Fix deep-review findings for prepare/closed-handle safety

Agent-Logs-Url: https://github.com/microsoft/regorus/sessions/c2c0f3ef-f641-4d83-94f8-a3cd3ede6db8

Co-authored-by: anakrish <35780660+anakrish@users.noreply.github.com>

bindings/cpp/regorus.hpp

@@ -80,6 +80,10 @@ namespace regorus {
 	    return std::unique_ptr<Engine>(new Engine(regorus_engine_clone(engine)));
 	}
 
+	Result prepare() {
+	    return Result(regorus_engine_prepare(engine));
+	}
+
 	Result set_rego_v0(bool enable) {
 		return Result(regorus_engine_set_rego_v0(engine, enable));
 	}

bindings/csharp/Regorus/Engine.cs

@@ -68,6 +68,18 @@ namespace Regorus
             });
         }
 
+        /// <summary>
+        /// Prepare internal evaluation structures without executing a query.
+        /// This is optional: if skipped, the first evaluation pays this setup cost.
+        /// </summary>
+        public void Prepare()
+        {
+            UseHandle(enginePtr =>
+            {
+                CheckAndDropResult(Regorus.Internal.API.regorus_engine_prepare((Regorus.Internal.RegorusEngine*)enginePtr));
+            });
+        }
+
         public void SetStrictBuiltinErrors(bool strict)
         {
             UseHandle(enginePtr =>

bindings/csharp/Regorus/NativeMethods.cs

@@ -92,6 +92,12 @@ namespace Regorus.Internal
         [DllImport(LibraryName, EntryPoint = "regorus_engine_clone", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
         internal static extern RegorusEngine* regorus_engine_clone(RegorusEngine* engine);
 
+        /// <summary>
+        /// Prepare a RegorusEngine for evaluation without executing a query.
+        /// </summary>
+        [DllImport(LibraryName, EntryPoint = "regorus_engine_prepare", CallingConvention = CallingConvention.Cdecl, ExactSpelling = true)]
+        internal static extern RegorusResult regorus_engine_prepare(RegorusEngine* engine);
+
         /// <summary>
         /// Compile an RVM program from the engine state with entry points.
         /// </summary>

bindings/ffi/src/engine.rs

@@ -199,6 +199,21 @@ pub extern "C" fn regorus_engine_clone(engine: *mut RegorusEngine) -> *mut Regor
     }
 }
 
+/// Prepare a [`RegorusEngine`] for evaluation without executing a query.
+///
+/// This is optional. If not called, first eval performs the same setup.
+/// If policy/data changes after preparation, setup is invalidated.
+#[no_mangle]
+pub extern "C" fn regorus_engine_prepare(engine: *mut RegorusEngine) -> RegorusResult {
+    with_unwind_guard(|| {
+        to_regorus_result(|| -> Result<()> {
+            let engine = to_ref(engine)?;
+            let mut guard = engine.try_write()?;
+            guard.prepare()
+        }())
+    })
+}
+
 #[no_mangle]
 pub extern "C" fn regorus_engine_drop(engine: *mut RegorusEngine) {
     if let Ok(e) = to_ref(engine) {

bindings/go/pkg/regorus/mod.go

@@ -28,6 +28,17 @@ func (e *Engine) Clone() *Engine {
 	return c
 }
 
+func (e *Engine) Prepare() error {
+	result := C.regorus_engine_prepare(e.e)
+	defer C.regorus_result_drop(result)
+
+	if result.status != C.Ok {
+		return fmt.Errorf("%s", C.GoString(result.error_message))
+	}
+
+	return nil
+}
+
 func (e *Engine) SetRegoV0(enable bool) error {
 	result := C.regorus_engine_set_rego_v0(e.e, C.bool(enable))
 	defer C.regorus_result_drop(result)

bindings/java/com_microsoft_regorus_Engine.h

@@ -23,6 +23,14 @@ JNIEXPORT jlong JNICALL Java_com_microsoft_regorus_Engine_nativeNewEngine
 JNIEXPORT jlong JNICALL Java_com_microsoft_regorus_Engine_nativeClone
   (JNIEnv *, jclass, jlong);
 
