trace: Add screencapture

Add a poller which takes a screenshot at a configurable interval
(`period`). Files are named based on the device timestamp, and are
placed into a configurable output directory.

devlib/trace/screencapture.py

@@ -0,0 +1,98 @@
+#    Copyright 2018 ARM Limited
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+#
+
+import logging
+import os
+import sys
+import threading
+import time
+
+from devlib.trace import TraceCollector
+from devlib.exception import WorkerThreadError
+
+
+class ScreenCapturePoller(threading.Thread):
+
+    def __init__(self, target, period, output_path=None, timeout=30):
+        super(ScreenCapturePoller, self).__init__()
+        self.target = target
+        self.logger = logging.getLogger('screencapture')
+        self.period = period
+        self.timeout = timeout
+        self.stop_signal = threading.Event()
+        self.lock = threading.Lock()
+        self.last_poll = 0
+        self.daemon = True
+        self.exc = None
+        self.output_path = output_path
+
+    def run(self):
+        self.logger.debug('Starting screen capture polling')
+        try:
+            while True:
+                if self.stop_signal.is_set():
+                    break
+                with self.lock:
+                    current_time = time.time()
+                    if (current_time - self.last_poll) >= self.period:
+                        self.poll()
+                time.sleep(0.5)
+        except Exception:  # pylint: disable=W0703
+            self.exc = WorkerThreadError(self.name, sys.exc_info())
+
+    def stop(self):
+        self.logger.debug('Stopping screen capture polling')
+        self.stop_signal.set()
+        self.join(self.timeout)
+        if self.is_alive():
+            self.logger.error('Could not join screen capture poller thread.')
+        if self.exc:
+            raise self.exc  # pylint: disable=E0702
+
+    def poll(self):
+        self.last_poll = time.time()
+        self.target.capture_screen(os.path.join(self.output_path, "screencap_{ts}.png"))
+
+
+class ScreenCaptureCollector(TraceCollector):
+
+    def __init__(self, target, output_path=None, period=None):
+        super(ScreenCaptureCollector, self).__init__(target)
+        self._collecting = False
+        self.output_path = output_path
+        self.period = period
+        self.target = target
+        self._poller = ScreenCapturePoller(self.target, self.period,
+                                           self.output_path)
+
+    def reset(self):
+        pass
+
+    def start(self):
+        """
+        Start collecting the screenshots
+        """
+        self._poller.start()
+        self._collecting = True
+
+    def stop(self):
+        """
+        Stop collecting the screenshots
+        """
+        if not self._collecting:
+            raise RuntimeError('Screen capture collector is not running, nothing to stop')
+
+        self._poller.stop()
+        self._collecting = False