fix: Support IPv6 bind addresses in http_server helper

Fixes URI::InvalidURIError when binding to IPv6 addresses like '::' or '::1'.
The http_server helper was generating invalid URIs such as 'http://:::24231'
instead of the RFC 3986 compliant 'http://[::]:24231'.

Changes:
- Wrap IPv6 addresses in brackets when constructing URIs
- Add unit tests for IPv6 localhost (::1) and wildcard (::) binding
- Update CHANGELOG.md

This bug affected all Fluentd versions including v1.19.1 and caused
crashes when attempting to bind HTTP servers to IPv6 addresses in
dual-stack environments.

Resolves: Invalid URI generation for IPv6 bind addresses

Author: I501307

CHANGELOG.md

@@ -1,5 +1,14 @@
 # v1.19
 
+## Release v1.19.1 - TBD
+
+### Bug fix
+
+* http_server helper: Fix IPv6 bind address support in URI construction
+  * Fixed `URI::InvalidURIError` when binding to IPv6 addresses (e.g., `::`)
+  * IPv6 addresses are now properly bracketed in URIs per RFC 3986 (e.g., `http://[::]:24231`)
+  * Affects all plugins using http_server helper with IPv6 bind addresses
+
 ## Release v1.19.0 - 2025/07/30
 
 ### Enhancement

lib/fluent/plugin_helper/http_server/server.rb

@@ -37,7 +37,8 @@ def initialize(addr:, port:, logger:, default_app: nil, tls_context: nil)
 
           # TODO: support http2
           scheme = tls_context ? 'https' : 'http'
-          @uri = URI("#{scheme}://#{@addr}:#{@port}").to_s
+          addr_display = @addr.include?(":") ? "[#{@addr}]" : @addr
+          @uri = URI("#{scheme}://#{addr_display}:#{@port}").to_s
           @router = Router.new(default_app)
           @server_task = nil
           Console.logger = Fluent::Log::ConsoleAdapter.wrap(@logger)

test/plugin_helper/test_http_server_helper.rb

@@ -21,6 +21,16 @@ def teardown
     @port = nil
   end
 
+  def ipv6_enabled?
+    begin
+      sock = Socket.new(Socket::AF_INET6, Socket::SOCK_STREAM, 0)
+      sock.close
+      true
+    rescue
+      false
+    end
+  end
+
   class Dummy < Fluent::Plugin::TestBase
     helpers :http_server
   end
@@ -364,6 +374,33 @@ def start_https_request(addr, port, verify: true, cert_path: nil, selfsigned: tr
       end
     end
 
+    test 'bind to IPv6 address' do
+      omit('IPv6 not supported') unless ipv6_enabled?
+      on_driver do |driver|
+        driver.http_server_create_http_server(:http_server_helper_test, addr: '::1', port: @port, logger: NULL_LOGGER) do |s|
+          s.get('/example/hello') { [200, { 'Content-Type' => 'text/plain' }, 'hello from ipv6'] }
+        end
+
+        resp = get("http://[::1]:#{@port}/example/hello")
+        assert_equal('200', resp.code)
+        assert_equal('hello from ipv6', resp.body)
+      end
+    end
+
+    test 'bind to IPv6 wildcard address' do
+      omit('IPv6 not supported') unless ipv6_enabled?
+      on_driver do |driver|
+        driver.http_server_create_http_server(:http_server_helper_test, addr: '::', port: @port, logger: NULL_LOGGER) do |s|
+          s.get('/example/hello') { [200, { 'Content-Type' => 'text/plain' }, 'hello from ipv6 wildcard'] }
+        end
+
+        # Can access via IPv4-mapped IPv6 or IPv6 loopback
+        resp = get("http://[::1]:#{@port}/example/hello")
+        assert_equal('200', resp.code)
+        assert_equal('hello from ipv6 wildcard', resp.body)
+      end
+    end
+
     test 'must be called #start and #stop' do
       on_driver do |driver|
         server = flexmock('Server') do |watcher|