support custom executor in execution_queue

src/bthread/execution_queue.cpp

@@ -101,6 +101,7 @@ void ExecutionQueueBase::start_execute(TaskNode* node) {
         }
     }
 
+    if (nullptr == _options.executor) {
         bthread_t tid;
         // We start the execution thread in background instead of foreground as
         // we can't determine whether the code after execute() is urgent (like
@@ -111,6 +112,12 @@ void ExecutionQueueBase::start_execute(TaskNode* node) {
             PLOG(FATAL) << "Fail to start bthread";
             _execute_tasks(node);
         }
+    } else {
+        if (_options.executor->submit(_execute_tasks, node) != 0) {
+            PLOG(FATAL) << "Fail to submit task";
+            _execute_tasks(node);
+        }
+    }
 }
 
 void* ExecutionQueueBase::_execute_tasks(void* arg) {

src/bthread/execution_queue.h

@@ -128,11 +128,24 @@ const static TaskOptions TASK_OPTIONS_NORMAL = TaskOptions(false, false);
 const static TaskOptions TASK_OPTIONS_URGENT = TaskOptions(true, false);
 const static TaskOptions TASK_OPTIONS_INPLACE = TaskOptions(false, true);
 
+class Executor {
+public:
+    virtual ~Executor() {}
+
+    // Return 0 on success.
+    virtual int submit(void * (*fn)(void*), void* args) = 0;
+};
+
 struct ExecutionQueueOptions {
     ExecutionQueueOptions();
     // Attribute of the bthread which execute runs on
     // default: BTHREAD_ATTR_NORMAL
     bthread_attr_t bthread_attr;
+
+    // Executor that tasks run on. bthread will be used when executor = NULL.
+    // Note that TaskOptions.in_place_if_possible = false will not work, if implementation of
+    // Executor is in-place(synchronous).
+    Executor * executor;
 };
 
 // Start a ExecutionQueue. If |options| is NULL, the queue will be created with

src/bthread/execution_queue_inl.h

@@ -317,7 +317,7 @@ public:
 };
 
 inline ExecutionQueueOptions::ExecutionQueueOptions()
-    : bthread_attr(BTHREAD_ATTR_NORMAL)
+    : bthread_attr(BTHREAD_ATTR_NORMAL), executor(NULL)
 {}
 
 template <typename T>