Merge #19132: qt: lock cs_main, m_cached_tip_mutex in that order

f46b678 qt: lock cs_main, m_cached_tip_mutex in that order (Vasil Dimov)

Pull request description:

  Always lock the mutexes `cs_main` and `m_cached_tip_mutex` in
  the same order: `cs_main`, `m_cached_tip_mutex`. Otherwise we may end up
  in a deadlock.

  `ClientModel::m_cached_tip_blocks` is protected by
  `ClientModel::m_cached_tip_mutex`. There are two access paths that
  lock the two mutexes in opposite order:

  ```
  validation.cpp:2868 CChainState::ActivateBestChain(): lock cs_main
  validation.cpp:2916 CChainState::ActivateBestChain(): call uiInterface.NotifyBlockTip()
  ui_interface.cpp:52 CClientUIInterface::NotifyBlockTip(): go deep in boost
  ...
  qt/clientmodel.cpp:255 BlockTipChanged(): lock m_cached_tip_mutex
  ```

  and

  ```
  qt/clientmodel.cpp:119 ClientModel::getBestBlockHash(): lock m_cached_tip_mutex
  qt/clientmodel.cpp:121 ClientModel::getBestBlockHash(): call m_node.getBestBlockHash()
  interfaces/node.cpp:200 NodeImpl::getBestBlockHash(): lock cs_main
  ```

  From `debug.log`:

  ```
  POTENTIAL DEADLOCK DETECTED
  Previous lock order was:
   m_cs_chainstate validation.cpp:2851
   (1) cs_main validation.cpp:2868
   ::mempool.cs validation.cpp:2868
   (2) clientmodel->m_cached_tip_mutex qt/clientmodel.cpp:255
  Current lock order is:
   (2) m_cached_tip_mutex qt/clientmodel.cpp:119
   (1) ::cs_main interfaces/node.cpp:200
  ```

  The possible deadlock was introduced in #17993

ACKs for top commit:
  jonasschnelli:
    Tested ACK f46b678

Tree-SHA512: 904f24b39bdc97c4d0ecb897a6980d8d479814535eb167e23105238800ea2f1f85273e3370cf894db58bc597f94c4f2e81fb68d0ff3362d468c16af5ce8f5d78

Author: jonasschnelli

src/qt/clientmodel.cpp

@@ -116,9 +116,23 @@ int ClientModel::getNumBlocks() const
 
 uint256 ClientModel::getBestBlockHash()
 {
+    uint256 tip{WITH_LOCK(m_cached_tip_mutex, return m_cached_tip_blocks)};
+
+    if (!tip.IsNull()) {
+        return tip;
+    }
+
+    // Lock order must be: first `cs_main`, then `m_cached_tip_mutex`.
+    // The following will lock `cs_main` (and release it), so we must not
+    // own `m_cached_tip_mutex` here.
+    tip = m_node.getBestBlockHash();
+
     LOCK(m_cached_tip_mutex);
+    // We checked that `m_cached_tip_blocks` is not null above, but then we
+    // released the mutex `m_cached_tip_mutex`, so it could have changed in the
+    // meantime. Thus, check again.
     if (m_cached_tip_blocks.IsNull()) {
-        m_cached_tip_blocks = m_node.getBestBlockHash();
+        m_cached_tip_blocks = tip;
     }
     return m_cached_tip_blocks;
 }