Merge bitcoin#29867: index: race fix, lock cs_main while 'm_synced' is subject to change

65951e0 index: race fix, lock cs_main while 'm_synced' is subject to change (Ryan Ofsky)

Pull request description:

  Fixes bitcoin#29831 and bitcoin#29863. Thanks to Marko for the detailed description of the issue.

  The race occurs because a block could be connected and its event signaled in-between reading the 'next block' and setting `m_synced` during the index initial synchronization. This is because `cs_main` is not locked through the process of determining the final index sync state.
  To address the issue, the `m_synced` flag set has been moved under `cs_main` guard.

ACKs for top commit:
  fjahr:
    Code review ACK 65951e0
  achow101:
    ACK 65951e0
  ryanofsky:
    Code review ACK 65951e0

Tree-SHA512: 77286e22de164a27939d2681b7baa6552eb75e99c541d3b9631f4340d7dd01742667c86899b6987fd2d97799d959e0a913a7749b2b69d9e50505128cd3ae0e69

Author: achow101

src/index/base.cpp

@@ -160,12 +160,24 @@ void BaseIndex::Sync()
             }
 
             const CBlockIndex* pindex_next = WITH_LOCK(cs_main, return NextSyncBlock(pindex, m_chainstate->m_chain));
+            // If pindex_next is null, it means pindex is the chain tip, so
+            // commit data indexed so far.
             if (!pindex_next) {
                 SetBestBlockIndex(pindex);
                 // No need to handle errors in Commit. See rationale above.
                 Commit();
-                m_synced = true;
-                break;
+
+                // If pindex is still the chain tip after committing, exit the
+                // sync loop. It is important for cs_main to be locked while
+                // setting m_synced = true, otherwise a new block could be
+                // attached while m_synced is still false, and it would not be
+                // indexed.
+                LOCK(::cs_main);
+                pindex_next = NextSyncBlock(pindex, m_chainstate->m_chain);
+                if (!pindex_next) {
+                    m_synced = true;
+                    break;
+                }
             }
             if (pindex_next->pprev != pindex && !Rewind(pindex, pindex_next->pprev)) {
                 FatalErrorf("%s: Failed to rewind index %s to a previous chain tip", __func__, GetName());