Better DuplicateHeaderError detailed message.

Author: ioquatix

lib/protocol/http/error.rb

@@ -19,12 +19,30 @@ class DuplicateHeaderError < Error
 			include BadRequest
 
 			# @parameter key [String] The header key that was duplicated.
-			def initialize(key)
+			def initialize(key, existing_value, new_value)
 				super("Duplicate singleton header key: #{key.inspect}")
+				
+				@key = key
+				@existing_value = existing_value
+				@new_value = new_value
 			end
 
 			# @attribute [String] key The header key that was duplicated.
 			attr :key
+			
+			# @attribute [String] existing_value The existing value for the duplicated header.
+			attr :existing_value
+			
+			# @attribute [String] new_value The new value for the duplicated header.
+			attr :new_value
+			
+			def detailed_message(highlight: false)
+				<<~MESSAGE
+					#{self.message}
+						Existing value: #{@existing_value.inspect}
+						New value: #{@new_value.inspect}
+				MESSAGE
+			end
 		end
 
 		# Raised when an invalid trailer header is encountered in headers.

lib/protocol/http/headers.rb

@@ -453,7 +453,7 @@ def delete(key)
 					end
 				else
 					if hash.key?(key)
-						raise DuplicateHeaderError, key
+						raise DuplicateHeaderError.new(key, hash[key], value)
 					end
 
 					hash[key] = value

releases.md

@@ -1,5 +1,9 @@
 # Releases
 
+## Unreleased
+
+  - `Protocol::HTTP::DuplicateHeaderError` now includes the existing and new values for better debugging.
+
 ## v0.58.0
 
   - Move trailer validation to `Headers#add` method to ensure all additions are checked at the time of addition as this is a hard requirement.

test/protocol/http/error.rb

@@ -0,0 +1,43 @@
+# frozen_string_literal: true
+
+# Released under the MIT License.
+# Copyright, 2018-2026, by Samuel Williams.
+
+require "protocol/http/error"
+
+describe Protocol::HTTP::DuplicateHeaderError do
+	let(:key) {"content-length"}
+	let(:existing_value) {"100"}
+	let(:new_value) {"200"}
+	let(:error) {subject.new(key, existing_value, new_value)}
+	
+	with "#initialize" do
+		it "should set the key and values" do
+			expect(error.key).to be == key
+			expect(error.existing_value).to be == existing_value
+			expect(error.new_value).to be == new_value
+		end
+		
+		it "should have a descriptive message" do
+			expect(error.message).to be =~ /Duplicate singleton header key: "content-length"/
+		end
+	end
+	
+	with "#detailed_message" do
+		it "should include the header key and both values" do
+			message = error.detailed_message
+			
+			expect(message).to be =~ /Duplicate singleton header key: "content-length"/
+			expect(message).to be =~ /Existing value: "100"/
+			expect(message).to be =~ /New value: "200"/
+		end
+		
+		it "should work with highlight parameter" do
+			message = error.detailed_message(highlight: true)
+			
+			expect(message).to be =~ /Duplicate singleton header key: "content-length"/
+			expect(message).to be =~ /Existing value: "100"/
+			expect(message).to be =~ /New value: "200"/
+		end
+	end
+end