sync: Add RWLocker from previous repo

sync/doc.go

@@ -0,0 +1,2 @@
+// Package usync extends sync.
+package usync

sync/locker.go

@@ -2,7 +2,7 @@ package usync
 
 import "sync"
 
-// Locker guards access to a value
+// Locker guards access to a value. It must not be copied after first use.
 type Locker[T any] struct {
 	value T
 	mutex sync.Mutex
@@ -34,3 +34,44 @@ func (this *Locker[T]) Borrow () (T, func ()) {
 	this.mutex.Lock()
 	return this.value, this.mutex.Unlock
 }
+
+// RWLocker guards separate read/write access to a value.
+type RWLocker[T any] struct {
+	value T
+	mutex sync.RWMutex
+}
+
+// NewRWLocker creates a new locker with the specified value. It must not be
+// copied after first use.
+func NewRWLocker[T any] (value T) RWLocker[T] {
+	return RWLocker[T] {
+		value: value,
+	}
+}
+
+// Set sets the value of the locker.
+func (this *RWLocker[T]) Set (value T) {
+	this.mutex.Lock()
+	defer this.mutex.Unlock()
+	this.value = value
+}
+
+// Borrow borrows the value from the locker for write access, and returns a
+// function that must immediately be deferred, like this:
+//
+//   value, done := locker.Borrow()
+//   defer done()
+//
+// From the time Borrow is called to the time the done function is called, it is
+// safe to access the locked object from within the current goroutine.
+func (this *RWLocker[T]) Borrow () (T, func ()) {
+	this.mutex.Lock()
+	return this.value, this.mutex.Unlock
+}
+
+// RBorrow is like Borrow, but returns the item for read access only. Do not
+// under any circumstances write to an item returned from this method.
+func (this *RWLocker[T]) RBorrow () (T, func ()) {
+	this.mutex.Lock()
+	return this.value, this.mutex.Unlock
+}