`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
```
{
        CTransactionRef tx;
        CReserveKey reservekey(wallet.get());
        CAmount fee;
        int changePos = -1;
        std::string error;
        CCoinControl dummy;
        BOOST_CHECK(wallet->CreateTransaction(*m_locked_chain, {recipient}, tx, reservekey, fee, changePos, error, dummy));
        CValidationState state;
        BOOST_CHECK(wallet->CommitTransaction(tx, {}, {}, reservekey, state));
        CMutableTransaction blocktx;
        {
            LOCK(wallet->cs_wallet);
            blocktx = CMutableTransaction(*wallet->mapWallet.at(tx->GetHash()).tx);
        }
        CreateAndProcessBlock({CMutableTransaction(blocktx)}, GetScriptForRawPubKey(coinbaseKey.GetPubKey()));
        LOCK(cs_main);
        LOCK(wallet->cs_wallet);
        auto it = wallet->mapWallet.find(tx->GetHash());
        BOOST_CHECK(it != wallet->mapWallet.end());
        it->second.SetMerkleBranch(::ChainActive().Tip()->GetBlockHash(), 1);
        return it->second;
    }
    std::unique_ptr<interfaces::Chain> m_chain = interfaces::MakeChain();
    std::unique_ptr<interfaces::Chain::Lock> m_locked_chain = m_chain->assumeLocked();  
    std::unique_ptr<CWallet> wallet;
};
BOOST_FIXTURE_TEST_CASE(ListCoins, ListCoinsTestingSetup)