浅析lua异常捕获处理机制

异常捕获是高级语言的1大特性，通过对异常的捕获和处理，可以有效提高系统的稳定性和硬朗性。由于不管再怎样改进代码，都不可避免出现1些异常，例如文件io毛病、网络毛病、内存毛病等等，就要求编码对毛病进行捕获，同时打印日志以便开发人员跟进问题的处理。固然，lua也提供了接口用于捕获运行时异常。

lua异常捕获函数

lua有两个函数可用于捕获异常： pcall 和 xpcall，这两个函数很类似，都会在保护模式下履行函数，效果类似try-catch，可捕获并处理异常。
两个函数的原型以下：
pcall (func [, arg1, ・・・]) xpcall (func, errfunc [, arg1, ・・・])
对照两个函数，xpcall多了1个异常处理函数参数 errfunc。对pcall，异常处理完时只简单记录毛病信息，然后释放调用栈空间，而对xpcall，这个参数可用于在调用栈释放前跟踪到这些数据。效果以下：
> f=function(...) error(...) end > pcall(f, 123) false stdin:1: 123 > xpcall(f, function(e) print(debug.traceback()) return e end, 123) stack traceback: stdin:1: in function[C]: in function error stdin:1: in function f [C]: in function xpcall stdin:1: in main chunk [C]: in ? false stdin:1: 123
值得注意的是，errfunc的传入参数是异常数据，函数结束时必须将这个数据返回，才能实现和 pcall 1样的返回值

lua异常捕获处理机制

下面，以 pcall 来讲明lua异常捕获是怎样实现的。这里，lua代码版本取5.3.1

pcall的实现

pcall的c实现函数为luaB_pcall，以下：
// lbaselib.c static int luaB_pcall (lua_State *L) { int status; luaL_checkany(L, 1); lua_pushboolean(L, 1); // 如果没毛病产生，返回true lua_insert(L, 1); /* put it in place */ status = lua_pcallk(L, lua_gettop(L) - 2, LUA_MULTRET, 0, 0, finishpcall); return finishpcall(L, status, 0); }
看下lua_pcallk，这是pcall的预处理函数：
// lapi.c LUA_API int lua_pcallk (lua_State *L, int nargs, int nresults, int errfunc, lua_KContext ctx, lua_KFunction k) { struct CallS c; int status; ptrdiff_t func; lua_lock(L); api_check(L, k == NULL || !isLua(L->ci), "cannot use continuations inside hooks"); api_checknelems(L, nargs+1); api_check(L, L->status == LUA_OK, "cannot do calls on non-normal thread"); checkresults(L, nargs, nresults); if (errfunc == 0) func = 0; else { StkId o = index2addr(L, errfunc); api_checkstackindex(L, errfunc, o); func = savestack(L, o); } c.func = L->top - (nargs+1); // 取到pcall要履行的函数 if (k == NULL || L->nny > 0) { /* no continuation or no yieldable? */ c.nresults = nresults; /* do a conventional protected call */ /* 处理pcall（非协程走这里，详见拓展浏览）*/ status = luaD_pcall(L, f_call, &c, savestack(L, c.func), func); } else { //当resume协程时履行，已在保护模式下 CallInfo *ci = L->ci; ci->u.c.k = k; /* save continuation */ ci->u.c.ctx = ctx; /* save context */ /* save information for error recovery */ ci->extra = savestack(L, c.func); ci->u.c.old_errfunc = L->errfunc; L->errfunc = func; // 记录异常处理函数 setoah(ci->callstatus, L->allowhook); /* save value of allowhook */ ci->callstatus |= CIST_YPCALL; // 打标记设置协程恢复点 luaD_call(L, c.func, nresults, 1); /* do the call */ ci->callstatus &= ~CIST_YPCALL; // 履行结束去掉标记 L->errfunc = ci->u.c.old_errfunc; status = LUA_OK; /* if it is here, there were no errors */ } adjustresults(L, nresults); lua_unlock(L); return status; }
这里重点看下luaD_pcall 的实现，这是pcall的核心处理函数：
// ldo.c int luaD_pcall (lua_State *L, Pfunc func, void *u, ptrdiff_t old_top, ptrdiff_t ef) { int status; CallInfo *old_ci = L->ci; lu_byte old_allowhooks = L->allowhook; unsigned short old_nny = L->nny; ptrdiff_t old_errfunc = L->errfunc; L->errfunc = ef; // 记录异常处理函数 status = luaD_rawrunprotected(L, func, u); // 以保护模式运行 if (status != LUA_OK) { // 当异常产生时 StkId oldtop = restorestack(L, old_top); // ‘释放’调用栈 luaF_close(L, oldtop); /* close possible pending closures */ seterrorobj(L, status, oldtop); L->ci = old_ci; L->allowhook = old_allowhooks; L->nny = old_nny; luaD_shrinkstack(L); } L->errfunc = old_errfunc; return status; }
看下 luaD_rawrunprotected 函数的实现：
// ldo.c int luaD_rawrunprotected (lua_State *L, Pfunc f, void *ud) { unsigned short oldnCcalls = L->nCcalls; struct lua_longjmp lj; lj.status = LUA_OK; lj.previous = L->errorJmp; /* chain new error handler */ L->errorJmp = &lj; LUAI_TRY(L, &lj, (*f)(L, ud); ); // 异常宏处理 L->errorJmp = lj.previous; // 设置毛病跳转点 L->nCcalls = oldnCcalls; return lj.status; }
LUAI_TRY 和文章后面出现的LUAI_THROW 都是宏实现的，目的是兼容主流c或c++版本的异常处理语法，表现为try-catch的语法结构。简单说就是，履行代码前先try，履行进程出错就throw，然后在catch的地方处理异常。
/* ** LUAI_THROW/LUAI_TRY define how Lua does exception handling. By ** default, Lua handles errors with exceptions when compiling as ** C++ code, with _longjmp/_setjmp when asked to use them, and with ** longjmp/setjmp otherwise. */ #if !defined(LUAI_THROW) /* { */ #if defined(__cplusplus) && !defined(LUA_USE_LONGJMP) /* { */ /* C++ exceptions */ #define LUAI_THROW(L,c) throw(c) #define LUAI_TRY(L,c,a) try { a } catch(...) { if ((c)->status == 0) (c)->status = ⑴; } #define luai_jmpbuf int /* dummy variable */ #elif defined(LUA_USE_POSIX) /* }{ */ /* in POSIX, try _longjmp/_setjmp (more efficient) */ #define LUAI_THROW(L,c) _longjmp((c)->b, 1) #define LUAI_TRY(L,c,a) if (_setjmp((c)->b) == 0) { a } #define luai_jmpbuf jmp_buf #else /* }{ */ /* ISO C handling with long jumps */ #define LUAI_THROW(L,c) longjmp((c)->b, 1) #define LUAI_TRY(L,c,a) if (setjmp((c)->b) == 0) { a } #define luai_jmpbuf jmp_buf #endif /* } */ #endif /* } */
当异常出现时，status 就会赋值为⑴，即不等于 LUA_OK

