Live tail of a file

livetail.py

'''
Live tail of a file. Uses mmap to try to be efficient with memory.

Requires fsmonitor: https://github.com/shaurz/fsmonitor

 ...which unfortunately lacks a decent monitor for non-Windows/Linux
systems, so burns up CPU unnecessarily on those platforms. Despite its
reputation, python makes it surprisingly difficult to write clean and
correct code.
'''

# all the python things we need
from contextlib import closing
from mmap import mmap, ACCESS_READ, ALLOCATIONGRANULARITY
from os import fdopen, stat, fstat
from sys import argv, exit, stdout

# the magic
from fsmonitor import FSMonitor


def dump_contents(fd, offset, output):
    '''
    Dump contents attached to a filedescriptor.

    fd - a file descriptor, needs to support mmap
    offset - offset into the file object to start at
    '''

    filesize = fstat(fd).st_size
    gap = filesize - offset
    if gap < 0:
        exit(-1)  # file shrank, can't do much useful

    if gap > 0:
        # some logic here to paper over mmap API's inherent ugliness
        pageoffset = offset % ALLOCATIONGRANULARITY
        mmapoffset = offset - pageoffset
        with closing(mmap(fd, filesize - mmapoffset, access=ACCESS_READ, offset=mmapoffset)) as mapped:
            # handle the mislaigned bits
            if (pageoffset > 0):
                mapped.seek(pageoffset)
                output.write(mapped.read(min(ALLOCATIONGRANULARITY, gap) - pageoffset))
            # iterate through a chunk at a time
            for _ in xrange(pageoffset, filesize, ALLOCATIONGRANULARITY):
                output.write(mapped.read(ALLOCATIONGRANULARITY))
    return filesize


def tail(infile=__file__, outfile=None):
    # disable buffering
    with open(outfile, 'wb', 0) if outfile else fdopen(stdout.fileno(), 'wb', 0) as output:
        with open(infile, 'rb') as f:
            m = FSMonitor()
            m.add_file_watch(infile)
            offset = stat(infile).st_size

            while True:
                for _ in m.read_events():
                    offset = dump_contents(f.fileno(), offset, output)


if __name__ == '__main__':
    tail(*argv[1:])