Java: Teach gen_java_decl about Futures for type-safety.

java/client/src/test/java/org/signal/libsignal/internal/FutureTest.java

@@ -50,13 +50,13 @@ public class FutureTest {
 
   @Test
   public void testSuccessFromRust() throws Exception {
-    Future<Integer> future = (Future<Integer>)Native.Future_success();
+    Future<Integer> future = Native.Future_success();
     assertEquals(42, (int) future.get());
   }
 
   @Test
   public void testFailureFromRust() throws Exception {
-    Future<Integer> future = (Future<Integer>)Native.Future_failure();
+    Future<Integer> future = Native.Future_failure();
     ExecutionException e = assertThrows(ExecutionException.class, () -> future.get());
     assertTrue(e.getCause() instanceof IllegalArgumentException);
   }

java/shared/java/org/signal/libsignal/internal/Native.java

@@ -23,6 +23,7 @@ import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
 import java.nio.file.Files;
+import java.util.concurrent.Future;
 import java.util.UUID;
 import java.util.Map;
 
@@ -199,8 +200,8 @@ public final class Native {
   public static native void ExpiringProfileKeyCredential_CheckValidContents(byte[] buffer);
   public static native long ExpiringProfileKeyCredential_GetExpirationTime(byte[] credential);
 
-  public static native Object Future_failure();
-  public static native Object Future_success();
+  public static native Future<Integer> Future_failure();
+  public static native Future<Integer> Future_success();
 
   public static native void GenericServerPublicParams_CheckValidContents(byte[] paramsBytes);
 

rust/bridge/jni/bin/Native.java.in

@@ -23,6 +23,7 @@ import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
 import java.nio.file.Files;
+import java.util.concurrent.Future;
 import java.util.UUID;
 import java.util.Map;
 

rust/bridge/jni/bin/gen_java_decl.py

@@ -40,7 +40,8 @@ ignore_this_warning = re.compile(
     r"WARN: Can't find .*\. This usually means that this type was incompatible or not found\.|"
     r"WARN: Missing `\[defines\]` entry for `feature = \".*\"` in cbindgen config\.|"
     r"WARN: Skip libsignal-bridge::.+ - \(not `(?:pub|no_mangle)`\)\.|"
-    r"WARN: Couldn't find path for Array\(Path\(GenericPath \{ .+ \}\), Name\(\"LEN\"\)\), skipping associated constants"
+    r"WARN: Couldn't find path for Array\(Path\(GenericPath \{ .+ \}\), Name\(\"LEN\"\)\), skipping associated constants|"
+    r"WARN: Cannot find a mangling for generic path GenericPath { path: Path { name: \"JavaFuture\" }.+"
     ")")
 
 unknown_warning = False
@@ -58,7 +59,23 @@ for l in stderr.split('\n'):
 if unknown_warning:
     sys.exit(1)
 
-java_decl = re.compile(r'([a-zA-Z]+) Java_org_signal_libsignal_internal_Native_([A-Z][a-zA-Z0-9]+)_1([A-Za-z0-9]+)\(JNIEnv .?env, JClass class_(, .*)?\);')
+java_decl = re.compile(r'([a-zA-Z]+(?:<.+>)?) Java_org_signal_libsignal_internal_Native_([A-Z][a-zA-Z0-9]+)_1([A-Za-z0-9]+)\(JNIEnv .?env, JClass class_(, .*)?\);')
+
+
+def box_primitives(typ):
+    type_map = {
+        "void": "Void",
+        "boolean": "Boolean",
+        "char": "Character",
+        "byte": "Byte",
+        "short": "Short",
+        "int": "Integer",
+        "long": "Long",
+        "float": "Float",
+        "double": "Double",
+    }
+
+    return type_map.get(typ, typ)
 
 
 def translate_to_java(typ):
@@ -78,7 +95,11 @@ def translate_to_java(typ):
     if typ in type_map:
         return type_map[typ]
 
