题记
Go 语言自发布以来，一直以高性能、高并发著称。因为标准库提供了 http 包，即使刚学不久的程序员，也能轻松写出 http 服务程序。
panicrecover初识 panic 和 recover
panicpanic恐慌、恐慌的panicjavathrowrecoverrecover恢复、复原recoverjavatry ... catchtry ... catchpanicrecoverif error then returnsetjumplongjump#include <setjmp.h>
#include <stdio.h>
static jmp_buf env;
double divide(double to, double by)
{
if(by == 0)
{
longjmp(env, 1);
}
return to / by;
}
void test_divide()
{
divide(2, 0);
printf("done\n");
}
int main()
{
if (setjmp(env) == 0)
{
test_divide();
}
else
{
printf("Cannot / 0\n");
return -1;
}
return 0;
}
dividemainCannot / 0donetry catchrecoversetjumprecoverdeferthrowpaniclongjump深入 panic 和 recover
源码
panicrecoversrc/runtime/panic.gogopanicgorecover// gopanic 的代码，在 src/runtime/panic.go 第 454 行
// 预定义函数 panic 的实现
func gopanic(e interface{}) {
gp := getg()
if gp.m.curg != gp {
print("panic: ")
printany(e)
print("\n")
throw("panic on system stack")
}
if gp.m.mallocing != 0 {
print("panic: ")
printany(e)
print("\n")
throw("panic during malloc")
}
if gp.m.preemptoff != "" {
print("panic: ")
printany(e)
print("\n")
print("preempt off reason: ")
print(gp.m.preemptoff)
print("\n")
throw("panic during preemptoff")
}
if gp.m.locks != 0 {
print("panic: ")
printany(e)
print("\n")
throw("panic holding locks")
}
var p _panic
p.arg = e
p.link = gp._panic
gp._panic = (*_panic)(noescape(unsafe.Pointer(&p)))
atomic.Xadd(&runningPanicDefers, 1)
for {
d := gp._defer
if d == nil {
break
}
// 如果触发 defer 的 panic 是在前一个 panic 或者 Goexit 的 defer 中触发的，那么将前一个 defer 从列表中去除。前一个 panic 或者 Goexit 将不再继续执行。
if d.started {
if d._panic != nil {
d._panic.aborted = true
}
d._panic = nil
d.fn = nil
gp._defer = d.link
freedefer(d)
continue
}
// 将 defer 标记为 started，但是保留在列表上，这样，如果在 reflectcall 开始执行 d.fn 之前发生了堆栈增长或垃圾回收，则 traceback 可以找到并更新 defer 的参数帧。
d.started = true
// 将正在执行 defer 的 panic 保存下来。如果在该 panic 的 defer 函数中触发了新的 panic ，则新 panic 在列表中将会找到 d 并将 d._panic 标记为 aborted 。
d._panic = (*_panic)(noescape(unsafe.Pointer(&p)))
p.argp = unsafe.Pointer(getargp(0))
reflectcall(nil, unsafe.Pointer(d.fn), deferArgs(d), uint32(d.siz), uint32(d.siz))
p.argp = nil
// reflectcall 不会 panic，移除 d 。
if gp._defer != d {
throw("bad defer entry in panic")
}
d._panic = nil
d.fn = nil
gp._defer = d.link
// 这里用 GC() 来触发堆栈收缩以测试堆栈拷贝。由于是测试代码，所以注释掉了。参考 stack_test.go:TestStackPanic
//GC()
pc := d.pc
sp := unsafe.Pointer(d.sp) // 必须是指针，以便在堆栈复制期间进行调整
// defer 处理函数的内存是动态分配的，在执行完后需要释放内存。所以，如果 defer 一直得不到执行（比如在死循环中一直创建 defer），将会导致内存泄露
freedefer(d)
if p.recovered {
atomic.Xadd(&runningPanicDefers, -1)
gp._panic = p.link
// 已退出的 panic 已经被标记，但还遗留在 g.panic 列表里，从列表里移除他们。
for gp._panic != nil && gp._panic.aborted {
gp._panic = gp._panic.link
}
if gp._panic == nil { // must be done with signal
gp.sig = 0
}
// 将正在恢复的栈帧传给 recovery。
gp.sigcode0 = uintptr(sp)
gp.sigcode1 = pc
mcall(recovery)
throw("recovery failed") // mcall 不应该返回
}
}
// 如果所有的 defer 都遍历完毕，意味着没有 recover（前面提到，mcall 执行 recovery 是不返回的），继续执行 panic 后续流程，如：输出调用栈信息和错误信息
// 由于在冻结世界之后调用任意用户代码是不安全的，因此我们调用preprintpanics来调用所有必要的Error和String方法以在startpanic之前准备 panic 输出的字符串。
preprintpanics(gp._panic)
fatalpanic(gp._panic) // 不应该返回
*(*int)(nil) = 0 // 因为 fatalpanic 不应该返回，正常情况下这里不会执行。如果执行到了，这行代码将触发 panic
}
// gorecover 的代码，在 src/runtime/panic.go 第 585 行
// 预定义函数 recover 的实现。
// 无法拆分堆栈，因为它需要可靠地找到其调用方的堆栈段。
//
// TODO(rsc): Once we commit to CopyStackAlways,
// this doesn't need to be nosplit.
