如何从文件描述符创建IO::Socket::INET对象？解决参数长度错误

问题现象 #

通过systemd套接字激活Plack应用后，客户端发送HTTP请求无响应，查看journalctl日志出现以下错误：

systemd: Started Socket-activated hello world service.
plackup: HTTP::Server::PSGI: Accepting connections at http://0:5000/
plackup: Bad arg length for Socket::unpack_sockaddr_in, length is 28, should be 16 at …perl/5.42.2/lib/5.42.2/x86_64-linux-ld/Socket.pm line 858.
systemd: demo.service: Main process exited, code=exited, status=29/n/a
systemd: demo.service: Failed with result 'exit-code'.

问题根源 #

补丁中使用IO::Socket::INET->new_from_fd(3, 'r+')获取systemd传递的套接字，但IO::Socket::INET是仅支持IPv4的套接字类。而systemd默认创建的是IPv4/IPv6双栈兼容套接字，其地址结构为28字节的IPv6格式，unpack_sockaddr_in只能解析16字节的IPv4地址结构，因此触发长度不匹配错误。

解决方案 #

将IO::Socket::INET替换为支持双栈的IO::Socket::IP，修改后的补丁如下：

diff --git a/lib/HTTP/Server/PSGI.pm b/lib/HTTP/Server/PSGI.pm
index 225d4ca..f060463 100644
--- a/lib/HTTP/Server/PSGI.pm
+++ b/lib/HTTP/Server/PSGI.pm
@@ -90,6 +90,12 @@ sub prepare_socket_class {
 sub setup_listener {
     my $self = shift;
 
+    # TO DO: also handle LISTEN_PID LISTEN_PIDFDID
+    if ($ENV{LISTEN_FDS} && 1 == $ENV{LISTEN_FDS}) {
+        $self->{listen_sock} ||= IO::Socket::IP->new_from_fd(3, 'r+');
+        goto DONE;
+    }
+
     $self->{listen_sock} ||= do {
         my %args = (
             Listen    => SOMAXCONN,
@@ -104,6 +110,7 @@ sub setup_listener {
             or die "failed to listen to port $self->{port}: $!";
     };
 
+DONE:
     $self->{server_ready}->({ %$self, proto => $self->{ssl} ? 'https' : 'http' });
 }

补充说明 #

1. 确保已安装IO::Socket::IP模块，可通过命令安装：cpanm IO::Socket::IP
2. 若需强制使用IPv4（放弃IPv6支持），可在创建套接字时指定域：

   $self->{listen_sock} ||= IO::Socket::INET->new_from_fd(3, 'r+', Domain => AF_INET);
   

验证步骤 #

1. 重新应用修改后的补丁并安装Plack
2. 重启服务：

   systemctl --user stop demo.service
   systemctl --user restart demo.socket
   

3. 再次发送HTTP请求，确认服务正常响应

内容的提问来源于stack exchange，提问作者daxim