java: Set up class loader for libsignal_jni_testing.so too

The class loader is a rare process-wide variable for us. When the
testing crates are compiled into the same .so as the main bridge
crates, they all use the same variable, but when we build for Android
we have two separate native library files that get loaded, and so we
need to initialize both.

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

@@ -40,6 +40,7 @@ private static void loadNativeCode() {
 
   static {
     loadNativeCode();
+    initializeLibrary();
   }
 
   private NativeTesting() {}
@@ -96,5 +97,7 @@ private NativeTesting() {}
 
   public static native void TestingHandleType_Destroy(long handle);
 
+  public static native void initializeLibrary();
+
   public static native int test_only_fn_returns_123();
 }

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

@@ -40,6 +40,7 @@ public final class NativeTesting {
 
   static {
     loadNativeCode();
+    initializeLibrary();
   }
 
   private NativeTesting() {}

rust/bridge/jni/testing/Cargo.toml

@@ -16,5 +16,7 @@ crate-type = ["cdylib"]
 
 [dependencies]
 libsignal-bridge-testing = { path = "../../shared/testing", features = ["jni"] }
+libsignal-bridge-types = { path = "../../shared/types", features = ["jni"] }
 
+jni = { workspace = true }
 log = { workspace = true }

rust/bridge/jni/testing/src/lib.rs

@@ -3,7 +3,37 @@
 // SPDX-License-Identifier: AGPL-3.0-only
 //
 
+#![allow(clippy::missing_safety_doc)]
+
+use jni::objects::JClass;
+use jni::JNIEnv;
 // Import bridged functions. Without this, the compiler and/or linker are too
 // smart and don't include the symbols in the library.
 #[allow(unused_imports)]
 use libsignal_bridge_testing::*;
+use libsignal_bridge_types::jni::run_ffi_safe;
+
+/// Initialize internal data structures.
+///
+/// Initialization function used to set up internal data structures. This should be called once when
+/// the library is first loaded. Must support being run in the same shared object as the
+/// `initializeLibrary` in libsignal-jni, as well as the intended use case of running in a different
+/// one.
+#[no_mangle]
+pub unsafe extern "C" fn Java_org_signal_libsignal_internal_NativeTesting_initializeLibrary<
+    'local,
+>(
+    mut env: JNIEnv<'local>,
+    class: JClass<'local>,
+) {
+    run_ffi_safe(&mut env, |env| {
+        #[cfg(target_os = "android")]
+        libsignal_bridge_types::jni::save_class_loader(env, &class)?;
+
+        // Silence the unused variable warning on non-Android.
+        _ = class;
+        _ = env;
+
+        Ok(())
+    })
+}