github.com/stolowski/snapd@v0.0.0-20210407085831-115137ce5a22/osutil/cmp.go (about)

     1  // -*- Mode: Go; indent-tabs-mode: t -*-
     2  
     3  /*
     4   * Copyright (C) 2014-2015 Canonical Ltd
     5   *
     6   * This program is free software: you can redistribute it and/or modify
     7   * it under the terms of the GNU General Public License version 3 as
     8   * published by the Free Software Foundation.
     9   *
    10   * This program is distributed in the hope that it will be useful,
    11   * but WITHOUT ANY WARRANTY; without even the implied warranty of
    12   * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    13   * GNU General Public License for more details.
    14   *
    15   * You should have received a copy of the GNU General Public License
    16   * along with this program.  If not, see <http://www.gnu.org/licenses/>.
    17   *
    18   */
    19  
    20  package osutil
    21  
    22  import (
    23  	"bytes"
    24  	"io"
    25  	"os"
    26  )
    27  
    28  const defaultChunkSize = 16 * 1024
    29  
    30  func filesAreEqualChunked(a, b string, chunkSize int) bool {
    31  	fa, err := os.Open(a)
    32  	if err != nil {
    33  		return false
    34  	}
    35  	defer fa.Close()
    36  
    37  	fb, err := os.Open(b)
    38  	if err != nil {
    39  		return false
    40  	}
    41  	defer fb.Close()
    42  
    43  	fia, err := fa.Stat()
    44  	if err != nil {
    45  		return false
    46  	}
    47  
    48  	fib, err := fb.Stat()
    49  	if err != nil {
    50  		return false
    51  	}
    52  
    53  	if fia.Size() != fib.Size() {
    54  		return false
    55  	}
    56  
    57  	return streamsEqualChunked(fa, fb, chunkSize)
    58  }
    59  
    60  // FilesAreEqual compares the two files' contents and returns whether
    61  // they are the same.
    62  func FilesAreEqual(a, b string) bool {
    63  	return filesAreEqualChunked(a, b, 0)
    64  }
    65  
    66  func streamsEqualChunked(a, b io.Reader, chunkSize int) bool {
    67  	if a == b {
    68  		return true
    69  	}
    70  	if chunkSize <= 0 {
    71  		chunkSize = defaultChunkSize
    72  	}
    73  	bufa := make([]byte, chunkSize)
    74  	bufb := make([]byte, chunkSize)
    75  	for {
    76  		ra, erra := io.ReadAtLeast(a, bufa, chunkSize)
    77  		rb, errb := io.ReadAtLeast(b, bufb, chunkSize)
    78  		if erra == io.EOF && errb == io.EOF {
    79  			return true
    80  		}
    81  		if erra != nil || errb != nil {
    82  			// if both files finished in the middle of a
    83  			// ReadAtLeast, (returning io.ErrUnexpectedEOF), then we
    84  			// still need to check what was read to know whether
    85  			// they're equal.  Otherwise, we know they're not equal
    86  			// (because we count any read error as a being non-equal
    87  			// also).
    88  			tailMightBeEqual := erra == io.ErrUnexpectedEOF && errb == io.ErrUnexpectedEOF
    89  			if !tailMightBeEqual {
    90  				return false
    91  			}
    92  		}
    93  		if !bytes.Equal(bufa[:ra], bufb[:rb]) {
    94  			return false
    95  		}
    96  	}
    97  }
    98  
    99  // StreamsEqual compares two streams and returns true if both
   100  // have the same content.
   101  func StreamsEqual(a, b io.Reader) bool {
   102  	return streamsEqualChunked(a, b, 0)
   103  }