Merge branch '3.1.x' into 3.2.x

Author: jameskleeh

grails-async/src/main/groovy/org/grails/async/factory/SynchronousPromise.groovy

@@ -30,6 +30,7 @@ import java.util.concurrent.TimeUnit
 class SynchronousPromise<T> implements Promise<T> {
     Closure<T> callable
     def value
+    boolean executed = false
 
     SynchronousPromise(Closure<T> callable) {
         this.callable = callable
@@ -51,7 +52,8 @@ class SynchronousPromise<T> implements Promise<T> {
     }
 
     T get() throws Throwable {
-        if (value == null) {
+        if (!executed) {
+            executed = true
             try {
                 value = callable.call()
             } catch (e) {

grails-async/src/test/groovy/grails/async/SynchronousPromiseFactorySpec.groovy

@@ -156,4 +156,18 @@ class SynchronousPromiseFactorySpec extends Specification {
         then:'the closure is executed'
         1 * callable.call()
     }
+
+    @Issue("GRAILS-10152")
+    void "Test promise closure is not executed multiple times if it returns null"() {
+        given:
+        Closure callable =  Mock(Closure) {
+            call() >> null
+        }
+
+        when:"A promise is created"
+        Promises.waitAll([Promises.createPromise(callable), Promises.createPromise(callable)])
+
+        then:'the closure is executed twice'
+        2 * callable.call()
+    }
 }