TaskTree: Fix For loop with empty body

Ensure the For loop iterates even when the loop's body is empty.
Simplify internals a bit.

Change-Id: I4a269d61fa324a9c36109e95e74a992e915a72b0
Reviewed-by: hjk <hjk@qt.io>

src/libs/solutions/tasking/tasktree.cpp

@@ -2185,13 +2185,6 @@ SetupResult TaskTreePrivate::start(RuntimeContainer *container)
             container->m_successBit = startAction == SetupResult::StopWithSuccess;
         }
     }
-    if (startAction == SetupResult::Continue
-        && (containerNode.m_children.empty()
-            || (containerNode.m_loop && !invokeLoopHandler(container)))) {
-        if (isProgressive(container))
-            advanceProgress(containerNode.m_taskCount);
-        startAction = toSetupResult(container->m_successBit);
-    }
     return continueStart(container, startAction);
 }
 
@@ -2225,14 +2218,14 @@ SetupResult TaskTreePrivate::startChildren(RuntimeContainer *container)
     const int childCount = int(containerNode.m_children.size());
 
     if (container->m_iterationCount == 0) {
+        if (container->m_shouldIterate && !invokeLoopHandler(container)) {
+            if (isProgressive(container))
+                advanceProgress(containerNode.m_taskCount);
+            return toSetupResult(container->m_successBit);
+        }
         container->m_iterations.emplace_back(
             std::make_unique<RuntimeIteration>(container->m_iterationCount, container));
         ++container->m_iterationCount;
-    } else if (containerNode.m_parallelLimit == 0) {
-        container->deleteFinishedIterations();
-        if (container->m_iterations.empty())
-            return toSetupResult(container->m_successBit);
-        return SetupResult::Continue;
     }
 
     GuardLocker locker(container->m_startGuard);
@@ -2241,17 +2234,20 @@ SetupResult TaskTreePrivate::startChildren(RuntimeContainer *container)
            || container->m_runningChildren < containerNode.m_parallelLimit) {
         container->deleteFinishedIterations();
         if (container->m_nextToStart == childCount) {
-            if (container->m_shouldIterate && invokeLoopHandler(container)) {
+            if (invokeLoopHandler(container)) {
                 container->m_nextToStart = 0;
                 container->m_iterations.emplace_back(
                     std::make_unique<RuntimeIteration>(container->m_iterationCount, container));
                 ++container->m_iterationCount;
+            } else if (container->m_iterations.empty()) {
+                return toSetupResult(container->m_successBit);
             } else {
-                if (container->m_iterations.empty())
-                    return toSetupResult(container->m_successBit);
                 return SetupResult::Continue;
             }
         }
+        if (containerNode.m_children.size() == 0) // Empty loop body.
+            continue;
+
         RuntimeIteration *iteration = container->m_iterations.back().get();
         RuntimeTask *newTask = new RuntimeTask{containerNode.m_children.at(container->m_nextToStart),
                                                iteration};

tests/auto/solutions/tasking/tst_tasking.cpp

@@ -43,7 +43,8 @@ enum class Handler {
     Sync,
     BarrierAdvance,
     Timeout,
-    Storage
+    Storage,
+    Iteration
 };
 Q_ENUM_NS(Handler);
 
@@ -2971,6 +2972,27 @@ void tst_Tasking::testTree_data()
             << TestData{storage, root, {}, 1, DoneWith::Success, 0};
     }
 
+    {
+        // Check if LoopUntil is executed with empty loop body.
+        const For root {
+            LoopUntil([storage](int iteration) {
+                storage->m_log.append({iteration, Handler::Iteration});
+                return iteration < 3;
+            }),
+            storage
+        };
+
+        const Log log {
+            {0, Handler::Iteration},
+            {1, Handler::Iteration},
+            {2, Handler::Iteration},
+            {3, Handler::Iteration} // The last iteration returns false
+        };
+
+        QTest::newRow("EmptyLoopUntil")
+            << TestData{storage, root, log, 0, DoneWith::Success, 0};
+    }
+
     {
         // Check if task tree finishes with the right progress value when onGroupSetup(false).
         const Group root {