netdata/netdata

// SPDX-License-Identifier: GPL-3.0-or-later

package logs

import (
    "errors"
    "os"

    "github.com/clbanning/rfile/v2"
)

const DefaultMaxLineWidth = 4 * 1024 // assume disk block size is 4K

var ErrTooLongLine = errors.New("too long line")

// ReadLastLine returns the last line of the file and any read error encountered.
// It expects last line width <= maxLineWidth.
// If maxLineWidth <= 0, it defaults to DefaultMaxLineWidth.
func ReadLastLine(filename string, maxLineWidth int64) ([]byte, error) {
    if maxLineWidth <= 0 {
        maxLineWidth = DefaultMaxLineWidth
    }
    f, err := os.Open(filename)
    if err != nil {
        return nil, err
    }
    defer func() { _ = f.Close() }()

    stat, _ := f.Stat()
    endPos := stat.Size()
    if endPos == 0 {
        return []byte{}, nil
    }
    startPos := endPos - maxLineWidth
    if startPos < 0 {
        startPos = 0
    }
    buf := make([]byte, endPos-startPos)
    n, err := f.ReadAt(buf, startPos)
    if err != nil {
        return nil, err
    }
    lnPos := 0
    foundLn := false
    for i := n - 2; i >= 0; i-- {
        ch := buf[i]
        if ch == '\n' {
            foundLn = true
            lnPos = i
            break
        }
    }
    if foundLn {
        return buf[lnPos+1 : n], nil
    }
    if startPos == 0 {
        return buf[0:n], nil
    }

    return nil, ErrTooLongLine
}

func ReadLastLines(filename string, n uint) ([]string, error) {
    return rfile.Tail(filename, int(n))
}