//go:nosplit
func gorecover(argp uintptr) interface{} {
// 在处理 panic 的时候，recover 函数的调用必须放在 defer 的顶层处理函数中。
// p.argp 是最顶层的延迟函数调用的参数指针，与调用方传递的argp进行比较，如果一致，则该调用方是可以恢复的。
gp := getg()
p := gp._panic
if p != nil && !p.recovered && argp == uintptr(p.argp) {
p.recovered = true
return p.arg
}
return nil
}
panicggoroutinegdeferdeferrecoverpanicpanicrecovereddeferrecovereddefergruntime.mcallm->g0recoverygrecoveryruntime.mcallsrc/runtime/asm_xxx.sxxxamd64// src/runtime/asm_amd64.s 第 274 行
// func mcall(fn func(*g))
// Switch to m->g0's stack, call fn(g).
// Fn must never return. It should gogo(&g->sched)
// to keep running g.
TEXT runtime·mcall(SB), NOSPLIT, $0-8
MOVQ fn+0(FP), DI
get_tls(CX)
MOVQ g(CX), AX // save state in g->sched
MOVQ 0(SP), BX // caller's PC
MOVQ BX, (g_sched+gobuf_pc)(AX)
LEAQ fn+0(FP), BX // caller's SP
MOVQ BX, (g_sched+gobuf_sp)(AX)
MOVQ AX, (g_sched+gobuf_g)(AX)
MOVQ BP, (g_sched+gobuf_bp)(AX)
// switch to m->g0 & its stack, call fn
MOVQ g(CX), BX
MOVQ g_m(BX), BX
MOVQ m_g0(BX), SI
CMPQ SI, AX // if g == m->g0 call badmcall
JNE 3(PC)
MOVQ $runtime·badmcall(SB), AX
JMP AX
MOVQ SI, g(CX) // g = m->g0
MOVQ (g_sched+gobuf_sp)(SI), SP // sp = m->g0->sched.sp
PUSHQ AX
MOVQ DI, DX
MOVQ 0(DI), DI
CALL DI
POPQ AX
MOVQ $runtime·badmcall2(SB), AX
JMP AX
RET
m->g0runtimegoroutinerecoveryruntimem->g0recoveryrecoveryggogoggrecover // gorecover 的代码，在 src/runtime/panic.go 第 637 行
// 在 panic 后，在延迟函数中调用 recover 的时候，将回溯堆栈，并且继续执行，就像延迟函数的调用者正常返回一样。
func recovery(gp *g) {
// Info about defer passed in G struct.
sp := gp.sigcode0
pc := gp.sigcode1
// 延迟函数的参数必须已经保存在堆栈中了（这里通过判断 sp 是否处于栈内存地址的范围内来保障参数的正确处理）
if sp != 0 && (sp < gp.stack.lo || gp.stack.hi < sp) {
print("recover: ", hex(sp), " not in [", hex(gp.stack.lo), ", ", hex(gp.stack.hi), "]\n")
throw("bad recovery")
}
// 让延迟函数的 deferproc 再次返回，这次返回 1 。调用函数将跳转到标准返回结尾。
gp.sched.sp = sp
gp.sched.pc = pc
gp.sched.lr = 0
gp.sched.ret = 1
gogo(&gp.sched)
}
// src/runtime/asm_amd64.s 第 274 行
// func gogo(buf *gobuf)
// restore state from Gobuf; longjmp
TEXT runtime·gogo(SB), NOSPLIT, $16-8
MOVQ buf+0(FP), BX // gobuf
MOVQ gobuf_g(BX), DX
MOVQ 0(DX), CX // make sure g != nil
get_tls(CX)
MOVQ DX, g(CX)
MOVQ gobuf_sp(BX), SP // 从 gobuf 中恢复 SP ，以便后面做跳转
MOVQ gobuf_ret(BX), AX
MOVQ gobuf_ctxt(BX), DX
MOVQ gobuf_bp(BX), BP
MOVQ $0, gobuf_sp(BX) // 这里开始清理 gobuf ，以便垃圾回收。
MOVQ $0, gobuf_ret(BX)
MOVQ $0, gobuf_ctxt(BX)
MOVQ $0, gobuf_bp(BX)
MOVQ gobuf_pc(BX), BX // 从 gobuf 中恢复 pc ，以便跳转
JMP BX
以上便是 Go 底层处理异常的流程，精简为三步便是：
deferrecoverpanicruntimedeferrecovergdeferrecover都有哪些坑
panicpanicruntimepanic接下来，笔者给大家细数下都有哪些坑。
-
数组( slice )下标越界
这个比较好理解，对于静态类型语言，数组下标越界是致命错误。如下代码可以验证：
package main
import (
"fmt"
)
func foo(){
defer func(){
if err := recover(); err != nil {
fmt.Println(err)
}
}()
var bar = []int{1}
fmt.Println(bar[1])
}
func main(){
foo()
fmt.Println("exit")
}
输出：
runtime error: index out of range
exit
recoverexitrecoverpanic: runtime error: index out of range
goroutine 1 [running]:
main.foo()
/home/letian/work/go/src/test/test.go:14 +0x3e
main.main()
/home/letian/work/go/src/test/test.go:18 +0x22
