`LockAnnotation lock(::cs_main)` is a guarantee to the compiler thread analysis that `::cs_main` is locked (when it couldn't be determined otherwise).

Despite being annotated with the locking guarantee ...

https://github.com/bitcoin/bitcoin/blob/65526fc8666fef35ef908dbc225f706bef642c7e/src/interfaces/chain.cpp#L134-L138

... `getTipLocator()` reads `chainActive` (via `::ChainActive()`) without holding `cs_main`.

This can be verified by adding the following `AssertLockHeld(cs_main)`:

```
$ git diff
diff --git a/src/interfaces/chain.cpp b/src/interfaces/chain.cpp
index 59623284d..9fc693a0f 100644
--- a/src/interfaces/chain.cpp
+++ b/src/interfaces/chain.cpp
@@ -134,6 +134,7 @@ class LockImpl : public Chain::Lock
     CBlockLocator getTipLocator() override
     {
         LockAnnotation lock(::cs_main);
+        AssertLockHeld(::cs_main);
         return ::ChainActive().GetLocator();
     }
     Optional<int> findLocatorFork(const CBlockLocator& locator) override
$ make check
../build-aux/test-driver: line 107: 12881 Aborted                 "$@" > $log_file 2>&1
FAIL: qt/test/test_bitcoin-qt
```
CWallet wallet(chain.get(), WalletLocation(), WalletDatabase::CreateDummy());
        AddKey(wallet, coinbaseKey);
        WalletRescanReserver reserver(&wallet);
        reserver.reserve();
        CWallet::ScanResult result = wallet.ScanForWalletTransactions({} , {} , reserver, false );
        BOOST_CHECK_EQUAL(result.status, CWallet::ScanResult::SUCCESS);
        BOOST_CHECK(result.last_failed_block.IsNull());
        BOOST_CHECK(result.last_scanned_block.IsNull());
        BOOST_CHECK(!result.last_scanned_height);
        BOOST_CHECK_EQUAL(wallet.GetBalance().m_mine_immature, 0);
    }
    
    
    {
        CWallet wallet(chain.get(), WalletLocation(), WalletDatabase::CreateDummy());
        AddKey(wallet, coinbaseKey);
        WalletRescanReserver reserver(&wallet);
        reserver.reserve();
        CWallet::ScanResult result = wallet.ScanForWalletTransactions(oldTip->GetBlockHash(), {} , reserver, false );
        BOOST_CHECK_EQUAL(result.status, CWallet::ScanResult::SUCCESS);
        BOOST_CHECK(result.last_failed_block.IsNull());
        BOOST_CHECK_EQUAL(result.last_scanned_block, newTip->GetBlockHash());
        BOOST_CHECK_EQUAL(*result.last_scanned_height, newTip->nHeight);
        BOOST_CHECK_EQUAL(wallet.GetBalance().m_mine_immature, 100 * COIN);
    }
    
    PruneOneBlockFile(oldTip->GetBlockPos().nFile);
    UnlinkPrunedFiles({oldTip->GetBlockPos().nFile});