Auto merge of #4390 - daira:fix-chaintip-race-condition, r=ebfull

Fix race conditions during init

Fix race conditions due to accessing `chainActive.Tip()` during init, and other minor cleanups.
Includes backport of https://github.com/bitcoin/bitcoin/pull/8063 .

src/init.cpp

@@ -869,6 +869,7 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
     }
 
     // Clean shutdown on SIGTERM
+    assert(fRequestShutdown.is_lock_free());
     struct sigaction sa;
     sa.sa_handler = HandleSIGTERM;
     sigemptyset(&sa.sa_mask);
@@ -877,6 +878,7 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
     sigaction(SIGINT, &sa, NULL);
 
     // Reopen debug.log on SIGHUP
+    assert(fReopenDebugLog.is_lock_free());
     struct sigaction sa_hup;
     sa_hup.sa_handler = HandleSIGHUP;
     sigemptyset(&sa_hup.sa_mask);
@@ -1570,10 +1572,17 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
     // original value of chainActive.Tip(), which corresponds with the wallet's
     // view of the chaintip, is passed to ThreadNotifyWallets before the chain
     // tip changes again.
-    boost::function<void()> threadnotifywallets = boost::bind(&ThreadNotifyWallets, chainActive.Tip());
-    threadGroup.create_thread(
-        boost::bind(&TraceThread<boost::function<void()>>, "txnotify", threadnotifywallets)
-    );
+    {
+        CBlockIndex *pindexLastTip;
+        {
+            LOCK(cs_main);
+            pindexLastTip = chainActive.Tip();
+        }
+        boost::function<void()> threadnotifywallets = boost::bind(&ThreadNotifyWallets, pindexLastTip);
+        threadGroup.create_thread(
+            boost::bind(&TraceThread<boost::function<void()>>, "txnotify", threadnotifywallets)
+        );
+    }
 
     // ********************************************************* Step 9: data directory maintenance
 
@@ -1609,10 +1618,21 @@ bool AppInit2(boost::thread_group& threadGroup, CScheduler& scheduler)
             vImportFiles.push_back(strFile);
     }
     threadGroup.create_thread(boost::bind(&ThreadImport, vImportFiles));
-    if (chainActive.Tip() == NULL) {
-        LogPrintf("Waiting for genesis block to be imported...\n");
-        while (!fRequestShutdown && chainActive.Tip() == NULL)
+
+    // Wait for genesis block to be processed
+    bool fHaveGenesis = false;
+    while (!fHaveGenesis && !fRequestShutdown) {
+        {
+            LOCK(cs_main);
+            fHaveGenesis = (chainActive.Tip() != NULL);
+        }
+
+        if (!fHaveGenesis) {
             MilliSleep(10);
+        }
+    }
+    if (!fHaveGenesis) {
+        return false;
     }
 
     // ********************************************************* Step 11: start node

src/utiltime.cpp

@@ -43,6 +43,8 @@ int64_t GetTimeMicros()
 
 void MilliSleep(int64_t n)
 {
+    // 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::milliseconds(n));
 }