Update policy parser to allow static methods for entitlement creation (…

…elastic#121706)

This updates the PolicyParser to allow static methods to have an ExternalEntitlement annotation. This 
removes a limitation where constructors cannot properly support type-erasure with different types of 
data structures for internal entitlement generation and external entitlement generation (for example 
List<Object> from the parser and List<SomeData> from an internal builder). We continue to enforce 
that only one constructor/method may be annotated with ExternalEntitlement per Entitlement class.

libs/entitlement/src/main/java/org/elasticsearch/entitlement/runtime/policy/ExternalEntitlement.java

@@ -22,7 +22,7 @@
  * using this annotation is considered parseable as part of a policy file
  * for entitlements.
  */
-@Target(ElementType.CONSTRUCTOR)
+@Target({ ElementType.CONSTRUCTOR, ElementType.METHOD })
 @Retention(RetentionPolicy.RUNTIME)
 public @interface ExternalEntitlement {
 

libs/entitlement/src/main/java/org/elasticsearch/entitlement/runtime/policy/PolicyParser.java

@@ -27,6 +27,8 @@
 import java.io.UncheckedIOException;
 import java.lang.reflect.Constructor;
 import java.lang.reflect.InvocationTargetException;
+import java.lang.reflect.Method;
+import java.lang.reflect.Modifier;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
@@ -147,6 +149,7 @@ protected Entitlement parseEntitlement(String scopeName, String entitlementType)
         }
 
         Constructor<?> entitlementConstructor = null;
+        Method entitlementMethod = null;
         ExternalEntitlement entitlementMetadata = null;
         for (var ctor : entitlementClass.getConstructors()) {
             var metadata = ctor.getAnnotation(ExternalEntitlement.class);
@@ -161,8 +164,27 @@ protected Entitlement parseEntitlement(String scopeName, String entitlementType)
                 entitlementConstructor = ctor;
                 entitlementMetadata = metadata;
             }
-
         }
+        for (var method : entitlementClass.getMethods()) {
+            var metadata = method.getAnnotation(ExternalEntitlement.class);
+            if (metadata != null) {
+                if (Modifier.isStatic(method.getModifiers()) == false) {
+                    throw new IllegalStateException(
+                        "entitlement class [" + entitlementClass.getName() + "] has non-static method annotated with ExternalEntitlement"
+                    );
+                }
+                if (entitlementMetadata != null) {
+                    throw new IllegalStateException(
+                        "entitlement class ["
+                            + entitlementClass.getName()
+                            + "] has more than one constructor and/or method annotated with ExternalEntitlement"
+                    );
+                }
+                entitlementMethod = method;
+                entitlementMetadata = metadata;
+            }
+        }
+
         if (entitlementMetadata == null) {
             throw newPolicyParserException(scopeName, "unknown entitlement type [" + entitlementType + "]");
         }
@@ -171,7 +193,9 @@ protected Entitlement parseEntitlement(String scopeName, String entitlementType)
             throw newPolicyParserException("entitlement type [" + entitlementType + "] is allowed only on modules");
         }
 
-        Class<?>[] parameterTypes = entitlementConstructor.getParameterTypes();
+        Class<?>[] parameterTypes = entitlementConstructor != null
+            ? entitlementConstructor.getParameterTypes()
+            : entitlementMethod.getParameterTypes();
         String[] parametersNames = entitlementMetadata.parameterNames();
 
         if (parameterTypes.length != 0 || parametersNames.length != 0) {
@@ -204,7 +228,11 @@ protected Entitlement parseEntitlement(String scopeName, String entitlementType)
         }
 
         try {
-            return (Entitlement) entitlementConstructor.newInstance(parameterValues);
+            if (entitlementConstructor != null) {
+                return (Entitlement) entitlementConstructor.newInstance(parameterValues);
+            } else {
+                return (Entitlement) entitlementMethod.invoke(null, parameterValues);
+            }
         } catch (InvocationTargetException | InstantiationException | IllegalAccessException e) {
             if (e.getCause() instanceof PolicyValidationException piae) {
                 throw newPolicyParserException(startLocation, scopeName, entitlementType, piae);

libs/entitlement/src/main/java/org/elasticsearch/entitlement/runtime/policy/entitlements/FileEntitlement.java

@@ -43,7 +43,7 @@ private static Mode parseMode(String mode) {
     }
 
     @ExternalEntitlement(parameterNames = { "path", "mode" }, esModulesOnly = false)
-    public FileEntitlement(String path, String mode) {
-        this(path, parseMode(mode));
+    public static FileEntitlement create(String path, String mode) {
+        return new FileEntitlement(path, parseMode(mode));
     }
 }

libs/entitlement/src/test/java/org/elasticsearch/entitlement/runtime/policy/FileAccessTreeTests.java