+/*
+ * Class:     com_microsoft_regorus_Engine
+ * Method:    nativePrepare
+ * Signature: (J)V
+ */
+JNIEXPORT void JNICALL Java_com_microsoft_regorus_Engine_nativePrepare
+  (JNIEnv *, jclass, jlong);
+
 /*
  * Class:     com_microsoft_regorus_Engine
  * Method:    nativeAddPolicy

bindings/java/src/lib.rs

@@ -27,13 +27,30 @@ pub extern "system" fn Java_com_microsoft_regorus_Engine_nativeNewEngine(
 
 #[no_mangle]
 pub extern "system" fn Java_com_microsoft_regorus_Engine_nativeClone(
-    _env: EnvUnowned,
+    env: EnvUnowned,
     _class: JClass,
     engine_ptr: jlong,
 ) -> jlong {
-    let engine = unsafe { &mut *(engine_ptr as *mut Engine) };
-    let c = engine.clone();
-    Box::into_raw(Box::new(c)) as jlong
+    let res = throw_err(env, |_env| {
+        let engine = unsafe { &mut *get_engine_ptr(engine_ptr)? };
+        let c = engine.clone();
+        Ok(Box::into_raw(Box::new(c)) as jlong)
+    });
+
+    res.unwrap_or_default()
+}
+
+#[no_mangle]
+pub extern "system" fn Java_com_microsoft_regorus_Engine_nativePrepare(
+    env: EnvUnowned,
+    _class: JClass,
+    engine_ptr: jlong,
+) {
+    let _ = throw_err(env, |_env| {
+        let engine = unsafe { &mut *get_engine_ptr(engine_ptr)? };
+        engine.prepare()?;
+        Ok(())
+    });
 }
 
 #[no_mangle]
@@ -437,6 +454,9 @@ pub extern "system" fn Java_com_microsoft_regorus_Engine_nativeDestroyEngine(
     _class: JClass,
     engine_ptr: jlong,
 ) {
+    if engine_ptr == 0 {
+        return;
+    }
     unsafe {
         let _engine = Box::from_raw(engine_ptr as *mut Engine);
     }
@@ -816,6 +836,13 @@ fn throw_err<T>(mut env: EnvUnowned, f: impl FnOnce(&mut Env) -> Result<T>) -> R
     }
 }
 
+fn get_engine_ptr(engine_ptr: jlong) -> Result<*mut Engine> {
+    if engine_ptr == 0 {
+        return Err(anyhow::anyhow!("Engine is closed"));
+    }
+    Ok(engine_ptr as *mut Engine)
+}
+
 fn get_string_array(env: &mut Env, array: jobjectArray) -> Result<Vec<String>> {
     if array.is_null() {
         return Ok(Vec::new());

bindings/java/src/main/java/com/microsoft/regorus/Engine.java

@@ -21,6 +21,7 @@ public class Engine implements AutoCloseable, Cloneable {
     // if you update the native API.
     private static native long nativeNewEngine();
     private static native long nativeClone(long enginePtr);
+    private static native void nativePrepare(long enginePtr);
     private static native void nativeSetRegoV0(long enginePtr, boolean enable);
     private static native String nativeAddPolicy(long enginePtr, String path, String rego);
     private static native String nativeAddPolicyFromFile(long enginePtr, String path);
@@ -45,7 +46,7 @@ public class Engine implements AutoCloseable, Cloneable {
 
     // Pointer to Engine allocated on Rust's heap, all native methods works on
     // engine expects this pointer. It is free'd in `close` method.
-    private final long enginePtr;
+    private long enginePtr;
 
     /**
      * Creates a new Regorus Engine.
@@ -63,7 +64,15 @@ public class Engine implements AutoCloseable, Cloneable {
      * Efficiently clones an Engine.
      */
     public Engine clone() {
-	return new Engine(nativeClone(enginePtr));
+	return new Engine(nativeClone(requireOpen()));
+    }
+
+    /**
+     * Prepares internal evaluation structures without executing a query.
+     * Optional: if skipped, first evaluation performs the same setup.
+     */
+    public void prepare() {
+        nativePrepare(requireOpen());
     }
     
      /**
@@ -73,7 +82,7 @@ public class Engine implements AutoCloseable, Cloneable {
      * 
      */
     public void setRegoV0(boolean enable) {
-        nativeSetRegoV0(enginePtr, enable);
+        nativeSetRegoV0(requireOpen(), enable);
     }
 
