crypto/rand: rand.Reader.Read is buffered on some systems

[zored]

What version of Go are you using (go version)?

$ go version
go version go1.16.2 darwin/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="/Users/user/Library/Caches/go-build"
GOENV="/Users/user/Library/Application Support/go/env"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOINSECURE=""
GOMODCACHE="/Users/user/go/pkg/mod"
GONOPROXY=""
GONOSUMDB=""
GOOS="darwin"
GOPATH="/Users/user/go/"
GOPRIVATE=""
GOPROXY="https://proxy.golang.org,direct"
GOROOT="/usr/local/Cellar/go/1.16.2/libexec"
GOSUMDB="sum.golang.org"
GOTMPDIR=""
GOTOOLDIR="/usr/local/Cellar/go/1.16.2/libexec/pkg/tool/darwin_amd64"
GOVCS=""
GOVERSION="go1.16.2"
GCCGO="gccgo"
AR="ar"
CC="clang"
CXX="clang++"
CGO_ENABLED="1"
GOMOD="/Users/user/git/gitlab.corp.mail.ru/mayday/payday/worker-service/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 -arch x86_64 -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/85/w4vtlcwn7qvccncnft_b1z1w0000gp/T/go-build752161639=/tmp/go-build -gno-record-gcc-switches -fno-common"

What did you do?

Go Playground:

package main

import (
	"crypto/rand"
	"fmt"
)

func main() {
	for i := 1; i <= 10000; i++ {
		n, err := rand.Reader.Read(make([]byte, 10))
		if n != 10 && err == nil {
			fmt.Printf("expected n=10, actial n=%d on i=%d", n, i)
			return
		}
	}
	fmt.Printf("ok")
}

What did you expect to see?

ok

What did you see instead?

expected n=10, actial n=6 on i=410

More info

Docs say nothing about this.

On Unix machine I get ok.

Seems that macOS uses some buffer limited with 4096 bytes. Is it an expected behaviour?

[mvdan]

This is fine as per the io.Reader godoc:

Read reads up to len(p) bytes into p. It returns the number of bytes read (0 <= n <= len(p)) and any error encountered. Even if Read returns n < len(p), it may use all of p as scratch space during the call. If some data is available but not len(p) bytes, Read conventionally returns what is available instead of waiting for more.

If you want to read an exact number of bytes, use https://golang.org/pkg/io/#ReadFull or https://golang.org/pkg/io/#ReadAtLeast.

[zored]

Thank you for explanations!