clair: add idle timeout

This will allow for Clair instances to spin down when they stop
receiving requests for some period. This is particularly useful when
combined with a systemd socket unit.

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

Author: hdonnay

cmd/clair/idle.go

@@ -0,0 +1,54 @@
+package main
+
+import (
+	"context"
+	"net"
+	"net/http"
+	"sync/atomic"
+	"time"
+)
+
+// IdleMontior holds idle tracking machinery.
+type idleMonitor struct {
+	timer   *time.Timer
+	timeout time.Duration
+	ct      atomic.Uint32
+}
+
+// NewIdleMonitor starts a goroutine to call "f" if the duration "timeout"
+// passes with no active HTTP connections.
+//
+// The goroutine will also exit if the passed context is canceled.
+func newIdleMonitor(ctx context.Context, timeout time.Duration, f context.CancelCauseFunc) idleMonitor {
+	timer := time.NewTimer(timeout)
+	go func() {
+		select {
+		case <-ctx.Done():
+		case <-timer.C:
+			f(nil) // TODO(hank) Add a specific "idle timeout" condition?
+		}
+	}()
+
+	return idleMonitor{
+		timer:   timer,
+		timeout: timeout,
+	}
+}
+
+// ServerHook is a function suitable for use as [http.Server.ConnState].
+//
+// This hook watches connection state changes, starting an idle timer when there
+// are no open connections. The timer will be stopped if new connections arrive
+// during the timeout period.
+func (m *idleMonitor) ServerHook(_ net.Conn, state http.ConnState) {
+	switch state {
+	case http.StateNew:
+		if m.ct.Add(1) == 1 {
+			m.timer.Stop()
+		}
+	case http.StateClosed, http.StateHijacked:
+		if m.ct.Add(^uint32(0)) == 0 {
+			m.timer.Reset(m.timeout)
+		}
+	}
+}

cmd/clair/main.go

@@ -115,8 +115,9 @@ func main() {
 	}()
 
 	srvs, srvctx := errgroup.WithContext(sig)
+	srvctx, teardown := context.WithCancelCause(srvctx)
 	srvs.Go(serveIntrospection(srvctx, &conf))
-	srvs.Go(serveAPI(srvctx, &conf))
+	srvs.Go(serveAPI(srvctx, &conf, teardown))
 
 	slog.InfoContext(ctx, "ready", "version", cmd.Version)
 	notify(msgReady,
@@ -127,7 +128,7 @@ func main() {
 	}
 }
 
-func serveAPI(ctx context.Context, cfg *config.Config) func() error {
+func serveAPI(ctx context.Context, cfg *config.Config, teardown context.CancelCauseFunc) func() error {
 	apicfg := &cfg.API.V1
 	if !*apicfg.Enabled {
 		return func() error {
@@ -146,6 +147,10 @@ func serveAPI(ctx context.Context, cfg *config.Config) func() error {
 				return context.WithoutCancel(ctx)
 			},
 		}
+		if t := time.Duration(apicfg.IdleTimeout); t != 0 {
+			idle := newIdleMonitor(ctx, t, teardown)
+			srv.ConnState = idle.ServerHook
+		}
 		srv.Handler, err = httptransport.New(ctx, cfg, srvs.Indexer, srvs.Matcher, srvs.Notifier)
 		if err != nil {
 			return fmt.Errorf("http transport configuration failed: %w", err)