`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
```
{
    int64_t nNow = GetTime();
    int64_t start_time = GetTimeMillis();
    assert(reserver.isReserved());
    uint256 block_hash = start_block;
    ScanResult result;
    WalletLogPrintf("Rescan started from block %s...\n", start_block.ToString());
    fAbortRescan = false;
    ShowProgress(strprintf("%s " + _("Rescanning..."), GetDisplayName()), 0); 
    uint256 tip_hash;
    
    Optional<int> block_height = MakeOptional(false, int());
    double progress_begin;
    double progress_end;
    {
        auto locked_chain = chain().lock();
        if (Optional<int> tip_height = locked_chain->getHeight()) {
            tip_hash = locked_chain->getBlockHash(*tip_height);
        }
        block_height = locked_chain->getBlockHeight(block_hash);
        progress_begin = chain().guessVerificationProgress(block_hash);
        progress_end = chain().guessVerificationProgress(stop_block.IsNull() ? tip_hash : stop_block);
    }
    double progress_current = progress_begin;
    while (block_height && !fAbortRescan && !chain().shutdownRequested()) {
        m_scanning_progress = (progress_current - progress_begin) / (progress_end - progress_begin);
        if (*block_height % 100 == 0 && progress_end - progress_begin > 0.0) {