github.com/containerd/nerdctl/v2@v2.0.0-beta.5.0.20240520001846-b5758f54fa28/pkg/logging/tail/tail.go (about)

     1  /*
     2     Copyright The containerd 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  /*
    18  	Forked from https://github.com/kubernetes/kubernetes/blob/master/pkg/util/tail/tail.go
    19  	Copyright The Kubernetes Authors.
    20  	Licensed under the Apache License, Version 2.0
    21  */
    22  
    23  package tail
    24  
    25  import (
    26  	"bytes"
    27  	"io"
    28  )
    29  
    30  const (
    31  	// blockSize is the block size used in tail.
    32  	blockSize = 1024
    33  )
    34  
    35  var (
    36  	// eol is the end-of-line sign in the log.
    37  	eol = []byte{'\n'}
    38  )
    39  
    40  // FindTailLineStartIndex returns the start of last nth line.
    41  // * If n <= 0, return the beginning of the file.
    42  // * If n > 0, return the beginning of last nth line.
    43  // Notice that if the last line is incomplete (no end-of-line), it will not be counted
    44  // as one line.
    45  func FindTailLineStartIndex(f io.ReadSeeker, n uint) (int64, error) {
    46  	if n <= 0 {
    47  		return 0, nil
    48  	}
    49  	size, err := f.Seek(0, io.SeekEnd)
    50  	if err != nil {
    51  		return 0, err
    52  	}
    53  	var left, cnt int64
    54  	buf := make([]byte, blockSize)
    55  	for right := size; right > 0 && uint(cnt) <= n; right -= blockSize {
    56  		left = right - blockSize
    57  		if left < 0 {
    58  			left = 0
    59  			buf = make([]byte, right)
    60  		}
    61  		if _, err := f.Seek(left, io.SeekStart); err != nil {
    62  			return 0, err
    63  		}
    64  		if _, err := f.Read(buf); err != nil {
    65  			return 0, err
    66  		}
    67  		cnt += int64(bytes.Count(buf, eol))
    68  	}
    69  	for ; uint(cnt) > n; cnt-- {
    70  		idx := bytes.Index(buf, eol) + 1
    71  		buf = buf[idx:]
    72  		left += int64(idx)
    73  	}
    74  	return left, nil
    75  }