taskschduler: fix a regression deadlock issue

This fix introduces a workaround to enforce synchronous tasking on worker threads.
Sometimes, out of threads get stuck in a deadlock condition.

@Issue: https://github.com/thorvg/thorvg/issues/1636

src/loaders/lottie/tvgLottieBuilder.cpp

@@ -25,6 +25,7 @@
 #include "tvgShape.h"
 #include "tvgLottieModel.h"
 #include "tvgLottieBuilder.h"
+#include "tvgTaskScheduler.h"
 
 
 /************************************************************************/
@@ -577,11 +578,17 @@ static void _updateImage(LottieGroup* parent, LottieImage* image, int32_t frameN
 
     if (!picture) {
         picture = Picture::gen().release();
+
+        //force to load a picture on the same thread
+        TaskScheduler::async(false);
+
         if (image->size > 0) {
             if (picture->load((const char*)image->b64Data, image->size, image->mimeType, false) != Result::Success) return;
         } else {
             if (picture->load(image->path) != Result::Success) return;
         }
+
+        TaskScheduler::async(true);
     }
 
     if (ctx.propagator) {

src/loaders/svg/tvgSvgSceneBuilder.cpp

@@ -550,12 +550,15 @@ static bool _isValidImageMimeTypeAndEncoding(const char** href, const char** mim
     return false;
 }
 
+#include "tvgTaskScheduler.h"
 
 static unique_ptr<Picture> _imageBuildHelper(SvgLoaderData& loaderData, SvgNode* node, const Box& vBox, const string& svgPath)
 {
     if (!node->node.image.href) return nullptr;
     auto picture = Picture::gen();
 
+    TaskScheduler::async(false);    //force to load a picture on the same thread
+
     const char* href = node->node.image.href;
     if (!strncmp(href, "data:", sizeof("data:") - 1)) {
         href += sizeof("data:") - 1;
@@ -567,12 +570,14 @@ static unique_ptr<Picture> _imageBuildHelper(SvgLoaderData& loaderData, SvgNode*
             auto size = b64Decode(href, strlen(href), &decoded);
             if (picture->load(decoded, size, mimetype, false) != Result::Success) {
                 free(decoded);
+                TaskScheduler::async(true);
                 return nullptr;
             }
         } else {
             auto size = svgUtilURLDecode(href, &decoded);
             if (picture->load(decoded, size, mimetype, false) != Result::Success) {
                 free(decoded);
+                TaskScheduler::async(true);
                 return nullptr;
             }
         }
@@ -584,6 +589,7 @@ static unique_ptr<Picture> _imageBuildHelper(SvgLoaderData& loaderData, SvgNode*
         const char *dot = strrchr(href, '.');
         if (dot && !strcmp(dot, ".svg")) {
             TVGLOG("SVG", "Embedded svg file is disabled.");
+            TaskScheduler::async(true);
             return nullptr;
         }
         string imagePath = href;
@@ -591,8 +597,13 @@ static unique_ptr<Picture> _imageBuildHelper(SvgLoaderData& loaderData, SvgNode*
             auto last = svgPath.find_last_of("/");
             imagePath = svgPath.substr(0, (last == string::npos ? 0 : last + 1)) + imagePath;
         }
-        if (picture->load(imagePath) != Result::Success) return nullptr;
+        if (picture->load(imagePath) != Result::Success) {
+            TaskScheduler::async(true);
+            return nullptr;
         }
+    }
+
+    TaskScheduler::async(true);
 
     float w, h;
     Matrix m = {1, 0, 0, 0, 1, 0, 0, 0, 1};
@@ -605,6 +616,7 @@ static unique_ptr<Picture> _imageBuildHelper(SvgLoaderData& loaderData, SvgNode*
     picture->transform(m);
 
     _applyComposition(loaderData, picture.get(), node, vBox, svgPath);
+
     return picture;
 }
 

src/renderer/tvgTaskScheduler.cpp

@@ -100,17 +100,18 @@ struct TaskQueue {
 };
 
 
+static thread_local bool _async = true;  //toggle async tasking for each thread on/off
+
+
 struct TaskSchedulerImpl
 {
     uint32_t                       threadCnt;
     vector<thread>                 threads;
     vector<TaskQueue>              taskQueues;
     atomic<uint32_t>               idx{0};
-    thread::id                     tid;
 
     TaskSchedulerImpl(unsigned threadCnt) : threadCnt(threadCnt), taskQueues(threadCnt)
     {
-        tid = this_thread::get_id();
         threads.reserve(threadCnt);
 
         for (unsigned i = 0; i < threadCnt; ++i) {
@@ -146,24 +147,13 @@ struct TaskSchedulerImpl
     void request(Task* task)
     {
         //Async
-        if (threadCnt > 0) {
+        if (threadCnt > 0 && _async) {
-            auto tid = this_thread::get_id();
-            if (tid == this->tid) {
             task->prepare();
             auto i = idx++;
             for (unsigned n = 0; n < threadCnt; ++n) {
                 if (taskQueues[(i + n) % threadCnt].tryPush(task)) return;
             }
             taskQueues[i % threadCnt].push(task);
-            //Not thread-safety now, it's requested from a worker-thread
-            } else {
-                for (unsigned i = 0; i < threadCnt; ++i) {
-                    if (tid == threads[i].get_id()) {
-                        task->prepare();
-                        (*task)(i + 1);
-                    }
-                }
-            }
         //Sync
         } else {
             task->run(0);
@@ -205,3 +195,9 @@ unsigned TaskScheduler::threads()
     if (inst) return inst->threadCnt;
     return 0;
 }
+
+
+void TaskScheduler::async(bool on)
+{
+    _async = on;
+}

src/renderer/tvgTaskScheduler.h

@@ -38,6 +38,7 @@ struct TaskScheduler
     static void init(unsigned threads);
     static void term();
     static void request(Task* task);
+    static void async(bool on);
 };
 
 struct Task