`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
```
{
        auto result = MakeUnique<LockingStateImpl>(::cs_main, "cs_main", __FILE__, __LINE__, try_lock);
        if (try_lock && result && !*result) return {};
        
        
        return std::move(result);
    }
    std::unique_ptr<Chain::Lock> assumeLocked() override { return MakeUnique<LockImpl>(); }
    bool findBlock(const uint256& hash, CBlock* block, int64_t* time, int64_t* time_max) override
    {
        CBlockIndex* index;
        {
            LOCK(cs_main);
            index = LookupBlockIndex(hash);
            if (!index) {
                return false;
            }
            if (time) {
                *time = index->GetBlockTime();
            }
            if (time_max) {
                *time_max = index->GetBlockTimeMax();
            }
        }
        if (block && !ReadBlockFromDisk(*block, index, Params().GetConsensus())) {
            block->SetNull();
        }
        return true;
    }
    void findCoins(std::map<COutPoint, Coin>& coins) override { return FindCoins(coins); }