@@ -85,6 +85,6 @@ public void testNormalizePath() {
 
     FileEntitlement entitlement(String path, String mode) {
         Path p = path(path);
-        return new FileEntitlement(p.toString(), mode);
+        return FileEntitlement.create(p.toString(), mode);
     }
 }

libs/entitlement/src/test/java/org/elasticsearch/entitlement/runtime/policy/PolicyParserTests.java

@@ -40,6 +40,35 @@ public ManyConstructorsEntitlement(String s) {}
         public ManyConstructorsEntitlement(int i) {}
     }
 
+    public static class ManyMethodsEntitlement implements Entitlement {
+        @ExternalEntitlement
+        public static ManyMethodsEntitlement create(String s) {
+            return new ManyMethodsEntitlement();
+        }
+
+        @ExternalEntitlement
+        public static ManyMethodsEntitlement create(int i) {
+            return new ManyMethodsEntitlement();
+        }
+    }
+
+    public static class ConstructorAndMethodEntitlement implements Entitlement {
+        @ExternalEntitlement
+        public static ConstructorAndMethodEntitlement create(String s) {
+            return new ConstructorAndMethodEntitlement(s);
+        }
+
+        @ExternalEntitlement
+        public ConstructorAndMethodEntitlement(String s) {}
+    }
+
+    public static class NonStaticMethodEntitlement implements Entitlement {
+        @ExternalEntitlement
+        public NonStaticMethodEntitlement create() {
+            return new NonStaticMethodEntitlement();
+        }
+    }
+
     public void testGetEntitlementTypeName() {
         assertEquals("create_class_loader", PolicyParser.getEntitlementTypeName(CreateClassLoaderEntitlement.class));
 
@@ -55,7 +84,7 @@ public void testPolicyBuilder() throws IOException {
             .parsePolicy();
         Policy expected = new Policy(
             "test-policy.yaml",
-            List.of(new Scope("entitlement-module-name", List.of(new FileEntitlement("test/path/to/file", "read_write"))))
+            List.of(new Scope("entitlement-module-name", List.of(FileEntitlement.create("test/path/to/file", "read_write"))))
         );
         assertEquals(expected, parsedPolicy);
     }
@@ -65,7 +94,7 @@ public void testPolicyBuilderOnExternalPlugin() throws IOException {
             .parsePolicy();
         Policy expected = new Policy(
             "test-policy.yaml",
-            List.of(new Scope("entitlement-module-name", List.of(new FileEntitlement("test/path/to/file", "read_write"))))
+            List.of(new Scope("entitlement-module-name", List.of(FileEntitlement.create("test/path/to/file", "read_write"))))
         );
         assertEquals(expected, parsedPolicy);
     }
@@ -174,4 +203,60 @@ public void testMultipleConstructorsAnnotated() throws IOException {
             )
         );
     }
+
+    public void testMultipleMethodsAnnotated() throws IOException {
+        var parser = new PolicyParser(new ByteArrayInputStream("""
+            entitlement-module-name:
+              - many_methods
+            """.getBytes(StandardCharsets.UTF_8)), "test-policy.yaml", true, Map.of("many_methods", ManyMethodsEntitlement.class));
+
+        var e = expectThrows(IllegalStateException.class, parser::parsePolicy);
+        assertThat(
+            e.getMessage(),
+            equalTo(
+                "entitlement class "
+                    + "[org.elasticsearch.entitlement.runtime.policy.PolicyParserTests$ManyMethodsEntitlement]"
+                    + " has more than one constructor and/or method annotated with ExternalEntitlement"
+            )
+        );
+    }
+
+    public void testConstructorAndMethodAnnotated() throws IOException {
+        var parser = new PolicyParser(
+            new ByteArrayInputStream("""
+                entitlement-module-name:
+                  - constructor_and_method
+                """.getBytes(StandardCharsets.UTF_8)),
+            "test-policy.yaml",
+            true,
+            Map.of("constructor_and_method", ConstructorAndMethodEntitlement.class)
+        );
+
+        var e = expectThrows(IllegalStateException.class, parser::parsePolicy);
+        assertThat(
+            e.getMessage(),
+            equalTo(
+                "entitlement class "
+                    + "[org.elasticsearch.entitlement.runtime.policy.PolicyParserTests$ConstructorAndMethodEntitlement]"
+                    + " has more than one constructor and/or method annotated with ExternalEntitlement"
+            )
+        );
+    }
+
+    public void testNonStaticMethodAnnotated() throws IOException {
+        var parser = new PolicyParser(new ByteArrayInputStream("""
+            entitlement-module-name:
+              - non_static
+            """.getBytes(StandardCharsets.UTF_8)), "test-policy.yaml", true, Map.of("non_static", NonStaticMethodEntitlement.class));
+
+        var e = expectThrows(IllegalStateException.class, parser::parsePolicy);
+        assertThat(
+            e.getMessage(),
+            equalTo(
+                "entitlement class "
+                    + "[org.elasticsearch.entitlement.runtime.policy.PolicyParserTests$NonStaticMethodEntitlement]"
+                    + " has non-static method annotated with ExternalEntitlement"
+            )
+        );
+    }
 }