fix: systemd socket support build handling for windows

pkg/http/systemd_socket.go

@@ -0,0 +1,55 @@
+//go:build !windows
+// +build !windows
+
+package http
+
+import (
+	"errors"
+	"fmt"
+	"net"
+	"strconv"
+
+	"github.com/coreos/go-systemd/activation"
+)
+
+// listenFdsStart corresponds to `SD_LISTEN_FDS_START`.
+// Since the 3 first file descriptors in every linux process is
+// stdin, stdout and stderr. The first usable file descriptor is 3.
+// systemd-socket-activate will always assume that the first socket will be
+// 3 and the rest follow.
+const (
+	listenFdsStart = 3
+)
+
+// convert a string filedescriptor to an actual listener
+func (s *server) fdToListener(bindAddress string) (net.Listener, error) {
+	fd, err := strconv.Atoi(bindAddress)
+	if err != nil {
+		return nil, errors.New("listen failed: fd with name is not implemented yet")
+	}
+	fdIndex := fd - listenFdsStart
+
+	if len(s.fdFiles) == 0 {
+		s.fdFiles = activation.Files(true)
+	}
+
+	l := len(s.fdFiles)
+
+	if fdIndex < 0 || fdIndex >= l || l == 0 {
+		return nil, errors.New("listen failed: fd outside of range of available file descriptors")
+	}
+
+	return net.FileListener(s.fdFiles[fdIndex])
+}
+
+func (s *server) checkSystemdSocketSupport(opts Opts) error {
+	listenAddr := opts.BindAddress[3:]
+	listener, err := s.fdToListener(listenAddr)
+	if err != nil {
+		err = fmt.Errorf("listen (file, %s) failed: %w", listenAddr, err)
+		return err
+	}
+
+	s.listener = listener
+	return nil
+}