`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
```
ShowProgress(strprintf("%s " + _("Rescanning..."), GetDisplayName()), std::max(1, std::min(99, (int)(m_scanning_progress * 100))));
        }
        if (GetTime() >= nNow + 60) {
            nNow = GetTime();
            WalletLogPrintf("Still rescanning. At block %d. Progress=%f\n", *block_height, progress_current);
        }
        CBlock block;
        if (chain().findBlock(block_hash, &block) && !block.IsNull()) {
            auto locked_chain = chain().lock();
            LOCK(cs_wallet);
            if (!locked_chain->getBlockHeight(block_hash)) {
                
                
                
                
                result.last_failed_block = block_hash;
                result.status = ScanResult::FAILURE;
                break;
            }
            for (size_t posInBlock = 0; posInBlock < block.vtx.size(); ++posInBlock) {
                SyncTransaction(block.vtx[posInBlock], block_hash, posInBlock, fUpdate);
            }
            
            result.last_scanned_block = block_hash;
            result.last_scanned_height = *block_height;
        } else {
            
            result.last_failed_block = block_hash;
            result.status = ScanResult::FAILURE;
        }