exit status 2
-
访问未初始化的指针或 nil 指针
对于有 c/c++ 开发经验的人来说，这个很好理解。但对于没用过指针的新手来说，这是最常见的一类错误。
如下代码可以验证：
package main
import (
"fmt"
)
func foo(){
defer func(){
if err := recover(); err != nil {
fmt.Println(err)
}
}()
var bar *int
fmt.Println(*bar)
}
func main(){
foo()
fmt.Println("exit")
}
输出：
runtime error: invalid memory address or nil pointer dereference
exit
recoverpanic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x4869ff]
goroutine 1 [running]:
main.foo()
/home/letian/work/go/src/test/test.go:14 +0x3f
main.main()
/home/letian/work/go/src/test/test.go:18 +0x22
exit status 2
chanchanpackage main
import (
"fmt"
)
func foo(){
defer func(){
if err := recover(); err != nil {
fmt.Println(err)
}
}()
var bar = make(chan int, 1)
close(bar)
bar<-1
}
func main(){
foo()
fmt.Println("exit")
}
输出：
send on closed channel
exit
如果注释掉 recover ，将输出：
panic: send on closed channel
goroutine 1 [running]:
main.foo()
/home/letian/work/go/src/test/test.go:15 +0x83
main.main()
/home/letian/work/go/src/test/test.go:19 +0x22
exit status 2
src/runtime/chan.gochansend// src/runtime/chan.go 第 269 行
// 如果 block 不为 nil ，则协议将不会休眠，但如果无法完成则返回。
// 当关闭休眠中的通道时，可以使用 g.param == nil 唤醒睡眠。
// 我们可以非常容易循环并重新运行该操作，并且将会看到它处于已关闭状态。
func chansend(c *hchan, ep unsafe.Pointer, block bool, callerpc uintptr) bool {
if c == nil {
if !block {
return false
}
gopark(nil, nil, waitReasonChanSendNilChan, traceEvGoStop, 2)
throw("unreachable")
}
if debugChan {
print("chansend: chan=", c, "\n")
}
if raceenabled {
racereadpc(c.raceaddr(), callerpc, funcPC(chansend))
}
// Fast path: check for failed non-blocking operation without acquiring the lock.
//
// After observing that the channel is not closed, we observe that the channel is
// not ready for sending. Each of these observations is a single word-sized read
// (first c.closed and second c.recvq.first or c.qcount depending on kind of channel).
// Because a closed channel cannot transition from 'ready for sending' to
// 'not ready for sending', even if the channel is closed between the two observations,
// they imply a moment between the two when the channel was both not yet closed
// and not ready for sending. We behave as if we observed the channel at that moment,
// and report that the send cannot proceed.
//
// It is okay if the reads are reordered here: if we observe that the channel is not
// ready for sending and then observe that it is not closed, that implies that the
// channel wasn't closed during the first observation.
if !block && c.closed == 0 && ((c.dataqsiz == 0 && c.recvq.first == nil) ||
(c.dataqsiz > 0 && c.qcount == c.dataqsiz)) {
return false
}
var t0 int64
if blockprofilerate > 0 {
t0 = cputicks()
}
lock(&c.lock)
if c.closed != 0 {
unlock(&c.lock)
panic(plainError("send on closed channel"))
}
if sg := c.recvq.dequeue(); sg != nil {
// Found a waiting receiver. We pass the value we want to send
// directly to the receiver, bypassing the channel buffer (if any).
send(c, sg, ep, func() { unlock(&c.lock) }, 3)
return true
}
if c.qcount < c.dataqsiz {
// Space is available in the channel buffer. Enqueue the element to send.
