mirror of https://github.com/containers/podman.git
67 lines
1.6 KiB
Go
67 lines
1.6 KiB
Go
package reversereader
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import (
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"errors"
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"fmt"
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"io"
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"os"
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)
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// ReverseReader structure for reading a file backwards
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type ReverseReader struct {
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reader *os.File
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offset int64
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readSize int64
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}
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// NewReverseReader returns a reader that reads from the end of a file
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// rather than the beginning. It sets the readsize to pagesize and determines
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// the first offset using modulus.
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func NewReverseReader(reader *os.File) (*ReverseReader, error) {
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// pagesize should be safe for memory use and file reads should be on page
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// boundaries as well
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pageSize := int64(os.Getpagesize())
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stat, err := reader.Stat()
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if err != nil {
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return nil, err
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}
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// figure out the last page boundary
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remainder := stat.Size() % pageSize
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end, err := reader.Seek(0, 2)
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if err != nil {
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return nil, err
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}
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// set offset (starting position) to the last page boundary or
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// zero if fits in one page
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startOffset := end - remainder
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if startOffset < 0 {
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startOffset = 0
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}
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rr := ReverseReader{
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reader: reader,
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offset: startOffset,
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readSize: pageSize,
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}
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return &rr, nil
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}
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// ReverseReader reads from a given offset to the previous offset and
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// then sets the newoff set one pagesize less than the previous read.
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func (r *ReverseReader) Read() (string, error) {
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if r.offset < 0 {
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return "", fmt.Errorf("at beginning of file: %w", io.EOF)
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}
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// Read from given offset
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b := make([]byte, r.readSize)
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n, err := r.reader.ReadAt(b, r.offset)
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if err != nil && !errors.Is(err, io.EOF) {
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return "", err
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}
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if int64(n) < r.readSize {
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b = b[0:n]
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}
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// Move the offset one pagesize up
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r.offset -= r.readSize
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return string(b), nil
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}
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