ianlancetaylor
commented
7 years ago This is not just using reflect. This similar program shows the same problem using an ordinary select statement.
package main
import (
"fmt"
)
func main() {
in1 := make(chan int, 15000)
in2 := make(chan int, 15000)
for i := 0; i < 15000; i++ {
in1 <- 1
}
for i := 0; i < 15000; i++ {
in2 <- 2
}
out := make(chan int)
go func() {
c1 := make(chan struct{})
c2 := make(chan struct{})
for {
var v int
select {
case <-c1:
case <-c2:
case v = <-in2:
case v = <-in1:
}
out <- v
}
}()
var nb1, nb2 int = 0, 0
ConsumerLoop:
for {
select {
case d := <-out:
switch d {
case 1:
nb1++
if nb1 > 5000 { // stop consume. At this point, messages should be the equidistribution
break ConsumerLoop
}
case 2:
nb2++
default:
panic(d)
}
}
}
print(nb1, "\n")
print(nb2, "\n")
if !areSameOrder(nb1, nb2) {
fmt.Printf("Should have as many nb1 as nb2, got %d nb1 and %d nb2\n", nb1, nb2)
}
}
func areSameOrder(a, b int) bool {
ratio := float32(a) / float32(b)
return ratio < 1.3 && ratio > 0.7
}
zigo101
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What version of Go are you using (
go version)?Does this issue reproduce with the latest release?
yes, on 1.9
What operating system and processor architecture are you using (
go env)?GOARCH="amd64" GOBIN="" GOEXE="" GOHOSTARCH="amd64" GOHOSTOS="linux" GOOS="linux" GOPATH="/home/bchenebault/DEV/git-oab.si.fr.intraorange/BU900102/ceo-be" GORACE="" GOROOT="/usr/local/go" GOTOOLDIR="/usr/local/go/pkg/tool/linux_amd64" GCCGO="gccgo" CC="gcc" GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build913166031=/tmp/go-build -gno-record-gcc-switches" CXX="g++" CGO_ENABLED="1" CGO_CFLAGS="-g -O2" CGO_CPPFLAGS="" CGO_CXXFLAGS="-g -O2" CGO_FFLAGS="-g -O2" CGO_LDFLAGS="-g -O2" PKG_CONFIG="pkg-config"
What did you do?
A Go select statement with reflect package function
reflect.Select(...)should block until at least one of the cases can proceed and make a uniform pseudo-random choice. It appears that the behavior has changed since returned values are no longer pseudo-random in specific cases only.Our case is the following : 2 signaling channels preceding 2 "logic" channels https://play.golang.org/p/0TiMtsRk03
But the same version with only one channel preceding the 2 others works https://play.golang.org/p/sy4RjEEjdg
This case perfectly works on go1.8.3 and below.
What did you expect to see?
A pseudo random distribution
What did you see instead?
A strange distribution, surely not pseudo random