Fix JNI stale reference error in getSeed method

- Extract seedOffset string content BEFORE acquiring mutex lock
- Prevent JNI reference invalidation by using std::string copy in critical section
- Add explicit scope boundaries for mutex-protected code
- Improve error handling with null checks after GetStringUTFChars
- Resolves 'attempt to use stale Global' crash by ensuring no JNI calls while mutex is held

Fixes issue where seedOffset jstring reference became stale when used across mutex boundary.

Author: niyid

app/src/main/cpp/monerujo.cpp

@@ -716,104 +716,90 @@ Java_com_m2049r_xmrwallet_model_Wallet_getSeed(
         JNIEnv *env,
         jobject instance,
         jstring seedOffset) {
-
+    
     LOGD("getSeed: ENTER - thread: %ld", (long)gettid());
-
+    
     // ============================================================
-    // PHASE 1: ALL JNI OPERATIONS - NO MUTEX YET
+    // PHASE 1: ALL JNI OPERATIONS BEFORE MUTEX
     // ============================================================
 
-    // Safety check: ensure env is valid
     if (env == nullptr) {
         LOGE("getSeed: ERROR - JNIEnv is null");
         return nullptr;
     }
-
-    // Get wallet handle BEFORE mutex (read-only operation, safe)
-    Monero::Wallet *wallet = nullptr;
-    try {
-        wallet = getHandle<Monero::Wallet>(env, instance);
-    } catch (...) {
-        LOGE("getSeed: EXCEPTION getting wallet handle");
-        return env->NewStringUTF("");
-    }
-
+    
+    // Get wallet handle
+    Monero::Wallet *wallet = getHandle<Monero::Wallet>(env, instance);
     if (!wallet) {
         LOGE("getSeed: ERROR - Wallet handle is null");
         return env->NewStringUTF("");
     }
-
-    // Extract seedOffset string IMMEDIATELY - this is the critical fix
-    // We MUST do this BEFORE acquiring any mutex
+    
+    // CRITICAL: Extract string content BEFORE mutex
     std::string offset;
     if (seedOffset != nullptr) {
         const char *off = env->GetStringUTFChars(seedOffset, nullptr);
-        
-        // Check for JNI exceptions
-        if (env->ExceptionCheck()) {
-            env->ExceptionClear();
-            LOGE("getSeed: JNI exception while reading seedOffset");
+        if (off == nullptr || env->ExceptionCheck()) {
+            if (env->ExceptionCheck()) {
+                env->ExceptionClear();
+            }
+            LOGE("getSeed: Failed to get UTF chars from seedOffset");
             return env->NewStringUTF("");
         }
-
-        if (off != nullptr) {
-            offset.assign(off);
-            env->ReleaseStringUTFChars(seedOffset, off);
-        }
+        
+        offset.assign(off);
+        env->ReleaseStringUTFChars(seedOffset, off);
+        // seedOffset jstring is now "dead" to us - never touch it again
     }
 
-    // IMPORTANT: seedOffset jstring is NEVER touched again after this point
-    // We only use the std::string copy (offset) from now on
-
     // ============================================================
-    // PHASE 2: CRITICAL SECTION - MUTEX HELD, NO JNI CALLS
+    // PHASE 2: CRITICAL SECTION - NO JNI CALLS WHILE LOCKED
     // ============================================================
 
-    // NOW acquire the mutex (after ALL JNI operations are complete)
-    std::lock_guard<std::mutex> lock(g_walletMutex);
-    LOGD("getSeed: acquired wallet mutex");
-
-    // Wallet status check
-    if (wallet->status() != Monero::Wallet::Status_Ok) {
-        std::string err;
-        int status;
-        wallet->statusWithErrorString(status, err);
-        LOGE("getSeed: Wallet status not OK (%d): %s", status, err.c_str());
-        // Return empty string on error
-        // Note: Returning while holding lock is fine - lock_guard destructor releases it
-        return env->NewStringUTF("");
-    }
-
-    // Get seed using the C++ string copy (NOT the jstring parameter)
     std::string seed;
-    try {
-        seed = wallet->seed(offset);
-    } catch (const std::exception& e) {
-        LOGE("getSeed: EXCEPTION in wallet->seed: %s", e.what());
-        return env->NewStringUTF("");
-    } catch (...) {
-        LOGE("getSeed: UNKNOWN EXCEPTION in wallet->seed");
-        return env->NewStringUTF("");
-    }
-
-    // Lock is automatically released here when lock_guard goes out of scope
+    {
+        std::lock_guard<std::mutex> lock(g_walletMutex);
+        LOGD("getSeed: acquired wallet mutex");
+        
+        // Wallet status check
+        if (wallet->status() != Monero::Wallet::Status_Ok) {
+            std::string err;
+            int status;
+            wallet->statusWithErrorString(status, err);
+            LOGE("getSeed: Wallet status not OK (%d): %s", status, err.c_str());
+            // Lock releases here when scope ends
+            return env->NewStringUTF("");
+        }
+        
+        // Get seed using C++ string (safe - no JNI involved)
+        try {
+            seed = wallet->seed(offset);
+        } catch (const std::exception& e) {
+            LOGE("getSeed: EXCEPTION in wallet->seed: %s", e.what());
+            return env->NewStringUTF("");
+        } catch (...) {
+            LOGE("getSeed: UNKNOWN EXCEPTION in wallet->seed");
+            return env->NewStringUTF("");
+        }
+    } // Lock automatically released here
 
     // ============================================================
-    // PHASE 3: CREATE RETURN VALUE - MUTEX RELEASED, JNI SAFE
+    // PHASE 3: CREATE RETURN VALUE - MUTEX RELEASED
     // ============================================================
 
-    // Create Java string from the result
     jstring result = env->NewStringUTF(seed.c_str());
-    
-    if (env->ExceptionCheck() || result == nullptr) {
-        env->ExceptionClear();
-        LOGE("getSeed: Failed to create jstring");
+    if (result == nullptr || env->ExceptionCheck()) {
+        if (env->ExceptionCheck()) {
+            env->ExceptionClear();
+        }
+        LOGE("getSeed: Failed to create return jstring");
         return env->NewStringUTF("");
     }
-
+    
     LOGD("getSeed: EXIT - success");
     return result;
 }
+
 JNIEXPORT jstring JNICALL
 Java_com_m2049r_xmrwallet_model_Wallet_getSeedLanguage(
         JNIEnv *env,