Fix fast-shutdown hang on ThreadImport+GenesisWait

If the user somehow manages to get into ShutdownRequested before
ThreadImport gets to ActivateBestChain() we may hang waiting on
condvar_GenesisWait forever. A simple wait_for and
ShutdownRequested resolves this case.

Github-Pull: #12367
Rebased-From: 1c9394a
Tree-SHA512: fb0751ef32d2005520738bf3b0a0f41ae3f9314d700d2a85eb50f023e87e109ce806cdcdf4a08f49a4d9c1001e27df7f461d3fd52b1f5a57885260ce9375260f

Author: TheBlueMatt

src/init.cpp

@@ -1645,12 +1645,19 @@ bool AppInitMain()
     // 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);
     }
 
+    if (ShutdownRequested()) {
+        return false;
+    }
+
     // ********************************************************* Step 11: start node
 
     int chain_active_height;