optimize OutputEventFilter for large stdout streams

update our event data search algorithm to be a bit lazier in event data
discovery; this drastically improves processing speeds for stdout >5MB

see: https://github.com/ansible/awx/issues/417

awx/main/utils/common.py

@@ -18,6 +18,7 @@ import contextlib
 import tempfile
 import six
 import psutil
+from StringIO import StringIO
 
 # Decorator
 from decorator import decorator
@@ -867,7 +868,8 @@ class OutputEventFilter(object):
         self._event_ct = 0
         self._counter = 1
         self._start_line = 0
-        self._buffer = ''
+        self._buffer = StringIO()
+        self._last_chunk = ''
         self._current_event_data = None
 
     def flush(self):
@@ -878,9 +880,19 @@ class OutputEventFilter(object):
         pass
 
     def write(self, data):
-        self._buffer += data
-        while True:
-            match = self.EVENT_DATA_RE.search(self._buffer)
+        self._buffer.write(data)
+
+        # keep a sliding window of the last chunk written so we can detect
+        # event tokens and determine if we need to perform a search of the full
+        # buffer
+        should_search = '\x1b[K' in (self._last_chunk + data)
+        self._last_chunk = data
+
+        # Only bother searching the buffer if we recently saw a start/end
+        # token (\x1b[K)
+        while should_search:
+            value = self._buffer.getvalue()
+            match = self.EVENT_DATA_RE.search(value)
             if not match:
                 break
             try:
@@ -888,13 +900,17 @@ class OutputEventFilter(object):
                 event_data = json.loads(base64.b64decode(base64_data))
             except ValueError:
                 event_data = {}
-            self._emit_event(self._buffer[:match.start()], event_data)
-            self._buffer = self._buffer[match.end():]
+            self._emit_event(value[:match.start()], event_data)
+            remainder = value[match.end():]
+            self._buffer = StringIO()
+            self._buffer.write(remainder)
+            self._last_chunk = remainder
 
     def close(self):
-        if self._buffer:
-            self._emit_event(self._buffer)
-            self._buffer = ''
+        value = self._buffer.getvalue()
+        if value:
+            self._emit_event(value)
+            self._buffer = StringIO()
         self._event_callback(dict(event='EOF'))
 
     def _emit_event(self, buffered_stdout, next_event_data=None):