include class like UNIX tail -f for displaying logs

This allows fdroidserver to easily log activity while displaying it at the
same time.

fdroidserver/tail.py

@@ -0,0 +1,103 @@
+#!/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, encoding='utf8') 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