github.com/mvdan/u-root-coreutils@v0.0.0-20230122170626-c2eef2898555/pkg/rand/random_urandom.go (about)

     1  // Copyright 2019 the u-root Authors. All rights reserved
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  //go:build aix || darwin || dragonfly || freebsd || nacl || netbsd || openbsd || solaris || linux
     6  // +build aix darwin dragonfly freebsd nacl netbsd openbsd solaris linux
     7  
     8  package rand
     9  
    10  import (
    11  	"context"
    12  	"fmt"
    13  	"sync"
    14  	"syscall"
    15  
    16  	"golang.org/x/sys/unix"
    17  )
    18  
    19  // urandomReader is a contextReader.
    20  type urandomReader struct {
    21  	once sync.Once
    22  
    23  	// fd is expected to be non-blocking.
    24  	fd int
    25  }
    26  
    27  func (r *urandomReader) init() error {
    28  	var realErr error
    29  	r.once.Do(func() {
    30  		fd, err := unix.Open("/dev/urandom", unix.O_RDONLY, 0)
    31  		if err != nil {
    32  			realErr = fmt.Errorf("open(/dev/urandom): %v", err)
    33  			return
    34  		}
    35  		r.fd = fd
    36  	})
    37  	return realErr
    38  }
    39  
    40  // ReadContext implements a cancelable read from /dev/urandom.
    41  func (r *urandomReader) ReadContext(ctx context.Context, b []byte) (int, error) {
    42  	if err := r.init(); err != nil {
    43  		return 0, err
    44  	}
    45  	for {
    46  		n, err := unix.Read(r.fd, b)
    47  		if err == nil {
    48  			return n, err
    49  		}
    50  		select {
    51  		case <-ctx.Done():
    52  			return 0, ctx.Err()
    53  
    54  		default:
    55  			if err != nil && err != syscall.EAGAIN && err != syscall.EINTR {
    56  				return n, err
    57  			}
    58  		}
    59  	}
    60  }