xpcall和pcall的区分

对照下xpcall对照pcall，有甚么区分？

// lbaselib.c static int luaB_pcall (lua_State *L) { int status; luaL_checkany(L, 1); lua_pushboolean(L, 1); /* first result if no errors */ lua_insert(L, 1); /* put it in place */ status = lua_pcallk(L, lua_gettop(L) - 2, LUA_MULTRET, 0, 0, finishpcall); return finishpcall(L, status, 0); } static int luaB_xpcall (lua_State *L) { int status; int n = lua_gettop(L); luaL_checktype(L, 2, LUA_TFUNCTION); /* check error function */ lua_pushboolean(L, 1); /* first result */ lua_pushvalue(L, 1); // 将异常处理函数 errfunc 写入调用栈 lua_rotate(L, 3, 2); /* move them below functions arguments */ status = lua_pcallk(L, n - 2, LUA_MULTRET, 2, 2, finishpcall); return finishpcall(L, status, 2); }
再回头看下lua_pcallk 
// lapi.c LUA_API int lua_pcallk (lua_State *L, int nargs, int nresults, int errfunc, lua_KContext ctx, lua_KFunction k) { struct CallS c; int status; ptrdiff_t func; lua_lock(L); api_check(L, k == NULL || !isLua(L->ci), "cannot use continuations inside hooks"); api_checknelems(L, nargs+1); api_check(L, L->status == LUA_OK, "cannot do calls on non-normal thread"); checkresults(L, nargs, nresults); if (errfunc == 0) func = 0; // pcall时 func为0 else { StkId o = index2addr(L, errfunc); api_checkstackindex(L, errfunc, o); func = savestack(L, o); // xpcall取到异常处理函数 errfunc }
然后， func 赋值给 L->errfunc，在异常产生时，就检查1下这个函数。