     /**
@@ -85,7 +94,7 @@ public class Engine implements AutoCloseable, Cloneable {
      * @return Rego package defined in the policy.
      */
     public String addPolicy(String filename, String rego) {
-        return nativeAddPolicy(enginePtr, filename, rego);
+        return nativeAddPolicy(requireOpen(), filename, rego);
     }
 
     /**
@@ -96,7 +105,7 @@ public class Engine implements AutoCloseable, Cloneable {
      * @return Rego package defined in the policy.
      */
     public String addPolicyFromFile(String path) {
-        return nativeAddPolicyFromFile(enginePtr, path);
+        return nativeAddPolicyFromFile(requireOpen(), path);
     }
 
     /**
@@ -105,7 +114,7 @@ public class Engine implements AutoCloseable, Cloneable {
      * @return List of Rego packages as a JSON array of strings.
      */
     public String getPackages() {
-        return nativeGetPackages(enginePtr);
+        return nativeGetPackages(requireOpen());
     }
     
     /**
@@ -114,14 +123,14 @@ public class Engine implements AutoCloseable, Cloneable {
      * @return List of Rego policies as a JSON array of sources.
      */
     public String getPolicies() {
-        return nativeGetPolicies(enginePtr);
+        return nativeGetPolicies(requireOpen());
     }
     
     /**
      * Clears the data  document.
      */
     public void clearData() {
-        nativeClearData(enginePtr);
+        nativeClearData(requireOpen());
     }
 
     /**
@@ -143,7 +152,7 @@ public class Engine implements AutoCloseable, Cloneable {
      * @param data Inline data document.
      */
     public void addDataJson(String data) throws RuntimeException {
-        nativeAddDataJson(enginePtr, data);
+        nativeAddDataJson(requireOpen(), data);
     }
 
     /**
@@ -160,7 +169,7 @@ public class Engine implements AutoCloseable, Cloneable {
      * @param path Path to JSON data document.
      */
     public void addDataJsonFromFile(String path) throws RuntimeException {
-        nativeAddDataJsonFromFile(enginePtr, path);
+        nativeAddDataJsonFromFile(requireOpen(), path);
     }
 
     /**
@@ -169,7 +178,7 @@ public class Engine implements AutoCloseable, Cloneable {
      * @param input inline JSON input.
      */
     public void setInputJson(String input) {
-        nativeSetInputJson(enginePtr, input);
+        nativeSetInputJson(requireOpen(), input);
     }
 
     /**
@@ -178,7 +187,7 @@ public class Engine implements AutoCloseable, Cloneable {
      * @param path Path to JSON input.
      */
     public void setInputJsonFromFile(String path) {
-        nativeSetInputJsonFromFile(enginePtr, path);
+        nativeSetInputJsonFromFile(requireOpen(), path);
     }
 
     /**
@@ -189,7 +198,7 @@ public class Engine implements AutoCloseable, Cloneable {
      * @return Query results as a JSON string.
      */
     public String evalQuery(String query) {
-        return nativeEvalQuery(enginePtr, query);
+        return nativeEvalQuery(requireOpen(), query);
     }
     
     /**
@@ -200,7 +209,7 @@ public class Engine implements AutoCloseable, Cloneable {
      * @return Value of the rule as a JSON string.
      */
     public String evalRule(String rule) {
-        return nativeEvalRule(enginePtr, rule);
+        return nativeEvalRule(requireOpen(), rule);
     }
     
     /**
@@ -210,7 +219,7 @@ public class Engine implements AutoCloseable, Cloneable {
      * 
      */
     public void setEnableCoverage(boolean enable) {
-        nativeSetEnableCoverage(enginePtr, enable);
+        nativeSetEnableCoverage(requireOpen(), enable);
     }
     
     /**
@@ -218,7 +227,7 @@ public class Engine implements AutoCloseable, Cloneable {
      * 
      */
     public void clearCoverageData() {
-        nativeClearCoverageData(enginePtr);
+        nativeClearCoverageData(requireOpen());
     }
     
