`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
```
{
    
    hashBlock = block_hash;
    
    nIndex = posInBlock;
}
int CMerkleTx::GetDepthInMainChain(interfaces::Chain::Lock& locked_chain) const
{
    if (hashUnset())
        return 0;
    return locked_chain.getBlockDepth(hashBlock) * (nIndex == -1 ? -1 : 1);
}
int CMerkleTx::GetBlocksToMaturity(interfaces::Chain::Lock& locked_chain) const
{
    if (!IsCoinBase())
        return 0;
    int chain_depth = GetDepthInMainChain(locked_chain);
    assert(chain_depth >= 0); 
    return std::max(0, (COINBASE_MATURITY+1) - chain_depth);
}
bool CMerkleTx::IsImmatureCoinBase(interfaces::Chain::Lock& locked_chain) const
{
    
    return GetBlocksToMaturity(locked_chain) > 0;
}
bool CWalletTx::AcceptToMemoryPool(interfaces::Chain::Lock& locked_chain, CValidationState& state)