tiny-test-fw: save DUT log in different thread:

We found some SD card on Raspberry Pi could have very bad performance.
It could take seconds to save small amount of data.
If the DUT receives data and save it as log, then it stops receiving data until log is saved.
This could lead to expect timeout. As an workaround to this issue,
``BaseDUT`` class will create a thread to save logs.
Then data will be passed to ``expect`` as soon as received.

tools/tiny-test-fw/DUT.py

@@ -154,6 +154,52 @@ class _DataCache(_queue.Queue):
             self.data_cache = self.data_cache[index:]
 
 
+class _LogThread(threading.Thread, _queue.Queue):
+    """
+    We found some SD card on Raspberry Pi could have very bad performance.
+    It could take seconds to save small amount of data.
+    If the DUT receives data and save it as log, then it stops receiving data until log is saved.
+    This could lead to expect timeout.
+    As an workaround to this issue, ``BaseDUT`` class will create a thread to save logs.
+    Then data will be passed to ``expect`` as soon as received.
+    """
+    def __init__(self):
+        threading.Thread.__init__(self, name="LogThread")
+        _queue.Queue.__init__(self, maxsize=0)
+        self.setDaemon(True)
+        self.flush_lock = threading.Lock()
+
+    def save_log(self, filename, data):
+        """
+        :param filename: log file name
+        :param data: log data. Must be ``bytes``.
+        """
+        self.put({"filename": filename, "data": data})
+
+    def flush_data(self):
+        with self.flush_lock:
+            data_cache = dict()
+            while True:
+                # move all data from queue to data cache
+                try:
+                    log = self.get_nowait()
+                    try:
+                        data_cache[log["filename"]] += log["data"]
+                    except KeyError:
+                        data_cache[log["filename"]] = log["data"]
+                except _queue.Empty:
+                    break
+            # flush data
+            for filename in data_cache:
+                with open(filename, "ab+") as f:
+                    f.write(data_cache[filename])
+
+    def run(self):
+        while True:
+            time.sleep(1)
+            self.flush_data()
+
+
 class _RecvThread(threading.Thread):
 
     PERFORMANCE_PATTERN = re.compile(r"\[Performance]\[(\w+)]: ([^\r\n]+)\r?\n")
@@ -216,6 +262,9 @@ class BaseDUT(object):
     DEFAULT_EXPECT_TIMEOUT = 5
     MAX_EXPECT_FAILURES_TO_SAVED = 10
 
+    LOG_THREAD = _LogThread()
+    LOG_THREAD.start()
+
     def __init__(self, name, port, log_file, app, **kwargs):
 
         self.expect_lock = threading.Lock()
@@ -233,10 +282,25 @@ class BaseDUT(object):
         return "DUT({}: {})".format(self.name, str(self.port))
 
     def _save_expect_failure(self, pattern, data, start_time):
+        """
+        Save expect failure. If the test fails, then it will print the expect failures.
+        In some cases, user will handle expect exceptions.
+        The expect failures could be false alarm, and test case might generate a lot of such failures.
+        Therefore, we don't print the failure immediately and limit the max size of failure list.
+        """
         self.expect_failures.insert(0, {"pattern": pattern, "data": data,
                                         "start": start_time, "end": time.time()})
         self.expect_failures = self.expect_failures[:self.MAX_EXPECT_FAILURES_TO_SAVED]
 
+    def _save_dut_log(self, data):
+        """
+        Save DUT log into file using another thread.
+        This is a workaround for some devices takes long time for file system operations.
+
+        See descriptions in ``_LogThread`` for details.
+        """
+        self.LOG_THREAD.save_log(self.log_file, data)
+
     # define for methods need to be overwritten by Port
     @classmethod
     def list_available_ports(cls):
@@ -336,6 +400,7 @@ class BaseDUT(object):
         if self.receive_thread:
             self.receive_thread.exit()
         self._port_close()
+        self.LOG_THREAD.flush_data()
 
     def write(self, data, eol="\r\n", flush=True):
         """
@@ -625,8 +690,7 @@ class SerialDUT(BaseDUT):
     def _port_read(self, size=1):
         data = self.port_inst.read(size)
         if data:
-            with open(self.log_file, "ab+") as _log_file:
-                _log_file.write(self._format_data(data))
+            self._save_dut_log(self._format_data(data))
         return data
 
     def _port_write(self, data):