     /**
@@ -228,7 +237,7 @@ public class Engine implements AutoCloseable, Cloneable {
      * 
      */
     public String getCoverageReport() {
-        return nativeGetCoverageReport(enginePtr);
+        return nativeGetCoverageReport(requireOpen());
     }
     
     /**
@@ -238,7 +247,7 @@ public class Engine implements AutoCloseable, Cloneable {
      * 
      */
     public String getCoverageReportPretty() {
-        return nativeGetCoverageReportPretty(enginePtr);
+        return nativeGetCoverageReportPretty(requireOpen());
     }
     
     /**
@@ -248,7 +257,7 @@ public class Engine implements AutoCloseable, Cloneable {
      * 
      */
     public void setGatherPrints(boolean b) {
-        nativeSetGatherPrints(enginePtr, b);
+        nativeSetGatherPrints(requireOpen(), b);
     }
     
     /**
@@ -258,7 +267,7 @@ public class Engine implements AutoCloseable, Cloneable {
      * 
      */
     public String takePrints() {
-        return nativeTakePrints(enginePtr);
+        return nativeTakePrints(requireOpen());
     }
 
     /**
@@ -267,24 +276,34 @@ public class Engine implements AutoCloseable, Cloneable {
      * @param config Policy length configuration.
      */
     public void setPolicyLengthConfig(PolicyLengthConfig config) {
-        nativeSetPolicyLengthConfig(enginePtr, config.maxCol, config.maxFileBytes, config.maxLines);
+        nativeSetPolicyLengthConfig(requireOpen(), config.maxCol, config.maxFileBytes, config.maxLines);
     }
 
     /**
      * Clear the policy length configuration, reverting to defaults.
      */
     public void clearPolicyLengthConfig() {
-        nativeClearPolicyLengthConfig(enginePtr);
+        nativeClearPolicyLengthConfig(requireOpen());
     }
 
     long getPtr() {
+        return requireOpen();
+    }
+
+    private long requireOpen() {
+        if (enginePtr == 0) {
+            throw new IllegalStateException("Engine is closed");
+        }
         return enginePtr;
     }
 
     
     @Override
     public void close() {
-        nativeDestroyEngine(enginePtr);
+        if (enginePtr != 0) {
+            nativeDestroyEngine(enginePtr);
+            enginePtr = 0;
+        }
     }
 
     // Loading native library from JAR is adapted from:

bindings/java/src/test/java/com/microsoft/regorus/EngineTest.java

@@ -22,8 +22,19 @@ public class EngineTest extends TestCase
                 "package test\nmessage = concat(\", \", [input.message, data.message])"
             );
             engine.addDataJson("{\"message\":\"World!\"}");
+            engine.prepare();
             engine.setInputJson("{\"message\":\"Hello\"}");
             resJson = engine.evalQuery("data.test.message");
+
+            try (Engine template = engine.clone()) {
+                template.setInputJson("{\"message\":\"Hi\"}");
+                String templateResJson = template.evalQuery("data.test.message");
+                Map templateRes = new Gson().fromJson(templateResJson, Map.class);
+                ArrayList templateResults = (ArrayList) templateRes.get("result");
+                ArrayList templateExpressions = (ArrayList) ((Map) templateResults.get(0)).get("expressions");
+                Map templateExpression = (Map) templateExpressions.get(0);
+                Assert.assertEquals("Hi, World!", templateExpression.get("value"));
+            }
         }
 
         Gson gson = new Gson();
@@ -33,4 +44,28 @@ public class EngineTest extends TestCase
         Map expression = (Map) expressions.get(0);
         Assert.assertEquals("Hello, World!", expression.get("value"));
     }
+
+    public void test_closed_engine_operations_throw()
+    {
+        Engine engine = new Engine();
+        engine.close();
+
+        try {
+            engine.prepare();
+            fail("prepare should fail on closed engine");
+        } catch (IllegalStateException expected) {
+        }
+
+        try {
+            engine.clone();
+            fail("clone should fail on closed engine");
+        } catch (IllegalStateException expected) {
+        }
+
+        try {
+            engine.evalQuery("data");
+            fail("evalQuery should fail on closed engine");
+        } catch (IllegalStateException expected) {
+        }
+    }
 }

bindings/python/src/lib.rs

@@ -463,6 +463,13 @@ impl Engine {
         self.engine.take_prints()
     }
 
+    /// Prepare internal evaluation structures without executing a query.
+    ///
+    /// Optional: if skipped, first evaluation performs the same setup.
+    pub fn prepare(&mut self) -> Result<()> {
+        self.engine.prepare()
+    }
+
     /// Clone a [`Engine`]
     ///
     /// To avoid having to parse same policy again, the engine can be cloned

bindings/python/test.py

@@ -87,6 +87,7 @@ report = engine.get_coverage_report_pretty()
 print(report)
 
