test: add test for index commits ahead of the last flushed block

diff --git a/src/test/CMakeLists.txt b/src/test/CMakeLists.txt
index e5619064..97a034de 100644
--- a/src/test/CMakeLists.txt
+++ b/src/test/CMakeLists.txt
@@ -15,6 +15,7 @@ add_executable(test_bitcoin
   base32_tests.cpp
   base58_tests.cpp
   base64_tests.cpp
+  baseindex_tests.cpp
   bech32_tests.cpp
   bip32_tests.cpp
   bip324_tests.cpp
diff --git a/src/test/baseindex_tests.cpp b/src/test/baseindex_tests.cpp
new file mode 100644
index 00000000..525309bf
--- /dev/null
+++ b/src/test/baseindex_tests.cpp
@@ -0,0 +1,59 @@
+// Copyright (c) 2020-present The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <index/coinstatsindex.h>
+#include <interfaces/chain.h>
+#include <script/script.h>
+#include <test/util/setup_common.h>
+#include <util/byte_units.h>
+#include <util/check.h>
+#include <validation.h>
+
+#include <boost/test/unit_test.hpp>
+
+// Tests of generic BaseIndex functionality that is independent of which
+// concrete index is being used. CoinStatsIndex is used here merely as a
+// convenient instantiation of BaseIndex.
+BOOST_AUTO_TEST_SUITE(baseindex_tests)
+
+// Test that the index does not commit ahead of the chainstate's last
+// flushed block. If it did, a subsequent unclean shutdown would corrupt
+// the index, because during reverting it would require blocks that were
+// never flushed to disk.
+BOOST_FIXTURE_TEST_CASE(baseindex_no_commit_ahead_of_flush, TestChain100Setup)
+{
+    Chainstate& chainstate = Assert(m_node.chainman)->ActiveChainstate();
+    auto sync_index = [&](bool do_flush, int expected_sync_height, int expected_commit_height) {
+        CoinStatsIndex index{interfaces::MakeChain(m_node), /*n_cache_size=*/1_MiB};
+        BOOST_REQUIRE(index.Init());
+        index.Sync();
+        if (do_flush) {
+            chainstate.ForceFlushStateToDisk();
+            m_node.chain->context()->validation_signals->SyncWithValidationInterfaceQueue();
+        }
+        BOOST_CHECK_EQUAL(index.GetSummary().best_block_height, expected_sync_height);
+        index.Stop();
+        // Reload index to see which block data was actually committed.
+        BOOST_REQUIRE(index.Init());
+        BOOST_CHECK_EQUAL(index.GetSummary().best_block_height, expected_commit_height);
+        index.Stop();
+    };
+
+    // Part 1: Sync, then "crash" (stop without flushing). Models a node that
+    // started up, had its index catch up, but never flushed before going down.
+    sync_index(false, 100, 100);
+
+    // Part 2: Restart cleanly. Sync, force a chainstate flush, and drain the
+    // validation queue so the index's ChainStateFlushed callback runs.
+    sync_index(true, 100, 100);
+
+    // Part 3: Connect a new block on the chain without flushing
+    // (m_last_flushed_block stays at 100). For a real node this would happen
+    // in parallel with Sync(). Here we do it before Sync() to make the race
+    // state deterministic.
+    CreateAndProcessBlock({}, CScript() << OP_TRUE);
+    sync_index(false, 101, 101);
+}
+
+BOOST_AUTO_TEST_SUITE_END()