sync: Add Monitor.BorrowReturn

sync/monitor.go

@@ -25,7 +25,7 @@ func (this *Monitor[T]) Set (value T) {
 // Borrow borrows the value from the Monitor, and returns a function that must
 // immediately be deferred, like this:
 //
-//   value, done := Monitor.Borrow()
+//   value, done := monitor.Borrow()
 //   defer done()
 //
 // From the time Borrow is called to the time the done function is called, it is
@@ -35,6 +35,19 @@ func (this *Monitor[T]) Borrow () (T, func ()) {
 	return this.value, this.mutex.Unlock
 }
 
+// BorrowReturn is like borrow, but returns a "done" function that takes in an
+// updated value. The intended use of this function is like this:
+//
+//   value, done := monitor.BorrowReturn()
+//   defer done(value)
+func (this *Monitor[T]) BorrowReturn () (T, func (T)) {
+	this.mutex.Lock()
+	return this.value, func (value T) {
+		defer this.mutex.Unlock()
+		this.value = value
+	}
+}
+
 // RWMonitor guards separate read/write access to a value.
 type RWMonitor[T any] struct {
 	value T
@@ -59,7 +72,7 @@ func (this *RWMonitor[T]) Set (value T) {
 // Borrow borrows the value from the Monitor for write access, and returns a
 // function that must immediately be deferred, like this:
 //
-//   value, done := Monitor.Borrow()
+//   value, done := monitor.Borrow()
 //   defer done()
 //
 // From the time Borrow is called to the time the done function is called, it is
@@ -69,6 +82,19 @@ func (this *RWMonitor[T]) Borrow () (T, func ()) {
 	return this.value, this.mutex.Unlock
 }
 
+// BorrowReturn is like borrow, but returns a "done" function that takes in an
+// updated value. The intended use of this function is like this:
+//
+//   value, done := monitor.BorrowReturn()
+//   defer done(value)
+func (this *RWMonitor[T]) BorrowReturn () (T, func (T)) {
+	this.mutex.Lock()
+	return this.value, func (value T) {
+		defer this.mutex.Unlock()
+		this.value = value
+	}
+}
+
 // RBorrow is like Borrow, but returns the item for read access only. Do not
 // under any circumstances modify anything returned by this method.
 func (this *RWMonitor[T]) RBorrow () (T, func ()) {