Store the migration operation id rather than the operation iteslf

src/wallet/asyncrpcoperation_saplingmigration.cpp

@@ -191,6 +191,10 @@ libzcash::SaplingPaymentAddress AsyncRPCOperation_saplingmigration::getMigration
     return toAddress;
 }
 
+void AsyncRPCOperation_saplingmigration::cancel() {
+    set_state(OperationStatus::CANCELLED);
+}
+
 UniValue AsyncRPCOperation_saplingmigration::getStatus() const {
     UniValue v = AsyncRPCOperation::getStatus();
     UniValue obj = v.get_obj();

src/wallet/asyncrpcoperation_saplingmigration.h

@@ -20,6 +20,8 @@ public:
 
     virtual void main();
 
+    virtual void cancel();
+
     virtual UniValue getStatus() const;
 
 private:

src/wallet/wallet.cpp

@@ -603,17 +603,20 @@ void CWallet::RunSaplingMigration(int blockHeight) {
     // height N, implementations SHOULD start generating the transactions at around
     // height N-5
     if (blockHeight % 500 == 495) {
-        if (saplingMigrationOperation != nullptr) {
-            saplingMigrationOperation->cancel();
+        std::shared_ptr<AsyncRPCQueue> q = getAsyncRPCQueue();
+        std::shared_ptr<AsyncRPCOperation> lastOperation = q->popOperationForId(saplingMigrationOperationId);
+        if (lastOperation != nullptr) {
+            lastOperation->cancel();
         }
         pendingSaplingMigrationTxs.clear();
-        std::shared_ptr<AsyncRPCQueue> q = getAsyncRPCQueue();
         std::shared_ptr<AsyncRPCOperation> operation(new AsyncRPCOperation_saplingmigration(blockHeight + 5));
-        saplingMigrationOperation = operation;
+        saplingMigrationOperationId = operation->getId();
         q->addOperation(operation);
     } else if (blockHeight % 500 == 499) {
-        if (saplingMigrationOperation != nullptr) {
-            saplingMigrationOperation->cancel();
+        std::shared_ptr<AsyncRPCQueue> q = getAsyncRPCQueue();
+        std::shared_ptr<AsyncRPCOperation> lastOperation = q->popOperationForId(saplingMigrationOperationId);
+        if (lastOperation != nullptr) {
+            lastOperation->cancel();
         }
         for (const CTransaction& transaction : pendingSaplingMigrationTxs) {
             // The following is taken from z_sendmany/z_mergetoaddress

src/wallet/wallet.h

@@ -757,7 +757,7 @@ private:
     TxNullifiers mapTxSaplingNullifiers;
 
     std::vector<CTransaction> pendingSaplingMigrationTxs;
-    std::shared_ptr<AsyncRPCOperation> saplingMigrationOperation = nullptr;
+    AsyncRPCOperationId saplingMigrationOperationId;
 
     void AddToTransparentSpends(const COutPoint& outpoint, const uint256& wtxid);
     void AddToSproutSpends(const uint256& nullifier, const uint256& wtxid);