Worker thread per plugin instance.

src/Worker.cpp

@@ -15,7 +15,7 @@
 */
 // (Borrows heavily from worker.c by David Robillard.)
 
-// Copyright (c) 2022 Robin Davies
+// Copyright (c) 2024 Robin Davies
 //
 // Permission is hereby granted, free of charge, to any person obtaining a copy of
 // this software and associated documentation files (the "Software"), to deal in
@@ -242,8 +242,19 @@ void HostWorkerThread::ThreadProc() noexcept
 HostWorkerThread::HostWorkerThread()
 {
     this->dataBuffer.resize(16*1024);
-    pThread = std::make_unique<std::thread>([this]()
-                                            { this->ThreadProc(); });
+}
+
+bool HostWorkerThread::StartThread()
+{
+    if (!pThread)
+    {
+        if (closed) {
+            return false;
+        }
+        pThread = std::make_unique<std::thread>([this]()
+                                                { this->ThreadProc(); });
+    }    
+    return true;
 }
 
 void HostWorkerThread::Close()
@@ -252,10 +263,12 @@ void HostWorkerThread::Close()
     bool sendClose = false;
     {
         std::lock_guard lock{submitMutex};
-        if (!closed)
-        {
-            closed = true;
-            sendClose = true;
+        if (pThread) {
+            if (!closed)
+            {
+                closed = true;
+                sendClose = true;
+            }
         }
     }
     if (sendClose)
@@ -278,7 +291,10 @@ LV2_Worker_Status HostWorkerThread::ScheduleWork(Worker *worker, size_t size, co
 {
     std::lock_guard lock(submitMutex);
 
-    if (exiting)
+    if (!StartThread()) { // ensures thread is running, when plugins haven't declared they want a worker thread.
+        return LV2_Worker_Status::LV2_WORKER_ERR_NO_SPACE;
+    }
+    if (exiting )
     {
         return LV2_Worker_Status::LV2_WORKER_ERR_NO_SPACE;
     }