Fix destroyThreadPool() not waiting for jobs to finish

src/ammonite/core/threadManager.cpp

@@ -35,6 +35,9 @@ namespace ammonite {
         //If transitioning between the two, it'll remain at its old value until complete
         std::atomic_flag threadsUnblockedFlag;
         std::atomic<unsigned int> blockedThreadCount;
+        //0 when an unblock starts, 1 when a block starts
+        //Any other value means the system is broken
+        std::atomic<int> blockBalance;
 
         std::queue<WorkItem> workQueue;
         std::mutex workQueueMutex;
@@ -181,13 +184,20 @@ namespace ammonite {
         unblockThreadsTrigger.test_and_set();
         threadsUnblockedFlag.test_and_set();
         blockedThreadCount = 0;
+        blockBalance = 0;
         extraThreadCount = extraThreads;
         return 0;
       }
 
       //Jobs submitted at the same time may execute, but the threads will block after
       //Guarantees work submitted after won't begin yet
       void blockThreads(bool sync) {
+        //Skip blocking if it's already blocked / going to block
+        if (blockBalance > 0) {
+          return;
+        }
+        blockBalance++;
+
         //Submit a job for each thread that waits for the trigger
         unblockThreadsTrigger.clear();
         workQueueMutex.lock();
@@ -206,6 +216,12 @@ namespace ammonite {
       }
 
       void unblockThreads(bool sync) {
+        //Only unblock if it's already blocked / blocking
+        if (blockBalance == 0) {
+          return;
+        }
+        blockBalance--;
+
         //Unblock threads and wake them up
         unblockThreadsTrigger.test_and_set();
         unblockThreadsTrigger.notify_all();
@@ -243,12 +259,13 @@ namespace ammonite {
       //Finish work already in the queue and kill the threads
       void destroyThreadPool() {
         //Finish existing work and block new work from starting
+        unblockThreads(true);
         blockThreads(true);
 
-        //Kill all threads when they wake up
+        //Kill all threads when the're unblocked wake up
         stayAlive = false;
 
-        //Unlock threads and wake them up
+        //Unblock threads and wake them up
         unblockThreads(true);
 
         //Wait until all threads are done