test: Add coverage for best block locator write in wallet_backup

test/functional/wallet_backup.py

@@ -140,6 +140,25 @@ class WalletBackupTest(BitcoinTestFramework):
         assert_raises_rpc_error(-36, error_message, node.restorewallet, wallet_name, backup_file)
         assert wallet_file.exists()
 
+    def test_pruned_wallet_backup(self):
+        self.log.info("Test loading backup on a pruned node when the backup was created close to the prune height of the restoring node")
+        node = self.nodes[3]
+        self.restart_node(3, ["-prune=1", "-fastprune=1"])
+        # Ensure the chain tip is at height 214, because this test assume it is.
+        assert_equal(node.getchaintips()[0]["height"], 214)
+        # We need a few more blocks so we can actually get above an realistic
+        # minimal prune height
+        self.generate(node, 50, sync_fun=self.no_op)
+        # Backup created at block height 264
+        node.backupwallet(node.datadir_path / 'wallet_pruned.bak')
+        # Generate more blocks so we can actually prune the older blocks
+        self.generate(node, 300, sync_fun=self.no_op)
+        # This gives us an actual prune height roughly in the range of 220 - 240
+        node.pruneblockchain(250)
+        # The backup should be updated with the latest height (locator) for
+        # the backup to load successfully this close to the prune height
+        node.restorewallet(f'pruned', node.datadir_path / 'wallet_pruned.bak')
+
     def run_test(self):
         self.log.info("Generating initial blockchain")
         self.generate(self.nodes[0], 1)
@@ -242,6 +261,8 @@ class WalletBackupTest(BitcoinTestFramework):
         for sourcePath in sourcePaths:
             assert_raises_rpc_error(-4, "backup failed", self.nodes[0].backupwallet, sourcePath)
 
+        self.test_pruned_wallet_backup()
+
 
 if __name__ == '__main__':
     WalletBackupTest(__file__).main()