On Mon, Mar 9, 2020 at 9:16 AM Yuu LongXue <longxue...@gmail.com> wrote:
>
> Hi all,
>
> Slice is not thread safe for write operation if without lock, this is true
> and I know this feature. but recently, I got a panic which caused by writing
> a slice concurrently with out lock occasionally and I can't figure out why.
>
> ### the process as below:
> 1. define a slice: var x []string
> 2. append to x concurrently without lock in multi goroutines
> 3. wait all goroutines to be done
> 4. iterate the x slice, panic occurred when read some element with index
> <strong>i (i > 0 && i < len(x)-1)</strong> in the x
>
> ### the question is:
> In this case, the element with index 0 and index len(x) - 1 is ok to read,
> but it exists a <strong>x[i](i > 0 && i < len(x)-1)</strong> that can't be
> read and it will lead to panic[invalid memory address or nil pointer
> dereference];
You have a race, so the behavior is undefined. One can only guess what
went wrong. It is possible that you have a corrupt string in the
slice, one with len>0 but with a nil backing array, because you have
many goroutines writing to the same element of the slice, and you
don't know which one wrote what in what order.
>
> I know that some data by the write operation missed, but how could the panic
> occur? I have tried my best to know why, but I still can't figure out;
> <strong>so could anyone tell me why the element in x has a invalid address?
> maybe it was collected by gc or not initialized yet and why?<strong>
>
> ### code
> ```
> func TestConcurrentWriteSlice(t *testing.T) {
> // panic do not occurred for each running, so I repeat until it occurred
> for i := 0; i < 100; i++ {
> testConcurrentWriteSlice()
> }
> }
>
> func testConcurrentWriteSlice() {
> var strs []string
>
> // write concurrently here
> var wg sync.WaitGroup
> for x := 0; x < 1000; x++ {
> wg.Add(1)
> go func() {
> defer func() { wg.Done() }()
> time.Sleep(2 * time.Second)
> for i := 65; i < 91; i++ {
> strs = append(strs, string(i))
> }
> }()
> }
> wg.Wait()
>
> l := len(strs)
> log.Printf("strs len: %v f=%v l=%v", l, strs[0], strs[l-1])
> for i := 0; i < l; i++ {
> log.Printf("strs len=%v i=%v first=%v last=%v
> ¤t[i]=%v", l, i, strs[0], strs[l-1], &strs[i])
>
> // panic here
> log.Printf("strs len=%v i=%v first=%v last=%v current=%v", l,
> i, strs[0], strs[l-1], strs[i])
> }
> }
> ```
>
> ### output log
> ```
> 2020/03/09 14:39:08 strs len=2607 i=519 first=A last=Z
> ¤t[i]=0xc000606070
> 2020/03/09 14:39:08 strs len=2607 i=519 first=A last=Z current=
> 2020/03/09 14:39:08 strs len=2607 i=520 first=A last=Z
> ¤t[i]=0xc000606080
> 2020/03/09 14:39:08 strs len=2607 i=520 first=A last=Z current=
> 2020/03/09 14:39:08 strs len=2607 i=521 first=A last=Z
> ¤t[i]=0xc000606090
> 2020/03/09 14:39:08 strs len=2607 i=521 first=A last=Z current=
> 2020/03/09 14:39:08 strs len=2607 i=522 first=A last=Z
> ¤t[i]=0xc0006060a0
> 2020/03/09 14:39:08 strs len=2607 i=522 first=A last=Z current=
> 2020/03/09 14:39:08 strs len=2607 i=523 first=A last=Z
> ¤t[i]=0xc0006060b0
> --- FAIL: TestConcurrentWriteSlice (19.72s)
> panic: runtime error: invalid memory address or nil pointer dereference
> [recovered]
> panic: runtime error: invalid memory address or nil pointer
> dereference
> [signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x105aa93]
>
> goroutine 33 [running]:
> testing.tRunner.func1(0xc0000d8100)
> /usr/local/go/src/testing/testing.go:874 +0x3a3
> panic(0x1127f20, 0x12494b0)
> /usr/local/go/src/runtime/panic.go:679 +0x1b2
> fmt.(*buffer).writeString(...)
> /usr/local/go/src/fmt/print.go:82
> fmt.(*fmt).padString(0xc000082040, 0x0, 0x115ced0)
> /usr/local/go/src/fmt/format.go:110 +0x8c
> fmt.(*fmt).fmtS(0xc000082040, 0x0, 0x115ced0)
> /usr/local/go/src/fmt/format.go:359 +0x61
> fmt.(*pp).fmtString(0xc000082000, 0x0, 0x115ced0, 0x76)
> /usr/local/go/src/fmt/print.go:447 +0x131
> fmt.(*pp).printArg(0xc000082000, 0x111c3a0, 0xc00011ed80, 0x76)
> /usr/local/go/src/fmt/print.go:698 +0x877
> fmt.(*pp).doPrintf(0xc000082000, 0x115b38e, 0x2c, 0xc0000c9ef8, 0x5, 0x5)
> /usr/local/go/src/fmt/print.go:1030 +0x15b
> fmt.Sprintf(0x115b38e, 0x2c, 0xc000840ef8, 0x5, 0x5, 0x1, 0xc00011ed80)
> /usr/local/go/src/fmt/print.go:219 +0x66
> log.Printf(0x115b38e, 0x2c, 0xc000840ef8, 0x5, 0x5)
> /usr/local/go/src/log/log.go:307 +0x53
> github.com/test/project/util.testConcurrentWriteSlice()
> /Users/zhangqinxue/work/github/demo/go/project/util/slice_test.go:36
> +0x323
> github.com/test/project/util.TestConcurrentWriteSlice(0xc0000d8100)
> /Users/zhangqinxue/work/github/demo/go/project/util/slice_test.go:12
> +0x2a
> testing.tRunner(0xc0000d8100, 0x115caf0)
> /usr/local/go/src/testing/testing.go:909 +0xc9
> created by testing.(*T).Run
> /usr/local/go/src/testing/testing.go:960 +0x350
> ```
>
> Thanks
>
> YUU
>
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