`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
```
if (nFeePerK > HIGH_TX_FEE_PER_KB) {
            chain.initWarning(AmountHighWarn("-discardfee") + " " +
                              _("This is the transaction fee you may discard if change is smaller than dust at this level"));
        }
        walletInstance->m_discard_rate = CFeeRate(nFeePerK);
    }
    if (gArgs.IsArgSet("-paytxfee")) {
        CAmount nFeePerK = 0;
        if (!ParseMoney(gArgs.GetArg("-paytxfee", ""), nFeePerK)) {
            chain.initError(AmountErrMsg("paytxfee", gArgs.GetArg("-paytxfee", "")));
            return nullptr;
        }
        if (nFeePerK > HIGH_TX_FEE_PER_KB) {
            chain.initWarning(AmountHighWarn("-paytxfee") + " " +
                              _("This is the transaction fee you will pay if you send a transaction."));
        }
        walletInstance->m_pay_tx_fee = CFeeRate(nFeePerK, 1000);
        if (walletInstance->m_pay_tx_fee < chain.relayMinFee()) {
            chain.initError(strprintf(_("Invalid amount for -paytxfee=<amount>: '%s' (must be at least %s)"),
                gArgs.GetArg("-paytxfee", ""), chain.relayMinFee().ToString()));
            return nullptr;
        }
    }
    if (gArgs.IsArgSet("-maxtxfee"))
    {
        CAmount nMaxFee = 0;
        if (!ParseMoney(gArgs.GetArg("-maxtxfee", ""), nMaxFee)) {
            chain.initError(AmountErrMsg("maxtxfee", gArgs.GetArg("-maxtxfee", "")));
            return nullptr;
        }