zcatfollow

#!/usr/bin/python
#
# Will uncompress a gzipped file in "streaming" mode,
# like `tail -b +0 -f ` would do.
#
# If you pass -n <int> parameter it will try to act like
# a smart "tail" command: reads until the end of the current
# file, shows you the last <int> lines, then "follows" the file.
#
# If the writer is writing faster than we read, then it may never
# "detect" the "end of the file"...
#
# License: BSD
# Author: Martin Langhoff <martin.langhoff@gmail.com>
#
import sys
import gzip
import zlib
import time
import getopt
from collections import deque
import signal

try:
    optslist, args = getopt.getopt(sys.argv[1:], "tn:")
    opts={}
    for k,v in optslist:                                                             
    	opts[k] = v

except getopt.GetoptError as err:
    # print help information and exit:
    print(err) # will print something like "option -a not recognized"
    #usage()
    sys.exit(2)

# lookbehind buffer, defined as a list
lbbuf=deque([])
buffering=False
bufferfull=False

if '-n' in opts:
	buffering=True
	lbbufsize=int(opts['-n'])

# tidy up - no errors on ctrl-C, etc
def sig_exit(signum, frame):
	# this will prompt fh cleanup, etc
	sys.exit(0)
signal.signal(signal.SIGHUP,  sig_exit)
signal.signal(signal.SIGINT,  sig_exit)
signal.signal(signal.SIGTERM, sig_exit)

f = gzip.open(args[0], 'r')

# while / readline() sidesteps magic input buffering
while 1:
	try:
		line = f.readline()
	except IOError:
		## we ignore  IOError("CRC check failed")
		## because we are reading an "unfinished"
		## gzip stream...
		# line won't get unset if this triggers
		line = None

	if not line:
		try:
			if f.closed:
				break
		except AttributeError:
			# old gzip does not have "closed"
			pass
		if buffering:
			# got to the current tail of it
			# dump our lookbehind buffer, stop buffering
			for buf_line in lbbuf:
				sys.stdout.write(buf_line)
			buffering=False
		# internal gzip API use?
		# f.decompress.flush()
		time.sleep(0.1)
	else:
		if not buffering:
			sys.stdout.write(line)
		else:
			if bufferfull:
				lbbuf.popleft()
				lbbuf.append(line)
			else:
				lbbuf.append(line)
				if len(lbbuf) >= lbbufsize:
					bufferfull=True