Python协程greenlet实现原理

def foo():
    bar()

def bar():
    a = 3 + 1
    gr2.switch()

def func():
    pass

gr1 = greenlet(foo)
gr2 = greenlet(func)
gr1.switch()

struct Frame {
    unsigned char *codes; // 存放代码指令
    size_t pc; // 当前执行的指令位置
    int *stack; // stack-based的VM会有一个栈用于存放指令操作数
};

void op_call(frame) {
    switch (OP_CODE()) {
        case OP_CALL:
            child_frame = new_frame()
            op_call(child_frame)
                ...
        case OP_ADD:
            op_add(...)
    }
}

op_add
op_call
op_call

579         if (!PyGreenlet_STARTED(target)) { // greenlet未启动，是一个需要新创建的greenlet
580             void* dummymarker; // 该局部变量的地址成为新的greenlet的栈底
581             ts_target = target;
582             err = g_initialstub(&dummymarker); // 创建该greenlet并运行

410 static int GREENLET_NOINLINE(slp_save_state)(char* stackref)
411 {
412     /* must free all the C stack up to target_stop */
413     char* target_stop = ts_target->stack_stop;
414     PyGreenlet* owner = ts_current;
415     assert(owner->stack_saved == 0);
416     if (owner->stack_start == NULL)
417         owner = owner->stack_prev;  /* not saved if dying */
418     else
419         owner->stack_start = stackref; // stack_start指向栈顶

__asm__ ("movl %%esp, %0" : "=g" (stackref));

460         PyThreadState* tstate = PyThreadState_GET(); // 获取当前线程的VM执行上下文
461         current->recursion_depth = tstate->recursion_depth;
462         current->top_frame = tstate->frame; // 保存当前正在执行的frame到当前正在执行的greenlet
    ...
473         slp_switch(); // 做栈切换
    ...
487         PyThreadState* tstate = PyThreadState_GET();
488         tstate->recursion_depth = target->recursion_depth;
489         tstate->frame = target->top_frame; // 切换回来

def test1():
    gr2.switch()

def test2():
    print('test2')

gr1 = greenlet(test1)
gr2 = greenlet(test2)
gr1.switch()

// g_switch 实现
574         if (PyGreenlet_ACTIVE(target)) {
575             ts_target = target; // 找到目标greenlet，也就是gr2
576             err = g_switchstack(); // 开始切换

// g_switchstack 实现
462         current->top_frame = tstate->frame;
    ...
473     err = slp_switch();

// slp_switch 实现，根据不同平台实现方式不同，原理相同
69         SLP_SAVE_STATE(stackref, stsizediff);
// 这个很重要，强行将当前的栈指针ESP/EBP (32位OS)通过加上一个与目标greenlet栈地址的偏移，而回到了
// 目标greenlet的栈空间。可以在下文看到stsizediff的获取实现
70         __asm__ volatile (
71             "addl %0, %%esp\n"
72             "addl %0, %%ebp\n"
73             :
74             : "r" (stsizediff)
75             );
76         SLP_RESTORE_STATE();

// SLP_SAVE_STATE 实现
316 #define SLP_SAVE_STATE(stackref, stsizediff)            \
317     stackref += STACK_MAGIC;                        \
318     if (slp_save_state((char*)stackref)) return -1; \
319     if (!PyGreenlet_ACTIVE(ts_target)) return 1;    \
// 获取目标greenlet的栈空间与当前栈地址的偏移，用于稍后设置当前栈地址回目标greenlet的栈地址
320     stsizediff = ts_target->stack_start - (char*)stackref 

// slp_save_state 没啥看的，前面也提过了，主要就是复制当前greenlet栈内容到堆内存

// SLP_RESTORE_STATE 也没什么看的，主要就是把greenlet堆内存复制回栈空间

g_switchstack
g_switch
...