Fix an issue where qa test wallet_shieldcoinbase could hang.

The mempool was not synced so a block could be generated and not
clear out the mempool.  This would then cause subsequent code
which expected the mempool of all nodes to be empty to hang.

qa/rpc-tests/wallet_shieldcoinbase.py

@@ -6,7 +6,7 @@
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.authproxy import JSONRPCException
 from test_framework.util import assert_equal, initialize_chain_clean, \
-    start_node, connect_nodes_bi, sync_blocks
+    start_node, connect_nodes_bi, sync_blocks, sync_mempools
 
 import time
 from decimal import Decimal
@@ -185,8 +185,10 @@ class WalletShieldCoinbaseTest (BitcoinTestFramework):
         self.wait_and_assert_operationid_status(0, opid2)
 
         # sync_all() invokes sync_mempool() but node 2's mempool limit will cause tx1 and tx2 to be rejected.
-        # So instead, we sync on blocks, and after a new block is generated, all nodes will have an empty mempool.
-        sync_blocks(self.nodes)
+        # So instead, we sync on blocks and mempool for node 0 and node 1, and after a new block is generated
+        # which mines tx1 and tx2, all nodes will have an empty mempool which can then be synced.
+        sync_blocks(self.nodes[:2])
+        sync_mempools(self.nodes[:2])
         self.nodes[1].generate(1)
         self.sync_all()
 
@@ -215,7 +217,9 @@ class WalletShieldCoinbaseTest (BitcoinTestFramework):
         assert_equal(result["shieldingUTXOs"], Decimal('33'))
         assert_equal(result["remainingUTXOs"], Decimal('17'))
         self.wait_and_assert_operationid_status(0, result['opid'])
-        sync_blocks(self.nodes)  # don't sync on mempool as node 2 will reject thix tx due to its mempooltxinputlimit
+        # Don't sync node 2 which rejects the tx due to its mempooltxinputlimit
+        sync_blocks(self.nodes[:2])
+        sync_mempools(self.nodes[:2])
         self.nodes[1].generate(1)
         self.sync_all()