Merge bitcoin#12367: Fix two fast-shutdown bugs

dd2de47 Fix fast-shutdown crash if genesis block was not loaded (Matt Corallo)
1c9394a Fix fast-shutdown hang on ThreadImport+GenesisWait (Matt Corallo)

Pull request description:

  The second commit is a much simpler alternative fix for the issue fixed in bitcoin#12349. To test I made ShutdownRequested() always StartShutdown() after a certain number of calls, which turned up one other hang, fixed in the first commit.

Tree-SHA512: 86bde6ac4b8b4e2cb99fff87dafeed02c0d9514acee6d94455637fb2da9ffc274b5ad31b0a6b9f5bd7b700ae35395f28ddb14ffc65ddda3619aa28df28a5607d

src/init.cpp

@@ -2191,16 +2191,18 @@ bool AppInitMain(boost::thread_group& threadGroup, CScheduler& scheduler)
     // Wait for genesis block to be processed
     {
         WaitableLock lock(cs_GenesisWait);
-        while (!fHaveGenesis) {
-            condvar_GenesisWait.wait(lock);
+        // We previously could hang here if StartShutdown() is called prior to
+        // ThreadImport getting started, so instead we just wait on a timer to
+        // check ShutdownRequested() regularly.
+        while (!fHaveGenesis && !ShutdownRequested()) {
+            condvar_GenesisWait.wait_for(lock, std::chrono::milliseconds(500));
         }
         uiInterface.NotifyBlockTip.disconnect(BlockNotifyGenesisWait);
     }
 
     // As importing blocks can take several minutes, it's possible the user
     // requested to kill the GUI during one of the last operations. If so, exit.
-    if (fRequestShutdown)
-    {
+    if (ShutdownRequested()) {
         LogPrintf("Shutdown requested. Exiting.\n");
         return false;
     }

src/validation.cpp

@@ -2850,8 +2850,6 @@ bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams,
     int nStopAtHeight = gArgs.GetArg("-stopatheight", DEFAULT_STOPATHEIGHT);
     do {
         boost::this_thread::interruption_point();
-        if (ShutdownRequested())
-            break;
 
         const CBlockIndex *pindexFork;
         bool fInitialDownload;
@@ -2899,6 +2897,13 @@ bool ActivateBestChain(CValidationState &state, const CChainParams& chainparams,
         }
 
         if (nStopAtHeight && pindexNewTip && pindexNewTip->nHeight >= nStopAtHeight) StartShutdown();
+
+        // We check shutdown only after giving ActivateBestChainStep a chance to run once so that we
+        // never shutdown before connecting the genesis block during LoadChainTip(). Previously this
+        // caused an assert() failure during shutdown in such cases as the UTXO DB flushing checks
+        // that the best block hash is non-null.
+        if (ShutdownRequested())
+            break;
     } while (pindexNewTip != pindexMostWork);
     CheckBlockIndex(chainparams.GetConsensus());