[go-nuts] append slice concurrently without lock，after that panic occurred when iterate the slice

[Yuu LongXue]

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];

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 &current[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 &current[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 &current[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 &current[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 &current[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 &current[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

-- 
You received this message because you are subscribed to the Google Groups 
"golang-nuts" group.
To unsubscribe from this group and stop receiving emails from it, send an email 
to golang-nuts+unsubscr...@googlegroups.com.
To view this discussion on the web visit 
https://groups.google.com/d/msgid/golang-nuts/122CE4C2-A82A-4819-BFBD-E6E98A71BF7F%40gmail.com.