#!/usr/bin/env python

'''
Python-Tail - Unix tail follow implementation in Python.

python-tail can be used to monitor changes to a file.

Example:
    import tail

    # Create a tail instance
    t = tail.Tail('file-to-be-followed')

    # Register a callback function to be called when a new line is found in the followed file.
    # If no callback function is registerd, new lines would be printed to standard out.
    t.register_callback(callback_function)

    # Follow the file with 5 seconds as sleep time between iterations.
    # If sleep time is not provided 1 second is used as the default time.
    t.follow(s=5) '''

# Author - Kasun Herath <kasunh01 at gmail.com>
# Source - https://github.com/kasun/python-tail

# modified by Hans-Christoph Steiner <hans@eds.org> to add the
# background thread and support reading multiple lines per read cycle

import os
import sys
import time
import threading


class Tail(object):
    ''' Represents a tail command. '''
    def __init__(self, tailed_file):
        ''' Initiate a Tail instance.
            Check for file validity, assigns callback function to standard out.

            Arguments:
                tailed_file - File to be followed. '''

        self.check_file_validity(tailed_file)
        self.tailed_file = tailed_file
        self.callback = sys.stdout.write
        self.t_stop = threading.Event()

    def start(self, s=1):
        '''Start tailing a file in a background thread.

        Arguments:
            s - Number of seconds to wait between each iteration; Defaults to 3.
        '''

        t = threading.Thread(target=self.follow, args=(s,))
        t.start()

    def stop(self):
        '''Stop a background tail.
        '''
        self.t_stop.set()

    def follow(self, s=1):
        ''' Do a tail follow. If a callback function is registered it is called with every new line.
        Else printed to standard out.

        Arguments:
            s - Number of seconds to wait between each iteration; Defaults to 1. '''

        with open(self.tailed_file) as file_:
            # Go to the end of file
            file_.seek(0, 2)
            while not self.t_stop.is_set():
                curr_position = file_.tell()
                lines = file_.readlines()
                if len(lines) == 0:
                    file_.seek(curr_position)
                else:
                    for line in lines:
                        self.callback(line)
                time.sleep(s)

    def register_callback(self, func):
        ''' Overrides default callback function to provided function. '''
        self.callback = func

    def check_file_validity(self, file_):
        ''' Check whether the a given file exists, readable and is a file '''
        if not os.access(file_, os.F_OK):
            raise TailError("File '%s' does not exist" % (file_))
        if not os.access(file_, os.R_OK):
            raise TailError("File '%s' not readable" % (file_))
        if os.path.isdir(file_):
            raise TailError("File '%s' is a directory" % (file_))


class TailError(Exception):

    def __init__(self, msg):
        self.message = msg

    def __str__(self):
        return self.message
