+        expect(body_write_callback.call(stream, 1, 1, nil)).to eq(-1)
       end
     end
   end

spec/ethon/easy/http_spec.rb

@@ -45,7 +45,7 @@
 
           it "notifies when headers are ready" do
             headers = []
-            easy.on_headers { |r| headers << r.response_headers }
+            easy.on_headers { |r| headers << r.response_headers.dup }
             easy.http_request(url, action, options)
             easy.perform
             expect(headers).to eq([easy.response_headers])