math/big: non-deterministic behaviour of big.Rand #42701
Description
What version of Go are you using (go version)?
$ go version go version go1.15.5 linux/amd64
Does this issue reproduce with the latest release?
Yes
What operating system and processor architecture are you using (go env)?
go env Output
$ go env GO111MODULE="" GOARCH="amd64" GOBIN="" GOCACHE="/home/user/.cache/go-build" GOENV="/home/user/.config/go/env" GOEXE="" GOFLAGS="" GOHOSTARCH="amd64" GOHOSTOS="linux" GOINSECURE="" GOMODCACHE="/home/user/go/pkg/mod" GONOPROXY="" GONOSUMDB="" GOOS="linux" GOPATH="/home/user/go" GOPRIVATE="" GOPROXY="https://proxy.golang.org,direct" GOROOT="/usr/local/go" GOSUMDB="sum.golang.org" GOTMPDIR="" GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64" GCCGO="gccgo" AR="ar" CC="gcc" CXX="g++" CGO_ENABLED="1" GOMOD="/home/user/go/src/github.com/ethereum/go-ethereum/go.mod" CGO_CFLAGS="-g -O2" CGO_CPPFLAGS="" CGO_CXXFLAGS="-g -O2" CGO_FFLAGS="-g -O2" CGO_LDFLAGS="-g -O2" PKG_CONFIG="pkg-config" GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build512723341=/tmp/go-build -gno-record-gcc-switches"
What did you do?
main.go (edit: I added the wrong code first):
package main
import (
"fmt"
"math/big"
"math/rand"
)
type rndTest struct{
count int
src rand.Source
}
func (r *rndTest) Int63() int64 {
r.count++
return r.src.Int63()
}
func (r *rndTest) Seed(seed int64) {
r.src.Seed(seed)
}
func main(){
shim := &rndTest{0, rand.NewSource(0)}
random := rand.New(shim)
max := new(big.Int).Exp(big.NewInt(2), big.NewInt(256), nil)
fmt.Printf("%v (%d reads)\n", new(big.Int).Rand(random, max), shim.count)
fmt.Printf("%v (%d reads)\n", new(big.Int).Rand(random, max), shim.count)
}output:
$ go run .
75882278871742073348705908634037840808905895098001936781526057361623699972085 (10 reads)
32980378865574341698424589833777768979918116503443097246085029326460293060374 (30 reads)
$ GOOS=linux GOARCH=386 go run .
75882278871742073348705908634037840808905895098001936781526057361623699972085 (9 reads)
80252028366164880526544510430800498761927255387255296632111478555188670606789 (27 reads)
What did you expect to see?
I expected the big.Rand to perform deterministic amount of reads from the source, regardless of whether it was executed on a 32-bit or 64-bit platform
What did you see instead?
That different amounts of read was performed.
In my case, this caused certain testcases to fail on 32-bit platforms, since although the exact random steps were unimportant, the differing amounts of reads caused the source to go "out of sync", thus turning what should have been a deterministic process into two different deterministic processes (one for 64-bit and one for 32-bit).
This seems wrong to me, but there are obviously ways to work around it, so can't really say it's a biggie. Maybe there are other projects bit by this in other ways though, so the behaviour should at least be documented.