Sleep for 200us before each ActivateBestChainStep call

This should lower the main thread's likelihood to immediately reacquire
cs_main after dropping it, which should help ThreadNotifyWallets and the
RPC methods to acquire cs_main more quickly.

Co-authored-by: Daira Hopwood <daira@jacaranda.org>
Co-authored-by: Kris Nuttycombe <kris@nutty.land>

src/main.cpp

@@ -27,6 +27,7 @@
 #include "policy/policy.h"
 #include "pow.h"
 #include "reverse_iterator.h"
+#include "time.h"
 #include "txmempool.h"
 #include "ui_interface.h"
 #include "undo.h"
@@ -4386,7 +4387,15 @@ bool ActivateBestChain(CValidationState& state, const CChainParams& chainparams,
     CBlockIndex *pindexMostWork = NULL;
     CBlockIndex *pindexNewTip = NULL;
     do {
-        boost::this_thread::interruption_point();
+        // Sleep briefly to allow other threads a chance at grabbing cs_main if
+        // we are connecting a long chain of blocks and would otherwise hold the
+        // lock almost continuously. As of 2023-02-03 the Zcash mainnet chain is
+        // around height 1,972,000; this sleep adds ~6.6 minutes to the time
+        // required to activate the best chain from genesis.
+        //
+        // This is defined to be an interruption point.
+        // <https://www.boost.org/doc/libs/1_70_0/doc/html/thread/thread_management.html#interruption_points>
+        boost::this_thread::sleep_for(boost::chrono::microseconds(200));
 
         bool fInitialDownload;
         int nNewHeight;