Fix fiber runtime assertion

Fixes #671

In D1 tangort calling `Fiber.reset` immediately puts it into HOLD state,
and it checks for TERM state in contract. Thus it is not legal to reset
fiber twice even if it is never started in between.

src/ocean/task/Task.d

@@ -285,10 +285,13 @@ public abstract class Task : ISuspendable
 
         Params:
             fiber = the fiber to assign the task to
+            entry_point = optional custom entry point to replace
+                `this.entryPoint`
 
     ***************************************************************************/
 
-    public void assignTo ( WorkerFiber fiber )
+    public void assignTo ( WorkerFiber fiber,
+        void delegate() entry_point = null )
     {
         this.state_bitmask = TaskState.None;
 
@@ -297,7 +300,8 @@ public abstract class Task : ISuspendable
         if (fiber.state == fiber.state.TERM)
         {
             // cast to ignore return value
-            this.fiber.reset(cast(void delegate()) &this.entryPoint);
+            this.fiber.reset(entry_point ? entry_point
+                : cast(void delegate()) &this.entryPoint);
         }
     }
 
@@ -784,9 +788,10 @@ public class TaskWith ( Extensions... ) : Task
 
     ***************************************************************************/
 
-    override public void assignTo ( WorkerFiber fiber )
+    override public void assignTo ( WorkerFiber fiber,
+        void delegate() entry_point = null )
     {
-        super.assignTo(fiber);
+        super.assignTo(fiber, entry_point);
 
         foreach (ref extension; this.extensions.tupleof)
         {

src/ocean/task/internal/FiberPool.d

@@ -86,7 +86,7 @@ public class FiberPool : ObjectPool!(WorkerFiber)
         auto fiber = super.get(if_missing);
         if (fiber is null)
             return null;
-        verify(fiber.state() != Fiber.State.EXEC);
+        verify(fiber.state() == Fiber.State.TERM);
         return fiber;
     }
 

src/ocean/task/internal/FiberPoolWithQueue.d

@@ -141,13 +141,12 @@ public class FiberPoolWithQueue : FiberPool
         auto fiber = this.get();
         debug_trace("running task <{}> via worker fiber <{}>",
             cast(void*) task, cast(void*) fiber);
-        task.assignTo(fiber);
         // `Task.entryPoint` is supposed to be entry method for worker fiber
         // but it does not allow to reuse worker fiber immediately for new tasks
         // waititing in the queue. Because of that a custom method is used as
         // entry point instead which will internally call `Task.entryPoint`
         // directly:
-        fiber.reset(&this.workerFiberMethod);
+        task.assignTo(fiber, &this.workerFiberMethod);
         task.resume();
     }