-    # Assume anything prefixed with "Java" refers to an object
+    if typ.startswith('JavaFuture<'):
+        assert typ.endswith('>')
+        return 'Future<' + box_primitives(translate_to_java(typ[11:-1])) + '>'
+
+    # Assume anything else prefixed with "Java" refers to an object
     if typ.startswith('Java'):
         return typ[4:]
 

rust/bridge/jni/src/lib.rs

@@ -7,6 +7,7 @@
 #![deny(clippy::unwrap_used)]
 
 use jni::objects::{JByteArray, JClass, JLongArray, JObject};
+use jni::sys::jint;
 use jni::JNIEnv;
 use std::convert::TryFrom;
 
@@ -53,7 +54,7 @@ pub unsafe extern "C" fn Java_org_signal_libsignal_internal_Native_keepAlive(
 pub unsafe extern "C" fn Java_org_signal_libsignal_internal_Native_Future_1success<'local>(
     mut env: JNIEnv<'local>,
     _class: JClass,
-) -> JObject<'local> {
+) -> JavaFuture<'local, jint> {
     run_ffi_safe(&mut env, |env| {
         let future = new_object(
             env,
@@ -62,7 +63,7 @@ pub unsafe extern "C" fn Java_org_signal_libsignal_internal_Native_Future_1succe
         )?;
         let completer = FutureCompleter::new(env, &future)?;
         std::thread::spawn(move || completer.complete(42));
-        Ok(future)
+        Ok(future.into())
     })
 }
 
@@ -70,7 +71,7 @@ pub unsafe extern "C" fn Java_org_signal_libsignal_internal_Native_Future_1succe
 pub unsafe extern "C" fn Java_org_signal_libsignal_internal_Native_Future_1failure<'local>(
     mut env: JNIEnv<'local>,
     _class: JClass,
-) -> JObject<'local> {
+) -> JavaFuture<'local, jint> {
     run_ffi_safe(&mut env, |env| {
         let future = new_object(
             env,
@@ -83,6 +84,6 @@ pub unsafe extern "C" fn Java_org_signal_libsignal_internal_Native_Future_1failu
                 "failure".to_string(),
             )))
         });
-        Ok(future)
+        Ok(future.into())
     })
 }

rust/bridge/shared/src/jni/mod.rs

@@ -15,6 +15,7 @@ use signal_pin::Error as PinError;
 use std::convert::{TryFrom, TryInto};
 use std::error::Error;
 use std::fmt::Display;
+use std::marker::PhantomData;
 
 pub(crate) use jni::objects::{
     AutoElements, JByteArray, JClass, JLongArray, JObject, JString, ReleaseMode,
@@ -49,6 +50,28 @@ pub type JavaUUID<'a> = JObject<'a>;
 pub type JavaCiphertextMessage<'a> = JObject<'a>;
 pub type JavaMap<'a> = JObject<'a>;
 
+#[derive(Default)]
+#[repr(transparent)] // This ensures that JavaFuture has the same representation as JObject.
+pub struct JavaFuture<'a, T> {
+    future_object: JObject<'a>,
+    result: PhantomData<fn(T)>,
+}
+
+impl<'a, T> From<JObject<'a>> for JavaFuture<'a, T> {
+    fn from(future_object: JObject<'a>) -> Self {
+        Self {
+            future_object,
+            result: PhantomData,
+        }
+    }
+}
+
+impl<'a, T> From<JavaFuture<'a, T>> for JObject<'a> {
+    fn from(value: JavaFuture<'a, T>) -> Self {
+        value.future_object
+    }
+}
+
 fn convert_to_exception<'a, F>(env: &mut JNIEnv<'a>, error: SignalJniError, consume: F)
 where
     F: FnOnce(&mut JNIEnv<'a>, SignalJniResult<JThrowable<'a>>, &dyn Display),