甚么地方履行这个毛病处理函数？
// ldebug.c l_noret luaG_errormsg (lua_State *L) { if (L->errfunc != 0) { // 如果有异常处理函数 errfunc StkId errfunc = restorestack(L, L->errfunc); setobjs2s(L, L->top, L->top - 1); /* move argument */ setobjs2s(L, L->top - 1, errfunc); /* push function */ L->top++; /* assume EXTRA_STACK */ luaD_call(L, L->top - 2, 1, 0); /* call it */ } luaD_throw(L, LUA_ERRRUN); }
luaD_throw则是调用 LUAI_THROW，跳到前面catch的位置
// ldo.c l_noret luaD_throw (lua_State *L, int errcode) { if (L->errorJmp) { /* thread has an error handler? */ L->errorJmp->status = errcode; /* set status */ LUAI_THROW(L, L->errorJmp); /* jump to it */ } else { /* thread has no error handler */ global_State *g = G(L); L->status = cast_byte(errcode); /* mark it as dead */ if (g->mainthread->errorJmp) { /* main thread has a handler? */ setobjs2s(L, g->mainthread->top++, L->top - 1); /* copy error obj. */ luaD_throw(g->mainthread, errcode); /* re-throw in main thread */ } else { /* no handler at all; abort */ if (g->panic) { /* panic function? */ seterrorobj(L, errcode, L->top); /* assume EXTRA_STACK */ if (L->ci->top < L->top) L->ci->top = L->top; /* pushing msg. can break this invariant */ lua_unlock(L); g->panic(L); /* call panic function (last chance to jump out) */ } abort(); } } }


两种情况会触发这个函数
1、主动调用error时：
// lapi.c LUA_API int lua_error (lua_State *L) { lua_lock(L); api_checknelems(L, 1); luaG_errormsg(L); /* code unreachable; will unlock when control actually leaves the kernel */ return 0; /* to avoid warnings */ }
2、出现运行时毛病时：
// ldebug.c l_noret luaG_runerror (lua_State *L, const char *fmt, ...) { CallInfo *ci = L->ci; const char *msg; va_list argp; va_start(argp, fmt); msg = luaO_pushvfstring(L, fmt, argp); /* format message */ va_end(argp); if (isLua(ci)) /* if Lua function, add source:line information */ luaG_addinfo(L, msg, ci_func(ci)->p->source, currentline(ci)); luaG_errormsg(L); }

