randutil.go

     1  // Copyright 2018 The Go 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  // Package randutil contains internal randomness utilities for various
     6  // crypto packages.
     7  package randutil
     8  
     9  import (
    10  	"io"
    11  	"sync"
    12  )
    13  
    14  var (
    15  	closedChanOnce sync.Once
    16  	closedChan     chan struct{}
    17  )
    18  
    19  // MaybeReadByte reads a single byte from r with ~50% probability. This is used
    20  // to ensure that callers do not depend on non-guaranteed behaviour, e.g.
    21  // assuming that rsa.GenerateKey is deterministic w.r.t. a given random stream.
    22  //
    23  // This does not affect tests that pass a stream of fixed bytes as the random
    24  // source (e.g. a zeroReader).
    25  func MaybeReadByte(r io.Reader) {
    26  	closedChanOnce.Do(func() {
    27  		closedChan = make(chan struct{})
    28  		close(closedChan)
    29  	})
    30  
    31  	select {
    32  	case <-closedChan:
    33  		return
    34  	case <-closedChan:
    35  		var buf [1]byte
    36  		r.Read(buf[:])
    37  	}
    38  }
    39
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