Added SVN logging observer.

It isn't triggered by anything yet. When the observer is called, it uses
Blinker to send out a signal for every [T12345] marker it sees in the
first line of each commit log. Those signals aren't connected to anything
yet.

NOTE: this requires the 'svn' Python module , which is a wrapper for the
'svn' commandline client. This client needs to be installed on our docker
when we deploy.

attract_server/subversion.py

@@ -0,0 +1,53 @@
+"""Subversion interface."""
+
+from __future__ import absolute_import
+
+import re
+
+import attr
+import blinker
+import svn.remote
+
+from . import attrs_extra
+
+task_logged = blinker.NamedSignal('task_logged')
+task_marker_re = re.compile(r'\[(?P<task_id>T\d+)\]')
+FETCH_AND_OBSERVE_CHUNK_SIZE = 10
+
+
+def obtain(server_location):
+    """Returns a Connection object for the given server location."""
+
+    return svn.remote.RemoteClient(server_location)
+
+
+@attr.s
+class CommitLogObserver(object):
+    svn_client = attr.ib(validator=attr.validators.instance_of(svn.remote.RemoteClient))
+    last_seen_revision = attr.ib(default=0, validator=attr.validators.instance_of(int))
+    _log = attrs_extra.log('%s.CommitLogObserver' % __name__)
+
+    def fetch_and_observe(self):
+        """Obtains task IDs from SVN logs."""
+
+        self._log.debug('%s: fetch_and_observe()', self)
+
+        svn_log = self.svn_client.log_default(
+            revision_from=self.last_seen_revision + 1,
+            revision_to=self.last_seen_revision + FETCH_AND_OBSERVE_CHUNK_SIZE)
+
+        for log_entry in svn_log:
+            self._log.debug('- %r', log_entry)
+
+            # assumption: revisions are always logged in strictly increasing order.
+            self.last_seen_revision = log_entry.revision
+
+            # Parse the commit log to see if there are any task markers.
+            lines = log_entry.msg.split('\n', 1)
+            first_line = lines[0]
+            for match in task_marker_re.finditer(first_line):
+                task_id = match.group('task_id')
+
+                task_logged.send(self,
+                                 task_id=task_id,
+                                 log_entry=log_entry)