qp := chanbuf(c, c.sendx)
if raceenabled {
raceacquire(qp)
racerelease(qp)
}
typedmemmove(c.elemtype, qp, ep)
c.sendx++
if c.sendx == c.dataqsiz {
c.sendx = 0
}
c.qcount++
unlock(&c.lock)
return true
}
if !block {
unlock(&c.lock)
return false
}
// Block on the channel. Some receiver will complete our operation for us.
gp := getg()
mysg := acquireSudog()
mysg.releasetime = 0
if t0 != 0 {
mysg.releasetime = -1
}
// No stack splits between assigning elem and enqueuing mysg
// on gp.waiting where copystack can find it.
mysg.elem = ep
mysg.waitlink = nil
mysg.g = gp
mysg.isSelect = false
mysg.c = c
gp.waiting = mysg
gp.param = nil
c.sendq.enqueue(mysg)
goparkunlock(&c.lock, waitReasonChanSend, traceEvGoBlockSend, 3)
// Ensure the value being sent is kept alive until the
// receiver copies it out. The sudog has a pointer to the
// stack object, but sudogs aren't considered as roots of the
// stack tracer.
KeepAlive(ep)
// someone woke us up.
if mysg != gp.waiting {
throw("G waiting list is corrupted")
}
gp.waiting = nil
if gp.param == nil {
if c.closed == 0 {
throw("chansend: spurious wakeup")
}
panic(plainError("send on closed channel"))
}
gp.param = nil
if mysg.releasetime > 0 {
blockevent(mysg.releasetime-t0, 2)
}
mysg.c = nil
releaseSudog(mysg)
return true
}
-
并发读写相同 map
对于刚学并发编程的同学来说，并发读写 map 也是很容易遇到的问题。如下代码可以验证：
package main
import (
"fmt"
)
func foo(){
defer func(){
if err := recover(); err != nil {
fmt.Println(err)
}
}()
var bar = make(map[int]int)
go func(){
defer func(){
if err := recover(); err != nil {
fmt.Println(err)
}
}()
for{
_ = bar[1]
}
}()
for{
bar[1]=1
}
}
func main(){
foo()
fmt.Println("exit")
}
输出：
fatal error: concurrent map read and map write
goroutine 5 [running]:
runtime.throw(0x4bd8b0, 0x21)
/home/letian/.gvm/gos/go1.12/src/runtime/panic.go:617 +0x72 fp=0xc00004c780 sp=0xc00004c750 pc=0x427f22
runtime.mapaccess1_fast64(0x49eaa0, 0xc000088180, 0x1, 0xc0000260d8)
/home/letian/.gvm/gos/go1.12/src/runtime/map_fast64.go:21 +0x1a8 fp=0xc00004c7a8 sp=0xc00004c780 pc=0x40eb58
main.foo.func2(0xc000088180)
/home/letian/work/go/src/test/test.go:21 +0x5c fp=0xc00004c7d8 sp=0xc00004c7a8 pc=0x48708c
runtime.goexit()
/home/letian/.gvm/gos/go1.12/src/runtime/asm_amd64.s:1337 +0x1 fp=0xc00004c7e0 sp=0xc00004c7d8 pc=0x450e51
created by main.foo
/home/letian/work/go/src/test/test.go:14 +0x68
goroutine 1 [runnable]:
main.foo()
/home/letian/work/go/src/test/test.go:25 +0x8b
main.main()
/home/letian/work/go/src/test/test.go:30 +0x22
exit status 2
exitsrc/runtime/map.go if h.flags&hashWriting != 0 {
throw("concurrent map read and map write")
}
runtimethrowrecoverinterfaceinterfacepackage main
import (
"fmt"
)
func foo(){
defer func(){
if err := recover(); err != nil {
fmt.Println(err)
}
}()
var i interface{} = "abc"
_ = i.([]string)
}
func main(){
foo()
fmt.Println("exit")
}
输出：
interface conversion: interface {} is string, not []string
exit
src/runtime/iface.go// panicdottypeE is called when doing an e.(T) conversion and the conversion fails.
// have = the dynamic type we have.
// want = the static type we're trying to convert to.
// iface = the static type we're converting from.
func panicdottypeE(have, want, iface *_type) {
panic(&TypeAssertionError{iface, have, want, ""})
}
// panicdottypeI is called when doing an i.(T) conversion and the conversion fails.
// Same args as panicdottypeE, but "have" is the dynamic itab we have.
func panicdottypeI(have *itab, want, iface *_type) {
var t *_type
if have != nil {
t = have._type
}
panicdottypeE(t, want, iface)
}
更多的 panic
panicpanicpanic(限于篇幅，本文暂不介绍填坑技巧，后面再开其他篇幅逐个介绍。
感谢阅读！
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Go语言踩坑记之channel与goroutine