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// Copyright 2010 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.
//go:build unix
// Unix cryptographically secure pseudorandom number
// generator.
package rand
import (
"crypto/internal/boring"
"errors"
"io"
"os"
"sync"
"sync/atomic"
"syscall"
"time"
)
const urandomDevice = "/dev/urandom"
func init() {
if boring.Enabled {
Reader = boring.RandReader
return
}
Reader = &reader{}
}
// A reader satisfies reads by reading from urandomDevice
type reader struct {
f io.Reader
mu sync.Mutex
used atomic.Uint32 // Atomic: 0 - never used, 1 - used, but f == nil, 2 - used, and f != nil
}
// altGetRandom if non-nil specifies an OS-specific function to get
// urandom-style randomness.
var altGetRandom func([]byte) (err error)
func warnBlocked() {
println("crypto/rand: blocked for 60 seconds waiting to read random data from the kernel")
}
func (r *reader) Read(b []byte) (n int, err error) {
boring.Unreachable()
if r.used.CompareAndSwap(0, 1) {
// First use of randomness. Start timer to warn about
// being blocked on entropy not being available.
t := time.AfterFunc(time.Minute, warnBlocked)
defer t.Stop()
}
if altGetRandom != nil && altGetRandom(b) == nil {
return len(b), nil
}
if r.used.Load() != 2 {
r.mu.Lock()
if r.used.Load() != 2 {
f, err := os.Open(urandomDevice)
if err != nil {
r.mu.Unlock()
return 0, err
}
r.f = hideAgainReader{f}
r.used.Store(2)
}
r.mu.Unlock()
}
return io.ReadFull(r.f, b)
}
// hideAgainReader masks EAGAIN reads from /dev/urandom.
// See golang.org/issue/9205
type hideAgainReader struct {
r io.Reader
}
func (hr hideAgainReader) Read(p []byte) (n int, err error) {
n, err = hr.r.Read(p)
if errors.Is(err, syscall.EAGAIN) {
err = nil
}
return
}
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