Use a signal to continue init after genesis activation

Zcash: We set the Sprout tree root on the genesis block's index file in
InitBlockIndex because we were implicitly relying on this occurring via
ActivateBestChain previously.

(cherry picked from commit bitcoin/bitcoin@0fd2a33648)

src/init.cpp

@@ -554,6 +554,21 @@ static void TxExpiryNotifyCallback(const uint256& txid)
     boost::thread t(runCommand, strCmd); // thread runs free
 }
 
+static bool fHaveGenesis = false;
+static boost::mutex cs_GenesisWait;
+static CConditionVariable condvar_GenesisWait;
+
+static void BlockNotifyGenesisWait(bool, const CBlockIndex *pBlockIndex)
+{
+    if (pBlockIndex != NULL) {
+        {
+            boost::unique_lock<boost::mutex> lock_GenesisWait(cs_GenesisWait);
+            fHaveGenesis = true;
+        }
+        condvar_GenesisWait.notify_all();
+    }
+}
+
 struct CImportingNow
 {
     CImportingNow() {
@@ -1705,7 +1720,7 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
                     break;
                 }
 
-                if (!fReindex) {
+                if (!fReindex && chainActive.Tip() != NULL) {
                     uiInterface.InitMessage(_("Rewinding blocks if needed..."));
                     if (!RewindBlockIndex(chainparams, clearWitnessCaches)) {
                         strLoadError = _("Unable to rewind the database to a pre-upgrade state. You will need to redownload the blockchain");
@@ -1862,6 +1877,14 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
 
     // ********************************************************* Step 10: import blocks
 
+    // Either install a handler to notify us when genesis activates, or set fHaveGenesis directly.
+    // No locking, as this happens before any background thread is started.
+    if (chainActive.Tip() == NULL) {
+        uiInterface.NotifyBlockTip.connect(BlockNotifyGenesisWait);
+    } else {
+        fHaveGenesis = true;
+    }
+
     if (mapArgs.count("-blocknotify"))
         uiInterface.NotifyBlockTip.connect(BlockNotifyCallback);
 
@@ -1874,19 +1897,16 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
         for (const std::string& strFile : mapMultiArgs["-loadblock"])
             vImportFiles.push_back(strFile);
     }
+
     threadGroup.create_thread(boost::bind(&ThreadImport, vImportFiles, chainparams));
 
     // Wait for genesis block to be processed
-    bool fHaveGenesis = false;
-    while (!fHaveGenesis && !fRequestShutdown) {
-        {
-            LOCK(cs_main);
-            fHaveGenesis = (chainActive.Tip() != NULL);
-        }
-
-        if (!fHaveGenesis) {
-            MilliSleep(10);
+    {
+        boost::unique_lock<boost::mutex> lock(cs_GenesisWait);
+        while (!fHaveGenesis) {
+            condvar_GenesisWait.wait(lock);
         }
+        uiInterface.NotifyBlockTip.disconnect(BlockNotifyGenesisWait);
     }
     if (!fHaveGenesis) {
         return false;

src/main.cpp

@@ -5818,8 +5818,11 @@ bool InitBlockIndex(const CChainParams& chainparams)
             CBlockIndex *pindex = AddToBlockIndex(block, chainparams.GetConsensus());
             if (!ReceivedBlockTransactions(block, state, chainparams, pindex, blockPos))
                 return error("LoadBlockIndex(): genesis block not accepted");
-            if (!ActivateBestChain(state, chainparams, &block))
-                return error("LoadBlockIndex(): genesis block cannot be activated");
+            // Before the genesis block, there was an empty tree. We set its root here so
+            // that the block import thread doesn't race other methods that need to query
+            // the Sprout tree (namely CWallet::ScanForWalletTransactions).
+            SproutMerkleTree tree;
+            pindex->hashSproutAnchor = tree.root();
             // Force a chainstate write so that when we VerifyDB in a moment, it doesn't check stale data
             return FlushStateToDisk(chainparams, state, FLUSH_STATE_ALWAYS);
         } catch (const std::runtime_error& e) {

src/test/test_bitcoin.cpp

@@ -90,6 +90,11 @@ TestingSetup::TestingSetup(const std::string& chainName) : BasicTestingSetup(cha
         pcoinsdbview = new CCoinsViewDB(1 << 23, true);
         pcoinsTip = new CCoinsViewCache(pcoinsdbview);
         InitBlockIndex(chainparams);
+        {
+            CValidationState state;
+            bool ok = ActivateBestChain(state, chainparams);
+            BOOST_CHECK(ok);
+        }
         nScriptCheckThreads = 3;
         for (int i=0; i < nScriptCheckThreads-1; i++)
             threadGroup.create_thread(&ThreadScriptCheck);