log_offset.py

SEEK_BEG = 0
SEEK_SET = 1
SEEK_END = 2
BLOCK_SIZE = 10

FILE_NAME = "console.log"


def get_console_output(length, offset, origin=SEEK_END):
    if origin == SEEK_END:
        offset *= -1

    with open(FILE_NAME, "rb") as f:
        offset -= length
        f.seek(offset, origin)
        bytes_read = f.read(length)
        pos = f.tell()

    # Return bytes and position of last unread byte from the begin.
    # Subtract length to pos since we will read in opposite direction
    return bytes_read, pos - length


if __name__ == "__main__":
    data = []

    bytes_array, pos = get_console_output(BLOCK_SIZE, 20, SEEK_END)
    data.append(bytes_array)
    bytes_array, pos = get_console_output(BLOCK_SIZE, pos, origin=SEEK_BEG)
    data.append(bytes_array)
    bytes_array, pos = get_console_output(BLOCK_SIZE, pos, origin=SEEK_BEG)
    data.append(bytes_array)
    print("".join(data[::-1]))

The code above returns:

106
107
1101
92
93
94

Which is not the expected data.

The issue is the content of the file which will be under constant change when log rotation is enabled:

1st      2nd      3rd
read     read     read
------------------------
  1        2        3        
  2        3        4
  3        4        5
  4        5        6
  5        6        7           
  6        7        8
  7        8        9
  8        9       10
  9       10       11
 10       11       12
------------------------