index: race fix, lock cs_main while 'm_synced' is subject to change

This ensures that the index does not miss any 'new block' signals
occurring in-between reading the 'next block' and setting 'm_synced'.
Because, if this were to happen, the ignored blocks would never be
indexed, thus stalling the index forever.

src/index/base.cpp

@@ -160,13 +160,25 @@ 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();
+
+                // 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());
                 return;