test: add forward-compat functional test for txindex

test/functional/feature_txindex_compatibility.py

@@ -0,0 +1,72 @@
+#!/usr/bin/env python3
+# Copyright (c) 2026 The Bitcoin Core developers
+# Distributed under the MIT software license, see the accompanying
+# file COPYING or http://www.opensource.org/licenses/mit-license.php.
+"""Test txindex forward compatibility.
+
+"""
+
+import shutil
+
+from test_framework.test_framework import BitcoinTestFramework
+from test_framework.util import assert_equal
+from test_framework.wallet import MiniWallet
+
+class TxIndexTest(BitcoinTestFramework):
+    def set_test_params(self):
+        self.num_nodes = 2
+        self.extra_args = [["-txindex"],["-txindex"]]
+
+    def skip_test_if_missing_module(self):
+        self.skip_if_no_previous_releases()
+
+    def setup_nodes(self):
+        self.add_nodes(
+            self.num_nodes,
+            extra_args=self.extra_args,
+            versions=[
+                None,
+                280200,
+            ],
+        )
+        self.start_nodes()
+
+    def run_test(self):
+        self._test_txindex_compatibility()
+
+    def _test_txindex_compatibility(self):
+        node = self.nodes[0]
+        legacy_node = self.nodes[1]
+        self.wallet = MiniWallet(self.nodes[0])
+        tx1 = self.wallet.send_self_transfer(from_node=self.nodes[0])
+        self.generate(self.nodes[0], 1)
+        txId1 = tx1['txid']
+
+        for n in self.nodes:
+            self.wait_until(lambda: n.getindexinfo()['txindex']['synced'])
+
+        self.log.info("Ensure that queries to the txindex are consistent between the different index versions")
+        assert_equal(node.getrawtransaction(txId1), tx1['hex'])
+        assert_equal(legacy_node.getrawtransaction(txId1), tx1['hex'])
+
+        self.log.info("Exercise the new index running on a datadir with the old version")
+        self.stop_nodes()
+        self.cleanup_folder(node.chain_path)
+        shutil.copytree(legacy_node.chain_path, node.chain_path)
+        msg = "txindex contains entries in the legacy format"
+        with node.assert_debug_log(expected_msgs=[msg]):
+            self.start_node(0)
+        self.wait_until(lambda: node.getindexinfo()['txindex']['synced'])
+        assert_equal(node.getrawtransaction(txId1), tx1['hex'])
+
+        self.log.info("Test that looking up a newly added transaction in a mixed-format db is possible")
+        tx2 = self.wallet.send_self_transfer(from_node=self.nodes[0])
+        self.generate(self.nodes[0], 1, sync_fun=self.no_op)
+        txId2 = tx2['txid']
+        self.wait_until(lambda: node.getindexinfo()['txindex']['synced'])
+        assert_equal(node.getrawtransaction(txId1), tx1['hex'])
+        assert_equal(node.getrawtransaction(txId2), tx2['hex'])
+
+
+if __name__ == '__main__':
+    TxIndexTest(__file__).main()

test/functional/test_runner.py

@@ -125,6 +125,7 @@ BASE_SCRIPTS = [
     'feature_segwit.py --v2transport',
     'feature_segwit.py --v1transport',
     'p2p_tx_download.py',
+    'feature_txindex_compatibility.py',
     'wallet_avoidreuse.py',
     'feature_abortnode.py',
     'wallet_address_types.py',