index: Use first block from locator instead of looking for fork point

The index sync code has logic to go back the chain to the forking point, while
also updating index-specific state, which is necessary to prevent
possible corruption of the coinstatsindex.

Also add a test for this (a reorg happens while the index is deactivated)
that would not pass before this change.

src/index/base.cpp

@@ -92,7 +92,13 @@ bool BaseIndex::Init()
     if (locator.IsNull()) {
         SetBestBlockIndex(nullptr);
     } else {
-        SetBestBlockIndex(m_chainstate->FindForkInGlobalIndex(locator));
+        // Setting the best block to the locator's top block. If it is not part of the
+        // best chain, we will rewind to the fork point during index sync
+        const CBlockIndex* locator_index{m_chainstate->m_blockman.LookupBlockIndex(locator.vHave.at(0))};
+        if (!locator_index) {
+            return InitError(strprintf(Untranslated("%s: best block of the index not found. Please rebuild the index."), GetName()));
+        }
+        SetBestBlockIndex(locator_index);
     }
 
     // Note: this will latch to true immediately if the user starts up with an empty

test/functional/feature_coinstatsindex.py

@@ -52,6 +52,7 @@ class CoinStatsIndexTest(BitcoinTestFramework):
         self._test_use_index_option()
         self._test_reorg_index()
         self._test_index_rejects_hash_serialized()
+        self._test_init_index_after_reorg()
 
     def block_sanity_check(self, block_info):
         block_subsidy = 50
@@ -60,6 +61,9 @@ class CoinStatsIndexTest(BitcoinTestFramework):
             block_info['new_outputs_ex_coinbase'] + block_info['coinbase'] + block_info['unspendable']
         )
 
+    def sync_index_node(self):
+        self.wait_until(lambda: self.nodes[1].getindexinfo()['coinstatsindex']['synced'] is True)
+
     def _test_coin_stats_index(self):
         node = self.nodes[0]
         index_node = self.nodes[1]
@@ -296,6 +300,26 @@ class CoinStatsIndexTest(BitcoinTestFramework):
         for use_index in {True, False, None}:
             assert_raises_rpc_error(-8, msg, self.nodes[1].gettxoutsetinfo, hash_type='hash_serialized_2', hash_or_height=111, use_index=use_index)
 
+    def _test_init_index_after_reorg(self):
+        self.log.info("Test a reorg while the index is deactivated")
+        index_node = self.nodes[1]
+        block = self.nodes[0].getbestblockhash()
+        self.generate(index_node, 2, sync_fun=self.no_op)
+        self.sync_index_node()
+
+        # Restart without index
+        self.restart_node(1, extra_args=[])
+        self.connect_nodes(0, 1)
+        index_node.invalidateblock(block)
+        self.generatetoaddress(index_node, 5, getnewdestination()[2])
+        res = index_node.gettxoutsetinfo(hash_type='muhash', hash_or_height=None, use_index=False)
+
+        # Restart with index that still has its best block on the old chain
+        self.restart_node(1, extra_args=self.extra_args[1])
+        self.sync_index_node()
+        res1 = index_node.gettxoutsetinfo(hash_type='muhash', hash_or_height=None, use_index=True)
+        assert_equal(res["muhash"], res1["muhash"])
+
 
 if __name__ == '__main__':
     CoinStatsIndexTest().main()