skip already parsed entries in dhcpd.leases

1 files changed, 49 insertions, 7 deletions

diff --git a/at.py b/at.py
index 0c5fb63..22bb220 100644
--- a/at.py
+++ b/at.py
@@ -167,36 +167,73 @@ class CapUpdater(Updater):
 class MtimeUpdater(Updater):
     def __init__(self, lease_file, *a, **kw):
         self.lease_file = lease_file
+        self.position = 0
         self.last_modified = 0
         Updater.__init__(self, *a, **kw)
 
     def file_changed(self, f):
-        pass
+        """Callback on changed lease file
+
+        Args:
+            f: Lease file. File offset can be used to skip already parsed lines.
+
+        Returns: New byte offset pointing after last parsed byte.
+        """
+        return f.tell()
 
     def _trigger_update(self):
         app.logger.info('Lease file changed, updating')
         with open(self.lease_file, 'r') as f:
-            self.file_changed(f)
+            f.seek(self.position)
+            self.position = self.file_changed(f)
 
     def run(self):
+        """Periodicaly check if file has changed
+
+        From ISC DHCPD manual:
+
+            New leases are appended to the end of the dhcpd.leases file.  In
+            order to prevent the file from becoming arbitrarily large, from
+            time to time dhcpd creates a new dhcpd.leases file from its in-core
+            lease database. Once this file has been written to disk, the old
+            file is renamed dhcpd.leases~, and the new file is renamed
+            dhcpd.leases.
+        """
         while True:
             try:
-                mtime = os.stat(self.lease_file).st_mtime
+                stat = os.stat(self.lease_file)
+                mtime = stat.st_mtime
+                size = stat.st_size
+                if size < self.position:
+                    app.logger.info('leases file changed - reseting pointer')
+                    self.position = 0
+                try:
+                    # checking if DHCPD performed cleanup
+                    # cleanup during operation seems to be currently broken
+                    # on customs so this could never execute
+                    purge_time = os.stat(self.lease_file + '~').st_mtime
+                    if purge_time > self.last_modified:
+                        app.logger.info('leases file purged - reseting pointer')
+                        self.position = 0
+                except FileNotFoundError:
+                    pass
                 if mtime > self.last_modified:
                     self._trigger_update()
                 self.last_modified = mtime
-                sleep(3.0)
+                sleep(5.0)
             except Exception as e:
-                app.logger.error('Updater got an exception:\n' +
-                                 traceback.format_exc(e))
+                app.logger.exception('Exception in updater')
                 sleep(10.0)
 
 
 class DnsmasqUpdater(MtimeUpdater):
     def file_changed(self, f):
+        raise NotImplementedError(
+            "This was not tested after adding differential update")
         for line in f:
             ts, hwaddr, ip, name, client_id = line.split(' ')
             self.update(hwaddr, int(ts), ip, name)
+        return f.tell()
 
 
 class DhcpdUpdater(MtimeUpdater):
@@ -206,7 +243,11 @@ class DhcpdUpdater(MtimeUpdater):
         ip = None
         hwaddr = None
         atime = None
-        for line in f:
+        while True:
+            # using readline because iter(file) blocks file.tell usage
+            line = f.readline()
+            if not f:
+                return offset
             line = line.split('#')[0]
             cmd = line.strip().split()
             if not cmd:
@@ -222,6 +263,7 @@ class DhcpdUpdater(MtimeUpdater):
                 if(field == 'hardware'):
                     hwaddr = cmd[2][:-1]
                 if(field.startswith('}')):
+                    offset = f.tell()
                     lease = False
                     if hwaddr is not None and atime is not None:
                         self.update(hwaddr, atime, ip, name)