Fix superclass functional interface resolution in Painless (#81698)

A recent change [1] to how we load our allow list changed the resolution for how Painless looks up 
methods of super classes. However, functional interface loading was not changed which caused a 
bug where a functional interface would not look at its super interfaces for the functional interface 
method [2].

This fixes the issue by going through each super interface until the functional interface method is 
found when the target interface doesn't have the functional interface method.

[1] #76955
[2] #81696

Also a big thanks to @megglos and @TheFireCookie for their help with this issue.

docs/changelog/81698.yaml

@@ -0,0 +1,5 @@
+pr: 81698
+summary: Fix subclass functional interface resolution in Painless
+area: Infra/Scripting
+type: regression
+issues: []

modules/lang-painless/src/main/java/org/elasticsearch/painless/lookup/PainlessLookupBuilder.java

@@ -2116,7 +2116,7 @@ public final class PainlessLookupBuilder {
         }
     }
 
-    private void setFunctionalInterfaceMethod(Class<?> targetClass, PainlessClassBuilder painlessClassBuilder) {
+    private void setFunctionalInterfaceMethod(Class<?> targetClass, PainlessClassBuilder targetPainlessClassBuilder) {
         if (targetClass.isInterface()) {
             List<java.lang.reflect.Method> javaMethods = new ArrayList<>();
 
@@ -2141,7 +2141,31 @@ public final class PainlessLookupBuilder {
             } else if (javaMethods.size() == 1) {
                 java.lang.reflect.Method javaMethod = javaMethods.get(0);
                 String painlessMethodKey = buildPainlessMethodKey(javaMethod.getName(), javaMethod.getParameterCount());
-                painlessClassBuilder.functionalInterfaceMethod = painlessClassBuilder.methods.get(painlessMethodKey);
+
+                List<Class<?>> superInterfaces = new ArrayList<>();
+                Set<Class<?>> resolvedInterfaces = new HashSet<>();
+
+                superInterfaces.add(targetClass);
+
+                while (superInterfaces.isEmpty() == false) {
+                    Class<?> superInterface = superInterfaces.remove(0);
+
+                    if (resolvedInterfaces.add(superInterface)) {
+                        PainlessClassBuilder functionalInterfacePainlessClassBuilder = classesToPainlessClassBuilders.get(superInterface);
+
+                        if (functionalInterfacePainlessClassBuilder != null) {
+                            targetPainlessClassBuilder.functionalInterfaceMethod = functionalInterfacePainlessClassBuilder.methods.get(
+                                painlessMethodKey
+                            );
+
+                            if (targetPainlessClassBuilder.functionalInterfaceMethod != null) {
+                                break;
+                            }
+                        }
+
+                        superInterfaces.addAll(Arrays.asList(superInterface.getInterfaces()));
+                    }
+                }
             }
         }
     }

modules/lang-painless/src/test/java/org/elasticsearch/painless/LambdaTests.java

@@ -309,4 +309,14 @@ public class LambdaTests extends ScriptTestCase {
             exec("def b = new StringBuilder(); List l = [1, 2]; l.stream().map(i -> b.setLength(i)).collect(Collectors.toList())")
         );
     }
+
+    public void testUnaryOperator() {
+        assertEquals("doremi", exec("List lst = ['abc', '123']; lst.replaceAll(f -> f.replace('abc', 'doremi')); lst.get(0);"));
+        assertEquals("doremi", exec("def lst = ['abc', '123']; lst.replaceAll(f -> f.replace('abc', 'doremi')); lst.get(0);"));
+    }
+
+    public void testBinaryOperator() {
+        assertEquals(1, exec("def list = new ArrayList(); list.add(1); list.add(2); list.stream().reduce((i1, i2) -> i1).orElse(0)"));
+        assertEquals(1, exec("List list = new ArrayList(); list.add(1); list.add(2); list.stream().reduce((i1, i2) -> i1).orElse(0)"));
+    }
 }