`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 (nSubtractFeeFromAmount == 0)
                    nValueToSelect += nFeeRet;
                
                coin_selection_params.tx_noinputs_size = 11; 
                for (const auto& recipient : vecSend)
                {
                    CTxOut txout(recipient.nAmount, recipient.scriptPubKey);
                    if (recipient.fSubtractFeeFromAmount)
                    {
                        assert(nSubtractFeeFromAmount != 0);
                        txout.nValue -= nFeeRet / nSubtractFeeFromAmount; 
                        if (fFirst) 
                        {
                            fFirst = false;
                            txout.nValue -= nFeeRet % nSubtractFeeFromAmount;
                        }
                    }
                    
                    coin_selection_params.tx_noinputs_size += ::GetSerializeSize(txout, PROTOCOL_VERSION);
                    if (IsDust(txout, chain().relayDustFee()))
                    {
                        if (recipient.fSubtractFeeFromAmount && nFeeRet > 0)
                        {
                            if (txout.nValue < 0)
                                strFailReason = _("The transaction amount is too small to pay the fee");
                            else
                                strFailReason = _("The transaction amount is too small to send after the fee has been deducted");
                        }
                        else
                            strFailReason = _("Transaction amount too small");