Update gomod and vendor

vendor/github.com/moby/buildkit/util/cond/cond.go

@@ -0,0 +1,40 @@
+package cond
+
+import (
+	"sync"
+)
+
+// NewStatefulCond returns a stateful version of sync.Cond . This cond will
+// never block on `Wait()` if `Signal()` has been called after the `Wait()` last
+// returned. This is useful for avoiding to take a lock on `cond.Locker` for
+// signalling.
+func NewStatefulCond(l sync.Locker) *StatefulCond {
+	sc := &StatefulCond{main: l}
+	sc.c = sync.NewCond(&sc.mu)
+	return sc
+}
+
+type StatefulCond struct {
+	main      sync.Locker
+	mu        sync.Mutex
+	c         *sync.Cond
+	signalled bool
+}
+
+func (s *StatefulCond) Wait() {
+	s.main.Unlock()
+	s.mu.Lock()
+	if !s.signalled {
+		s.c.Wait()
+	}
+	s.signalled = false
+	s.mu.Unlock()
+	s.main.Lock()
+}
+
+func (s *StatefulCond) Signal() {
+	s.mu.Lock()
+	s.signalled = true
+	s.c.Signal()
+	s.mu.Unlock()
+}