Output unix socket credentials where applicable

thanks @ajaspers!

irc/utils/net.go

@@ -9,6 +9,7 @@ import (
 	"net"
 	"regexp"
 	"strings"
+	"syscall"
 )
 
 var (
@@ -195,10 +196,24 @@ func HandleXForwardedFor(remoteAddr string, xForwardedFor string, whitelist []ne
 	return
 }
 
-func DescribeConn(conn net.Conn) string {
-	// XXX for unix domain sockets, this is not informative enough for an operator
-	// to determine who holds the other side of the connection. there seems to be
-	// no way to get either the correct file descriptor of the connection, or the
-	// udiag_ino from `man 7 sock_diag`. maybe there's something else we can do?
+// Output a description of a connection that can identify it to other systems
+// administration tools.
+func DescribeConn(c net.Conn) (description string) {
+	description = "<error>"
+	switch conn := c.(type) {
+	case *net.UnixConn:
+		f, err := conn.File()
+		if err != nil {
+			return
+		}
+		defer f.Close()
+		ucred, err := syscall.GetsockoptUcred(int(f.Fd()), syscall.SOL_SOCKET, syscall.SO_PEERCRED)
+		if err != nil {
+			return
+		}
+		return fmt.Sprintf("%s <-> %s [pid=%d, uid=%d]", conn.LocalAddr().String(), conn.RemoteAddr().String(), ucred.Pid, ucred.Uid)
+	default:
+		// *net.TCPConn or *tls.Conn
 		return fmt.Sprintf("%s <-> %s", conn.LocalAddr().String(), conn.RemoteAddr().String())
 	}
+}