 # Clone engine
+engine.prepare()
 engine1 = engine.clone()
 
 

bindings/ruby/ext/regorusrb/src/lib.rs

@@ -115,6 +115,13 @@ impl Engine {
         Ok(())
     }
 
+    fn prepare(&self) -> Result<(), Error> {
+        self.engine
+            .borrow_mut()
+            .prepare()
+            .map_err(|e| Error::new(runtime_error(), format!("Failed to prepare engine: {e}")))
+    }
+
     fn get_packages(&self) -> Result<Vec<String>, Error> {
         self.engine
             .borrow()
@@ -373,6 +380,7 @@ fn init(ruby: &Ruby) -> Result<(), Error> {
         method!(Engine::add_data_from_json_file, 1),
     )?;
     engine_class.define_method("clear_data", method!(Engine::clear_data, 0))?;
+    engine_class.define_method("prepare", method!(Engine::prepare, 0))?;
 
     // input operations
     engine_class.define_method("set_input", method!(Engine::set_input, 1))?;

bindings/ruby/test/test_regorus.rb

@@ -150,6 +150,7 @@ class TestRegorus < Minitest::Test
   end
 
   def test_engine_cloning
+    @engine.prepare
     cloned_engine = @engine.clone
 
     assert_instance_of ::Regorus::Engine, cloned_engine

bindings/wasm/README.md

@@ -21,3 +21,9 @@ Run `cargo xtask build-wasm` to invoke wasm-pack with sensible defaults, or `car
 ## Usage
 
 See [test.js](https://github.com/microsoft/regorus/blob/main/bindings/wasm/test.js) for example usage.
+
+For best performance with large policies, call `engine.prepare()` after loading
+policy/data, then use `engine.clone()` to create per-request engines. If
+`prepare()` is skipped, the first `eval*` call performs the same one-time
+setup. Adding/changing policy or data after `prepare()` invalidates the
+prepared state.

bindings/wasm/src/lib.rs

@@ -138,6 +138,17 @@ impl Engine {
         self.engine.set_rego_v0(enable)
     }
 
+    /// Clone this engine.
+    ///
+    /// Useful for creating per-request engines after loading policy/data once.
+    ///
+    /// Clone is designed to avoid reparsing policy text and reloading immutable
+    /// policy structures. Mutable evaluation state is copied for isolation.
+    #[wasm_bindgen(js_name = "clone")]
+    pub fn cloneEngine(&self) -> Engine {
+        Clone::clone(self)
+    }
+
     /// Add a policy
     ///
     /// The policy is parsed into AST.
@@ -158,6 +169,20 @@ impl Engine {
         self.engine.add_data(data).map_err(error_to_jsvalue)
     }
 
+    /// Prepare the engine for evaluation.
+    ///
+    /// The first evaluation on an unprepared engine performs one-time setup.
+    /// Calling `prepare()` performs that setup eagerly.
+    ///
+    /// This is optional for correctness. If omitted, the first `eval*` call
+    /// implicitly performs preparation.
+    ///
+    /// If policies/data are modified after `prepare()`, preparation is
+    /// invalidated and must be performed again (explicitly or via first eval).
+    pub fn prepare(&mut self) -> Result<(), JsValue> {
+        self.engine.prepare().map_err(error_to_jsvalue)
+    }
+
     /// Get the list of packages defined by loaded policies.
     ///
     /// See https://docs.rs/regorus/latest/regorus/struct.Engine.html#method.get_packages
@@ -487,6 +512,9 @@ mod tests {
         )?;
         assert_eq!(pkg, "data.test");
 
