Update

vendor/github.com/creachadair/msync/trigger/trigger.go

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+// Package trigger implements channel-based condition variable.
+package trigger
+
+import "sync"
+
+// A Cond is a condition shared by multiple goroutines.  The [Cond.Ready]
+// method returns a channel that is closed when the condition is activated.
+//
+// When a condition is first created it is inactive. While inactive, reads on
+// the ready channel will block.  The condition remains inactive until
+// [Cond.Set] or [Cond.Signal] is called, either of which causes the current
+// ready channel to be closed (and thus deliver a zero value). Once a condition
+// has been activated, it remains active until it is reset. Use [Cond.Reset] to
+// make it inactive again.
+//
+// The [Cond.Signal] method activates and then immediately resets the
+// condition, acting as Set and Reset done in a single step.
+//
+// A zero Cond is ready for use, and is inactive, but must not be copied after
+// any of its methods have been called.
+type Cond struct {
+	μ      sync.Mutex
+	ch     chan struct{}
+	closed bool
+
+	// The signal channel is lazily initialized by the first waiter.
+}
+
+// New constructs a new inactive [Cond].
+func New() *Cond { return new(Cond) }
+
+// Signal activates and immediately resets the condition.  If the condition was
+// already active, this is equivalent to [Cond.Reset].
+func (c *Cond) Signal() {
+	c.μ.Lock()
+	defer c.μ.Unlock()
+
+	if c.ch != nil && !c.closed {
+		close(c.ch) // wake any pending waiters
+	}
+	c.ch = nil
+	c.closed = false
+}
+
+// Set activates the condition. If it was already active, Set has no effect.
+func (c *Cond) Set() {
+	c.μ.Lock()
+	defer c.μ.Unlock()
+
+	if c.ch == nil {
+		c.ch = make(chan struct{})
+	}
+	if !c.closed {
+		close(c.ch)
+		c.closed = true
+	}
+}
+
+// Reset resets the condition. If it was already inactive, Reset has no effect.
+func (c *Cond) Reset() {
+	c.μ.Lock()
+	defer c.μ.Unlock()
+
+	if c.closed {
+		c.ch = nil
+		c.closed = false
+	}
+}
+
+// Ready returns a channel that is closed when c is activated.  If c is active
+// when Ready is called, the returned channel will already be closed.
+func (c *Cond) Ready() <-chan struct{} {
+	c.μ.Lock()
+	defer c.μ.Unlock()
+
+	if c.ch == nil {
+		c.ch = make(chan struct{})
+		c.closed = false
+	}
+	return c.ch
+}