clair: add supervisor notification mechanism

This allows Clair to inform a process supervisor about its state.

Currently, only systemd is supported and only "ready" and "stopping"
messages are used.

Signed-off-by: Hank Donnay <hdonnay@redhat.com>

Author: hdonnay

cmd/clair/notify.go

@@ -0,0 +1,13 @@
+package main
+
+type notifyMsg uint
+
+const (
+	_ notifyMsg = iota
+	msgReady
+	msgReloading
+	msgStopping
+	msgStatus
+	msgSocketAPI
+	msgSocketIntrospection
+)

cmd/clair/notify_linux.go

@@ -0,0 +1,106 @@
+package main
+
+import (
+	"bytes"
+	"errors"
+	"fmt"
+	"net"
+	"os"
+	"strings"
+
+	"golang.org/x/sys/unix"
+)
+
+// Notify sends information back to a supervisor process.
+//
+// # Linux
+//
+// The supervisor is assumed to be systemd, and this function implements the
+// sd_notify(3) protocol. Some messages require an additional argument.
+func notify(args ...any) error {
+	const key = `NOTIFY_SOCKET`
+
+	sockpath, ok := os.LookupEnv(key)
+	if !ok {
+		return nil
+	}
+	sock, err := net.ResolveUnixAddr("unix", sockpath)
+	if err != nil {
+		return err
+	}
+	conn, err := net.DialUnix("unix", nil, sock)
+	if err != nil {
+		return err
+	}
+	defer conn.Close()
+	rc, err := conn.SyscallConn()
+	if err != nil {
+		return err
+	}
+	var ctlErr error
+	err = rc.Control(func(fd uintptr) {
+		const szTgt = 8 * 1026 * 1026 // 8 MiB
+
+		_, ctlErr = unix.FcntlInt(fd, unix.F_SETFD, unix.FD_CLOEXEC)
+		if ctlErr != nil {
+			return
+		}
+
+		var sz int
+		sz, ctlErr = unix.GetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_SNDBUF)
+		if ctlErr != nil {
+			return
+		}
+		if sz < szTgt {
+			ctlErr = unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_SNDBUF, szTgt)
+			if ctlErr != nil {
+				return
+			}
+		}
+	})
+	if err := errors.Join(err, ctlErr); err != nil {
+		return err
+	}
+
+	// build message
+	var buf bytes.Buffer
+	var oob []byte
+	for i := 0; i < len(args); i++ {
+		var fdname string
+		msg := args[i].(notifyMsg)
+		switch msg {
+		case msgReady:
+			buf.WriteString("READY=1\n")
+		case msgReloading:
+			buf.WriteString("RELOADING=1\n")
+		case msgStopping:
+			buf.WriteString("STOPPING=1\n")
+		case msgStatus:
+			buf.WriteString("STATUS=")
+			i++
+			s := args[i].(string)
+			buf.WriteString(strings.TrimSpace(s))
+			buf.WriteByte('\n')
+		case msgSocketAPI:
+			fdname = "api"
+		case msgSocketIntrospection:
+			fdname = "introspection"
+		default:
+			panic(fmt.Sprintf("programmer error: unknown msg kind: %v", msg))
+		}
+		if fdname != "" {
+			if oob != nil {
+				panic("programmer error: sending multiple file descriptors")
+			}
+			buf.WriteString("FDSTORE=1\n")
+			buf.WriteString("FDNAME=")
+			buf.WriteString(fdname)
+			buf.WriteByte('\n')
+			i++
+			oob = unix.UnixRights(args[i].(int))
+		}
+	}
+
+	_, _, err = conn.WriteMsgUnix(buf.Bytes(), oob, sock)
+	return err
+}

cmd/clair/notify_other.go

@@ -0,0 +1,8 @@
+//go:build !linux
+
+package main
+
+// Notify sends information back to a supervisor process.
+func notify(_ ...any) error {
+	return nil
+}