lua源码学习--协程系列

lua协程库共提供了8个api，其中最重要的是luaB_cocreate,luaB_coresume,luaB_yield.

luaB_coresume主要是工作是
1.获取待消费协程实例co
2.通过auxresume完成主要工作
3.返回值处理，旋转栈，返回true+args或者false+errormag

static int luaB_coresume (lua_State *L) {
  lua_State *co = getco(L);
  int r;
  r = auxresume(L, co, lua_gettop(L) - 1);
  if (l_unlikely(r < 0)) {
    lua_pushboolean(L, 0);
    lua_insert(L, -2);
    return 2;  /* return false + error message */
  }
  else {
    lua_pushboolean(L, 1);
    lua_insert(L, -(r + 1));
    return r + 1;  /* return true + 'resume' returns */
  }
}

接下来看auxresume实现
这个函数主要工作是
1.从上一个协程栈L传递参数到待消费线程栈co
2.调用lua_resume完成主要工作
3.结果返回，与步骤一相反

static int auxresume (lua_State *L, lua_State *co, int narg) {
  int status, nres;
  if (l_unlikely(!lua_checkstack(co, narg))) {
    lua_pushliteral(L, "too many arguments to resume");
    return -1;  /* error flag */
  }
  lua_xmove(L, co, narg);
  status = lua_resume(co, L, narg, &nres);
  if (l_likely(status == LUA_OK || status == LUA_YIELD)) {
    if (l_unlikely(!lua_checkstack(L, nres + 1))) {
      lua_pop(co, nres);  /* remove results anyway */
      lua_pushliteral(L, "too many results to resume");
      return -1;  /* error flag */
    }
    lua_xmove(co, L, nres);  /* move yielded values */
    return nres;
  }
  else {
    lua_xmove(co, L, 1);  /* move error message */
    return -1;  /* error flag */
  }
}

lua_resume主要有两点，核心是在保护模式下调用resume消费协程，这里是lua协程最为复杂的逻辑。
其二是precover用来恢复pcall，pcall的恢复也是相当直接，顺着previous方向查找所有pcall，并以保护模式unroll栈上剩余的函数。

LUA_API int lua_resume (lua_State *L, lua_State *from, int nargs,
                                      int *nresults) {
  int status;
  lua_lock(L);
  if (L->status == LUA_OK) {  /* may be starting a coroutine */
    if (L->ci != &L->base_ci)  /* not in base level? */
      return resume_error(L, "cannot resume non-suspended coroutine", nargs);
    else if (L->top.p - (L->ci->func.p + 1) == nargs)  /* no function? */
      return resume_error(L, "cannot resume dead coroutine", nargs);
  }
  else if (L->status != LUA_YIELD)  /* ended with errors? */
    return resume_error(L, "cannot resume dead coroutine", nargs);
  L->nCcalls = (from) ? getCcalls(from) : 0;
  if (getCcalls(L) >= LUAI_MAXCCALLS)
    return resume_error(L, "C stack overflow", nargs);
  L->nCcalls++;
  luai_userstateresume(L, nargs);
  api_checknelems(L, (L->status == LUA_OK) ? nargs + 1 : nargs);
  status = luaD_rawrunprotected(L, resume, &nargs);
   /* continue running after recoverable errors */
  status = precover(L, status);
  if (l_likely(!errorstatus(status)))
    lua_assert(status == L->status);  /* normal end or yield */
  else {  /* unrecoverable error */
    L->status = cast_byte(status);  /* mark thread as 'dead' */
    luaD_seterrorobj(L, status, L->top.p);  /* push error message */
    L->ci->top.p = L->top.p;
  }
  *nresults = (status == LUA_YIELD) ? L->ci->u2.nyield
                                    : cast_int(L->top.p - (L->ci->func.p + 1));
  lua_unlock(L);
  return status;
}

resume函数按照是否第一次执行可以分为两部分，第一次执行resume，lua状态必然是LUA_OK状态，和普通函数执行无异，通过ccall即可处理。比较精彩的是resume对于挂起状态的处理，对于非hook的一般场景来说，执行到这一步的时候lua的当前调用栈ci必然是上一次lua_yieldk的调用栈，只需通过poscall将返回值调整到合适位置，退回到上上一层调用栈后unroll栈上剩余部分，hook部分暂不讨论

static void resume (lua_State *L, void *ud) {
  int n = *(cast(int*, ud));  /* number of arguments */
  StkId firstArg = L->top.p - n;  /* first argument */
  CallInfo *ci = L->ci;
  if (L->status == LUA_OK)  /* starting a coroutine? */
    ccall(L, firstArg - 1, LUA_MULTRET, 0);  /* just call its body */
  else {  /* resuming from previous yield */
    lua_assert(L->status == LUA_YIELD);
    L->status = LUA_OK;  /* mark that it is running (again) */
    if (isLua(ci)) {  /* yielded inside a hook? */
      /* undo increment made by 'luaG_traceexec': instruction was not
         executed yet */
      lua_assert(ci->callstatus & CIST_HOOKYIELD);
      ci->u.l.savedpc--;
      L->top.p = firstArg;  /* discard arguments */
      luaV_execute(L, ci);  /* just continue running Lua code */
    }
    else {  /* 'common' yield */
      if (ci->u.c.k != NULL) {  /* does it have a continuation function? */
        lua_unlock(L);
        n = (*ci->u.c.k)(L, LUA_YIELD, ci->u.c.ctx); /* call continuation */
        lua_lock(L);
        api_checknelems(L, n);
      }
      luaD_poscall(L, ci, n);  /* finish 'luaD_call' */
    }
    unroll(L, NULL);  /* run continuation */
  }
}

lua_yield相较于resume则要简单很多，置玩LUA_YIELD标志位，保存断点后，直接跳出到上一个jump点，就完成了yield挂起过程

LUA_API int lua_yieldk (lua_State *L, int nresults, lua_KContext ctx,
                        lua_KFunction k) {
  CallInfo *ci;
  luai_userstateyield(L, nresults);
  lua_lock(L);
  ci = L->ci;
  api_checknelems(L, nresults);
  if (l_unlikely(!yieldable(L))) {
    if (L != G(L)->mainthread)
      luaG_runerror(L, "attempt to yield across a C-call boundary");
    else
      luaG_runerror(L, "attempt to yield from outside a coroutine");
  }
  L->status = LUA_YIELD;
  ci->u2.nyield = nresults;  /* save number of results */
  if (isLua(ci)) {  /* inside a hook? */
    lua_assert(!isLuacode(ci));
    api_check(L, nresults == 0, "hooks cannot yield values");
    api_check(L, k == NULL, "hooks cannot continue after yielding");
  }
  else {
    if ((ci->u.c.k = k) != NULL)  /* is there a continuation? */
      ci->u.c.ctx = ctx;  /* save context */
    luaD_throw(L, LUA_YIELD);
  }
  lua_assert(ci->callstatus & CIST_HOOKED);  /* must be inside a hook */
  lua_unlock(L);
  return 0;  /* return to 'luaD_hook' */
}

未完待续