clair: add Shutdown struct

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

cmd/clair/shutdown.go

@@ -0,0 +1,44 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+	"sync"
+
+	"golang.org/x/sync/errgroup"
+)
+
+// Shutdown aggregates http.Sever Shutdown methods.
+type Shutdown struct {
+	mu sync.Mutex
+	m  map[*http.Server]struct{}
+}
+
+// Add registers a server.
+func (s *Shutdown) Add(srv *http.Server) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	if s.m == nil {
+		s.m = make(map[*http.Server]struct{})
+	}
+	s.m[srv] = struct{}{}
+}
+
+// Shutdown calls Shutdown on all added Servers. If a timeout is needed, it
+// should be done via the passed Context.
+func (s *Shutdown) Shutdown(ctx context.Context) error {
+	s.mu.Lock() // Leave locked forever
+	eg := &errgroup.Group{}
+	for srv := range s.m {
+		srv := srv
+		eg.Go(func() error {
+			if err := srv.Shutdown(ctx); err != nil {
+				return fmt.Errorf("unable to shutdown %q: %w", srv.Addr, err)
+			}
+			return nil
+		})
+		delete(s.m, srv)
+	}
+	return eg.Wait()
+}