instrument: Add support for Monsoon Power Monitor

This adds support for the Monsoon Power Monitor:

https://www.msoon.com/LabEquipment/PowerMonitor/

This device officialy supports only MS Windows so we use a tool from AOSP to
communicate with it from Unix targets:

https://android.googlesource.com/platform/cts/+/master/tools/utils/monsoon.py

The user is required to install this tool and its dependencies, then pass the
path to it as the monsoon_bin parameter.

devlib/__init__.py

@@ -14,6 +14,7 @@ from devlib.instrument import MEASUREMENT_TYPES, INSTANTANEOUS, CONTINUOUS
 from devlib.instrument.daq import DaqInstrument
 from devlib.instrument.energy_probe import EnergyProbeInstrument
 from devlib.instrument.hwmon import HwmonInstrument
+from devlib.instrument.monsoon import MonsoonInstrument
 from devlib.instrument.netstats import NetstatsInstrument
 
 from devlib.trace.ftrace import FtraceCollector

devlib/instrument/monsoon.py

@@ -0,0 +1,132 @@
+import csv
+import os
+import signal
+from subprocess import Popen, PIPE
+from tempfile import NamedTemporaryFile
+from devlib.instrument import Instrument, CONTINUOUS, MeasurementsCsv
+from devlib.exception import HostError
+from devlib.host import PACKAGE_BIN_DIRECTORY
+from devlib.utils.misc import which
+
+INSTALL_INSTRUCTIONS="""
+MonsoonInstrument requires the monsoon.py tool, available from AOSP:
+
+https://android.googlesource.com/platform/cts/+/master/tools/utils/monsoon.py
+
+Download this script and put it in your $PATH (or pass it as the monsoon_bin
+parameter to MonsoonInstrument). `pip install gflags pyserial` to install the
+dependencies.
+"""
+
+class MonsoonInstrument(Instrument):
+    """Instrument for Monsoon Solutions power monitor
+
+    To use this instrument, you need to install the monsoon.py script available
+    from the Android Open Source Project. As of May 2017 this is under the CTS
+    repository:
+
+    https://android.googlesource.com/platform/cts/+/master/tools/utils/monsoon.py
+
+    Collects power measurements only, from a selection of two channels, the USB
+    passthrough channel and the main output channel.
+
+    :param target: Ignored
+    :param monsoon_bin: Path to monsoon.py executable. If not provided,
+                        ``$PATH`` is searched.
+    :param tty_device: TTY device to use to communicate with the Power
+                       Monitor. If not provided, a sane default is used.
+    """
+
+    mode = CONTINUOUS
+
+    def __init__(self, target, monsoon_bin=None, tty_device=None):
+        super(MonsoonInstrument, self).__init__(target)
+        self.monsoon_bin = monsoon_bin or which('monsoon.py')
+        if not self.monsoon_bin:
+            raise HostError(INSTALL_INSTRUCTIONS)
+
+        self.tty_device = tty_device
+
+        self.process = None
+        self.output = None
+
+        self.sample_rate_hz = 500
+        self.add_channel('output', 'power')
+        self.add_channel('USB', 'power')
+
+    def reset(self, sites=None, kinds=None, channels=None):
+        super(MonsoonInstrument, self).reset(sites, kinds)
+
+    def start(self):
+        if self.process:
+            self.process.kill()
+
+        cmd = [self.monsoon_bin,
+               '--hz', str(self.sample_rate_hz),
+               '--samples', '-1', # -1 means sample indefinitely
+               '--includeusb']
+        if self.tty_device:
+            cmd += ['--device', self.tty_device]
+
+        self.logger.debug(' '.join(cmd))
+        self.buffer_file = NamedTemporaryFile(prefix='monsoon', delete=False)
+        self.process = Popen(cmd, stdout=self.buffer_file, stderr=PIPE)
+
+    def stop(self):
+        process = self.process
+        self.process = None
+        if not process:
+            raise RuntimeError('Monsoon script not started')
+
+        process.poll()
+        if process.returncode is not None:
+            stdout, stderr = process.communicate()
+            raise HostError(
+                'Monsoon script exited unexpectedly with exit code {}.\n'
+                'stdout:\n{}\nstderr:\n{}'.format(process.returncode,
+                                                  stdout, stderr))
+
+        process.send_signal(signal.SIGINT)
+
+        stderr =  process.stderr.read()
+
+        self.buffer_file.close()
+        with open(self.buffer_file.name) as f:
+            stdout = f.read()
+        os.remove(self.buffer_file.name)
+        self.buffer_file = None
+
+        self.output = (stdout, stderr)
+
+    def get_data(self, outfile):
+        if self.process:
+            raise RuntimeError('`get_data` called before `stop`')
+
+        stdout, stderr = self.output
+
+        with open(outfile, 'wb') as f:
+            writer = csv.writer(f)
+            active_sites = [c.site for c in self.active_channels]
+
+            # Write column headers
+            row = []
+            if 'output' in active_sites:
+                row.append('output_power')
+            if 'USB' in active_sites:
+                row.append('USB_power')
+            writer.writerow(row)
+
+            # Write data
+            for line in stdout.splitlines():
+                # Each output line is a main_output, usb_output measurement pair.
+                # (If our user only requested one channel we still collect both,
+                # and just ignore one of them)
+                output, usb = line.split()
+                row = []
+                if 'output' in active_sites:
+                    row.append(output)
+                if 'USB' in active_sites:
+                    row.append(usb)
+                writer.writerow(row)
+
+        return MeasurementsCsv(outfile, self.active_channels)