`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
```
{
        LockAnnotation lock(::cs_main);
        CBlockIndex* block = ::ChainActive()[height];
        return block && ((block->nStatus & BLOCK_HAVE_DATA) != 0) && block->nTx > 0;
    }
    Optional<int> findFirstBlockWithTimeAndHeight(int64_t time, int height, uint256* hash) override
    {
        LockAnnotation lock(::cs_main);
        CBlockIndex* block = ::ChainActive().FindEarliestAtLeast(time, height);
        if (block) {
            if (hash) *hash = block->GetBlockHash();
            return block->nHeight;
        }
        return nullopt;
    }
    Optional<int> findPruned(int start_height, Optional<int> stop_height) override
    {
        LockAnnotation lock(::cs_main);
        if (::fPruneMode) {
            CBlockIndex* block = stop_height ? ::ChainActive()[*stop_height] : ::ChainActive().Tip();
            while (block && block->nHeight >= start_height) {
                if ((block->nStatus & BLOCK_HAVE_DATA) == 0) {
                    return block->nHeight;
                }
                block = block->pprev;
            }
        }
        return nullopt;
    }
    Optional<int> findFork(const uint256& hash, Optional<int>* height) override