bytes+strings: Repeat can appear to succeed with an unreasonably large count

[pat42smith]

1. What version of Go are you using (go version)?
   go version go1.6.2 linux/amd64
2. What operating system and processor architecture are you using (go env)?
   GOARCH="amd64"
   GOBIN=""
   GOEXE=""
   GOHOSTARCH="amd64"
   GOHOSTOS="linux"
   GOOS="linux"
   GOPATH="/home/pat/go"
   GORACE=""
   GOROOT="/usr/lib/go"
   GOTOOLDIR="/usr/lib/go/pkg/tool/linux_amd64"
   GO15VENDOREXPERIMENT="1"
   CC="gcc"
   GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0"
   CXX="g++"
   CGO_ENABLED="1"
3. What did you do?
   https://play.golang.org/p/o-5UiG_cKc
4. What did you expect to see?
   A panic because the program tries to allocate too much memory.
5. What did you see instead?
   Both bytes.Repeat and strings.Repeat appear to succeed, but the result is shorter than the input string.

Looking at https://golang.org/src/bytes/bytes.go?s=9819:9858#L381, the cause seems to be that len(b)*count overflows, and happens to produce a result that is a small positive integer. Perhaps the routine should check for overflow and panic if one occurs.

[odeke-em]

I was just looking into fixing this issue but am wondering if a panic might cause incompatibility with pre-go1.8 programs.

[ianlancetaylor]

It's better to panic than to return the wrong answer. Given that we have no way to return an error from these functions, I think panicing is our only option here.

[pat42smith]

I tried https://play.golang.org/p/qQ37uU4mGT to see what might happen in other cases that require more memory than is available. On the playground, that produces a fatal runtime error that seems not to be a panic:

2147483647
runtime: memory allocated by OS (0x7ef60000) not in usable range [0x10400000,0x90400000)
runtime: out of memory: cannot allocate 2147483648-byte block (1048576 in use)
fatal error: out of memory
...

On my desktop, with 64 bit ints, the same program produces a panic:
9223372036854775807
panic: runtime error: makeslice: len out of range

goroutine 1 [running]:
panic(0x4e1f20, 0xc82000a340)
/usr/lib/go/src/runtime/panic.go:481 +0x3e6
strings.Repeat(0x5058a8, 0x1, 0x7fffffffffffffff, 0x0, 0x0)
/usr/lib/go/src/strings/strings.go:427 +0x5f
main.main()
/home/pat/xx.go:9 +0xfc
exit status 2

So a panic for this case would be at least somewhat consistent.

[odeke-em]

Thank you @ianlancetaylor and @pat42smith for the answers, sounds like a plan.

[gopherbot]

CL https://golang.org/cl/29954 mentions this issue.