拓展浏览

lua state之nny 

在lua state中，nny记录了调用栈上不能被挂起的次数，定义在 lua_State结构：
struct lua_State { CommonHeader; lu_byte status; StkId top; /* first free slot in the stack */ global_State *l_G; CallInfo *ci; /* call info for current function */ const Instruction *oldpc; /* last pc traced */ StkId stack_last; /* last free slot in the stack */ StkId stack; /* stack base */ UpVal *openupval; /* list of open upvalues in this stack */ GCObject *gclist; struct lua_State *twups; /* list of threads with open upvalues */ struct lua_longjmp *errorJmp; /* current error recover point */ CallInfo base_ci; /* CallInfo for first level (C calling Lua) */ lua_Hook hook; ptrdiff_t errfunc; /* current error handling function (stack index) */ int stacksize; int basehookcount; int hookcount; unsigned short nny; /* number of non-yieldable calls in stack */ unsigned short nCcalls; /* number of nested C calls */ lu_byte hookmask; lu_byte allowhook; };
为何lua state需要这个字段？
这是由于pcall调用函数中还可以有pcall，支持多层嵌套，所以需要记录1个次数值，在pcall履行前+1，结束后⑴
/* ** Call a function (C or Lua). The function to be called is at *func. ** The arguments are on the stack, right after the function. ** When returns, all the results are on the stack, starting at the original ** function position. */ void luaD_call (lua_State *L, StkId func, int nResults, int allowyield) { fprintf(stderr, "luaD_call %d ", allowyield); if (++L->nCcalls >= LUAI_MAXCCALLS) { if (L->nCcalls == LUAI_MAXCCALLS) luaG_runerror(L, "C stack overflow"); else if (L->nCcalls >= (LUAI_MAXCCALLS + (LUAI_MAXCCALLS>>3))) luaD_throw(L, LUA_ERRERR); /* error while handing stack error */ } if (!allowyield) L->nny++; if (!luaD_precall(L, func, nResults)) /* is a Lua function? */ luaV_execute(L); /* call it */ if (!allowyield) L->nny--; L->nCcalls--; }
这样，通过判断 nny的值，就能够知道当前进程能否挂起。
仔细的同学会发现，luaD_call是在pcall的履行进程调用到的，为何 L->nny会大于0？
这是由于lua 虚拟机（lua state）启动时 L->nny 就赋值1了
// lstate.c LUA_API lua_State *lua_newstate (lua_Alloc f, void *ud) { int i; lua_State *L; global_State *g; LG *l = cast(LG *, (*f)(ud, NULL, LUA_TTHREAD, sizeof(LG))); if (l == NULL) return NULL; L = &l->l.l; g = &l->g; L->next = NULL; L->tt = LUA_TTHREAD; g->currentwhite = bitmask(WHITE0BIT); L->marked = luaC_white(g); preinit_thread(L, g); // 预处理协程时将 L->nny 赋值1 g->frealloc = f; g->ud = ud; g->mainthread = L; g->seed = makeseed(L); g->gcrunning = 0; /* no GC while building state */ g->GCestimate = 0; g->strt.size = g->strt.nuse = 0; g->strt.hash = NULL; setnilvalue(&g->l_registry); luaZ_initbuffer(L, &g->buff); g->panic = NULL; g->version = NULL; g->gcstate = GCSpause; g->gckind = KGC_NORMAL; g->allgc = g->finobj = g->tobefnz = g->fixedgc = NULL; g->sweepgc = NULL; g->gray = g->grayagain = NULL; g->weak = g->ephemeron = g->allweak = NULL; g->twups = NULL; g->totalbytes = sizeof(LG); g->GCdebt = 0; g->gcfinnum = 0; g->gcpause = LUAI_GCPAUSE; g->gcstepmul = LUAI_GCMUL; for (i=0; i < LUA_NUMTAGS; i++) g->mt[i] = NULL; if (luaD_rawrunprotected(L, f_luaopen, NULL) != LUA_OK) { /* memory allocation error: free partial state */ close_state(L); L = NULL; } return L; }
看下preinit_thread 函数：
// lstate.c /* ** preinitialize a thread with consistent values without allocating ** any memory (to avoid errors) */ static void preinit_thread (lua_State *L, global_State *g) { G(L) = g; L->stack = NULL; L->ci = NULL; L->stacksize = 0; L->twups = L; /* thread has no upvalues */ L->errorJmp = NULL; L->nCcalls = 0; L->hook = NULL; L->hookmask = 0; L->basehookcount = 0; L->allowhook = 1; resethookcount(L); L->openupval = NULL; L->nny = 1; //将 L->nny 赋值1 L->status = LUA_OK; L->errfunc = 0; }
那协程 lua_newthread时也会调用 preinit_thread ，但是二者对pcall的实现却有很多的差异。
以下，协程创建时调用 preinit_thread 预处理协程：
// lstate.c LUA_API lua_State *lua_newthread (lua_State *L) { global_State *g = G(L); lua_State *L1; lua_lock(L); luaC_checkGC(L); /* create new thread */ L1 = &cast(LX *, luaM_newobject(L, LUA_TTHREAD, sizeof(LX)))->l; L1->marked = luaC_white(g); L1->tt = LUA_TTHREAD; /* link it on list allgc */ L1->next = g->allgc; g->allgc = obj2gco(L1); /* anchor it on L stack */ setthvalue(L, L->top, L1); api_incr_top(L); preinit_thread(L1, g); // 预处理协程时将 L->nny 赋值1 L1->hookmask = L->hookmask; L1->basehookcount = L->basehookcount; L1->hook = L->hook; resethookcount(L1); /* initialize L1 extra space */ memcpy(lua_getextraspace(L1), lua_getextraspace(g->mainthread), LUA_EXTRASPACE); luai_userstatethread(L, L1); stack_init(L1, L); /* init stack */ lua_unlock(L); return L1; }
但是，协程在lua中是这样调用的：
local co = coroutine.create(func) coroutine.resume(co)
再看下 resume的实现：
LUA_API int lua_resume (lua_State *L, lua_State *from, int nargs) { int status; int oldnny = L->nny; /* save "number of non-yieldable" calls */ lua_lock(L); luai_userstateresume(L, nargs); L->nCcalls = (from) ? from->nCcalls + 1 : 1; L->nny = 0; // 将 nny赋值 0，允许挂起 api_checknelems(L, (L->status == LUA_OK) ? nargs + 1 : nargs); status = luaD_rawrunprotected(L, resume, &nargs); // 在保护模式启动协程 if (status == ⑴) /* error calling lua_resume? */ status = LUA_ERRRUN; else { /* continue running after recoverable errors */ while (errorstatus(status) && recover(L, status)) { // 检查是不是有可恢复点 /* unroll continuation */ status = luaD_rawrunprotected(L, unroll, &status); } if (errorstatus(status)) { /* unrecoverable error? */ L->status = cast_byte(status); /* mark thread as dead */ seterrorobj(L, status, L->top); /* push error message */ L->ci->top = L->top; } else lua_assert(status == L->status); /* normal end or yield */ } L->nny = oldnny; // 恢复协程的nny值 L->nCcalls--; lua_assert(L->nCcalls == ((from) ? from->nCcalls : 0)); lua_unlock(L); return status; }
可以看出协程的pcall处理有些不同，为何会这样？
这是由于协程可挂起（yield），可恢复上下文（resume），就会有pcall履行进程有协程挂起的情况，所以需要记录还原点，然后从上下文恢复。
所以到这里也能够发现，除创建的协程，lua state原生的协程是没法被挂起，通常只有在lua代码履行完时才会退出。

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