+        // Prepare before first evaluation.
+        engine.prepare()?;
+
         let results = engine.evalQuery("data".to_string())?;
         let r = regorus::Value::from_json_str(&results).map_err(error_to_jsvalue)?;
 

bindings/wasm/test.js

@@ -40,6 +40,13 @@ engine.addDataJson(`
  }
 `);
 
+// Prepare internal evaluation structures once.
+engine.prepare();
+
+// Clone a prepared template engine for reuse.
+var template = engine.clone();
+engine = template.clone();
+
 // Set policy input
 engine.setInputJson(`
  {

src/engine.rs

@@ -505,6 +505,47 @@ impl Engine {
         self.add_data(Value::from_json_str(data_json)?)
     }
 
+    /// Prepare the engine for evaluation without executing a query or rule.
+    ///
+    /// The first evaluation on an unprepared engine performs one-time setup
+    /// (analysis, scheduling, imports/rules processing, and initialization of
+    /// internal evaluation structures). Calling this method performs that work
+    /// eagerly so a later call to [`Engine::eval_rule`] / [`Engine::eval_query`]
+    /// does not pay that startup cost.
+    ///
+    /// This method is optional for correctness. If omitted, the first
+    /// evaluation will implicitly prepare the engine.
+    ///
+    /// Preparation is invalidated when policy/data that affects evaluation is
+    /// changed (for example: [`Engine::add_policy`], [`Engine::add_policy_from_file`],
+    /// [`Engine::add_data`], [`Engine::clear_data`]). In those cases, the next
+    /// evaluation (or another explicit call to `prepare`) performs setup again.
+    ///
+    /// This is especially useful before cloning template engines used for
+    /// repeated evaluations.
+    ///
+    /// ```
+    /// # use regorus::*;
+    /// # fn main() -> anyhow::Result<()> {
+    /// let mut engine = Engine::new();
+    /// engine.add_policy("test.rego".to_string(), r#"
+    ///   package test
+    ///   import rego.v1
+    ///   allow if input.user == "alice"
+    /// "#.to_string())?;
+    ///
+    /// engine.prepare()?;
+    /// let mut cloned = engine.clone();
+    ///
+    /// cloned.set_input_json(r#"{"user":"alice"}"#)?;
+    /// assert_eq!(cloned.eval_rule("data.test.allow".to_string())?, Value::from(true));
+    /// # Ok(())
+    /// # }
+    /// ```
+    pub fn prepare(&mut self) -> Result<()> {
+        self.prepare_for_eval(false, false)
+    }
+
     /// Set whether builtins should raise errors strictly or not.
     ///
     /// Regorus differs from OPA in that by default builtins will
@@ -1084,9 +1125,10 @@ impl Engine {
         limits::enforce_memory_limit().map_err(|err| anyhow!(err))?;
 
         self.interpreter.set_traces(enable_tracing);
+        let newly_prepared = !self.prepared;
 
         // if the data/policies have changed or the interpreter has never been prepared
-        if !self.prepared {
+        if newly_prepared {
             // Analyze the modules and determine how statements must be scheduled.
             let analyzer = Analyzer::new();
             let schedule = Rc::new(analyzer.analyze(&self.modules)?);
@@ -1116,23 +1158,28 @@ impl Engine {
 
             // Set schedule after hoisting completes
             self.interpreter.set_schedule(Some(schedule));
+        }
 
-            #[cfg(feature = "azure_policy")]
+        #[cfg(feature = "azure_policy")]
+        {
             if for_target {
-                // Resolve and validate target specifications across all modules
+                // Resolve and validate target specifications across all modules.
+                // This must run for target-aware compilation even if generic prepare()
+                // was already called.
                 crate::interpreter::target::resolve::resolve_and_apply_target(
                     &mut self.interpreter,
                 )?;
                 // Infer resource types
                 crate::interpreter::target::infer::infer_resource_type(&mut self.interpreter)?;
-            }
-
-            if !for_target {
-                // Check if any module specifies a target and warn if so
-                #[cfg(feature = "azure_policy")]
+            } else if newly_prepared {
+                // Check if any module specifies a target and warn if so.
                 self.warn_if_targets_present();
             }
+        }
+        #[cfg(not(feature = "azure_policy"))]
+        let _ = for_target;
 
