Hold cs_main while calling UpdatedBlockTip() and ui.NotifyBlockTip

Ensures that callbacks are invoked in the order in which the chain is updated
Resolves bitcoin#12978

Github-Pull: #2290
Rebased-From: ef24337

src/validation.cpp

@@ -2264,16 +2264,17 @@ bool ActivateBestChain(CValidationState& state, std::shared_ptr<const CBlock> pb
                 assert(trace.pblock && trace.pindex);
                 GetMainSignals().BlockConnected(trace.pblock, trace.pindex, trace.conflictedTxs);
             }
-        }
 
-        // Notify external listeners about the new tip.
-        GetMainSignals().UpdatedBlockTip(pindexNewTip, pindexFork, fInitialDownload);
+            // Notify external listeners about the new tip.
+            // Enqueue while holding cs_main to ensure that UpdatedBlockTip is called in the order in which blocks are connected
+            GetMainSignals().UpdatedBlockTip(pindexNewTip, pindexFork, fInitialDownload);
 
-        // Always notify the UI if a new block tip was connected
-        if (pindexFork != pindexNewTip) {
-            // Notify the UI
-            uiInterface.NotifyBlockTip(fInitialDownload, pindexNewTip);
+            // Always notify the UI if a new block tip was connected
+            if (pindexFork != pindexNewTip) {
+                uiInterface.NotifyBlockTip(fInitialDownload, pindexNewTip);
+            }
         }
+        // When we reach this point, we switched to a new tip (stored in pindexNewTip).
 
     } while (pindexMostWork != chainActive.Tip());
 

src/validationinterface.cpp

@@ -146,6 +146,10 @@ void CMainSignals::MempoolEntryRemoved(CTransactionRef ptx, MemPoolRemovalReason
 }
 
 void CMainSignals::UpdatedBlockTip(const CBlockIndex* pindexNew, const CBlockIndex* pindexFork, bool fInitialDownload) {
+    // Dependencies exist that require UpdatedBlockTip events to be delivered in the order in which
+    // the chain actually updates. One way to ensure this is for the caller to invoke this signal
+    // in the same critical section where the chain is updated
+
     m_internals->m_schedulerClient.AddToProcessQueue([pindexNew, pindexFork, fInitialDownload, this] {
         m_internals->UpdatedBlockTip(pindexNew, pindexFork, fInitialDownload);
     });