tail.go

Documentation: k8s.io/kubernetes/pkg/util/tail

     1  /*
     2  Copyright 2017 The Kubernetes Authors.
     3  
     4  Licensed under the Apache License, Version 2.0 (the "License");
     5  you may not use this file except in compliance with the License.
     6  You may obtain a copy of the License at
     7  
     8      http://www.apache.org/licenses/LICENSE-2.0
     9  
    10  Unless required by applicable law or agreed to in writing, software
    11  distributed under the License is distributed on an "AS IS" BASIS,
    12  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    13  See the License for the specific language governing permissions and
    14  limitations under the License.
    15  */
    16  
    17  package tail
    18  
    19  import (
    20  	"bytes"
    21  	"io"
    22  	"os"
    23  )
    24  
    25  const (
    26  	// blockSize is the block size used in tail.
    27  	blockSize = 1024
    28  )
    29  
    30  var (
    31  	// eol is the end-of-line sign in the log.
    32  	eol = []byte{'\n'}
    33  )
    34  
    35  // ReadAtMost reads at most max bytes from the end of the file identified by path or
    36  // returns an error. It returns true if the file was longer than max. It will
    37  // allocate up to max bytes.
    38  func ReadAtMost(path string, max int64) ([]byte, bool, error) {
    39  	f, err := os.Open(path)
    40  	if err != nil {
    41  		return nil, false, err
    42  	}
    43  	defer f.Close()
    44  	fi, err := f.Stat()
    45  	if err != nil {
    46  		return nil, false, err
    47  	}
    48  	size := fi.Size()
    49  	if size == 0 {
    50  		return nil, false, nil
    51  	}
    52  	if size < max {
    53  		max = size
    54  	}
    55  	offset, err := f.Seek(-max, io.SeekEnd)
    56  	if err != nil {
    57  		return nil, false, err
    58  	}
    59  	data, err := io.ReadAll(f)
    60  	return data, offset > 0, err
    61  }
    62  
    63  // FindTailLineStartIndex returns the start of last nth line.
    64  // * If n < 0, return the beginning of the file.
    65  // * If n >= 0, return the beginning of last nth line.
    66  // Notice that if the last line is incomplete (no end-of-line), it will not be counted
    67  // as one line.
    68  func FindTailLineStartIndex(f io.ReadSeeker, n int64) (int64, error) {
    69  	if n < 0 {
    70  		return 0, nil
    71  	}
    72  	size, err := f.Seek(0, io.SeekEnd)
    73  	if err != nil {
    74  		return 0, err
    75  	}
    76  	var left, cnt int64
    77  	buf := make([]byte, blockSize)
    78  	for right := size; right > 0 && cnt <= n; right -= blockSize {
    79  		left = right - blockSize
    80  		if left < 0 {
    81  			left = 0
    82  			buf = make([]byte, right)
    83  		}
    84  		if _, err := f.Seek(left, io.SeekStart); err != nil {
    85  			return 0, err
    86  		}
    87  		if _, err := f.Read(buf); err != nil {
    88  			return 0, err
    89  		}
    90  		cnt += int64(bytes.Count(buf, eol))
    91  	}
    92  	for ; cnt > n; cnt-- {
    93  		idx := bytes.Index(buf, eol) + 1
    94  		buf = buf[idx:]
    95  		left += int64(idx)
    96  	}
    97  	return left, nil
    98  }
    99
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