+        if newly_prepared {
             self.prepared = true;
         }
 

tests/engine/mod.rs

@@ -102,6 +102,135 @@ fn extension_with_state() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn prepare_then_clone_without_initial_eval() -> Result<()> {
+    let mut engine = Engine::new();
+    engine.add_policy(
+        "test.rego".to_string(),
+        r#"package test
+           import rego.v1
+
+           default allow := false
+
+           allow if {
+             input.user in data.allowed_users
+           }
+        "#
+        .to_string(),
+    )?;
+    engine.add_data(Value::from_json_str(
+        r#"{"allowed_users":["alice","bob"]}"#,
+    )?)?;
+
+    // Prepare once and clone without running an initial evaluation.
+    engine.prepare()?;
+
+    let mut alice_engine = engine.clone();
+    alice_engine.set_input_json(r#"{"user":"alice"}"#)?;
+    assert_eq!(
+        alice_engine.eval_rule("data.test.allow".to_string())?,
+        Value::from(true)
+    );
+
+    let mut mallory_engine = engine.clone();
+    mallory_engine.set_input_json(r#"{"user":"mallory"}"#)?;
+    assert_eq!(
+        mallory_engine.eval_rule("data.test.allow".to_string())?,
+        Value::from(false)
+    );
+
+    Ok(())
+}
+
+#[test]
+#[cfg(feature = "azure_policy")]
+#[cfg_attr(docsrs, doc(cfg(feature = "azure_policy")))]
+fn prepare_then_compile_for_target() -> Result<()> {
+    if !registry::targets::contains("target.tests.sample_test_target") {
+        let target = Target::from_json_str(include_str!(
+            "../interpreter/cases/target/definitions/sample_target.json"
+        ))?;
+        registry::targets::register(Rc::new(target))?;
+    }
+
+    let mut engine = Engine::new();
+    engine.add_policy(
+        "test.rego".to_string(),
+        r#"package test
+           import rego.v1
+           __target__ := "target.tests.sample_test_target"
+
+           default allow := false
+
+           allow if {
+               input.type == "test_resource"
+           }
+        "#
+        .to_string(),
+    )?;
+
+    engine.prepare()?;
+    let compiled = engine.compile_for_target()?;
+    let info = compiled.get_policy_info()?;
+    assert_eq!(
+        info.target_name.as_deref(),
+        Some("target.tests.sample_test_target")
+    );
+
+    let result = compiled.eval_with_input(Value::from_json_str(
+        r#"{"name":"resource-1","type":"test_resource"}"#,
+    )?)?;
+    assert_eq!(result, Value::from(true));
+
+    Ok(())
+}
+
+#[test]
+#[cfg(feature = "azure_policy")]
+#[cfg_attr(docsrs, doc(cfg(feature = "azure_policy")))]
+fn prepare_then_compile_for_target_error_recovery() -> Result<()> {
+    let target_name = "target.tests.prepare_recovery_test_target";
+
+    let mut engine = Engine::new();
+    engine.add_policy(
+        "test.rego".to_string(),
+        format!(
+            r#"package test
+           import rego.v1
+           __target__ := "{target_name}"
+
+           default allow := false
+
+           allow if {{
+               input.type == "test_resource"
+           }}
+        "#
+        ),
+    )?;
+
+    engine.prepare()?;
+    assert!(engine.compile_for_target().is_err());
+
+    if !registry::targets::contains(target_name) {
+        let target_json =
+            include_str!("../interpreter/cases/target/definitions/sample_target.json")
+                .replace("target.tests.sample_test_target", target_name);
+        let target = Target::from_json_str(&target_json)?;
+        registry::targets::register(Rc::new(target))?;
+    }
+
+    let compiled = engine.compile_for_target()?;
+    let info = compiled.get_policy_info()?;
+    assert_eq!(info.target_name.as_deref(), Some(target_name));
+
+    let result = compiled.eval_with_input(Value::from_json_str(
+        r#"{"name":"resource-1","type":"test_resource"}"#,
+    )?)?;
+    assert_eq!(result, Value::from(true));
+
+    Ok(())
+}
+
 #[test]
 #[cfg(feature = "azure_policy")]
 #[cfg_attr(docsrs, doc(cfg(feature = "azure_policy")))]