Replace multiprocessing with threading in memory_inspector

This patch is part of an ongoing process to unbundle memory_inspector
and make it a chrome app. It replaces multiprocessing, which is
currently not supported by naclports/python, with threading.

BUG=448399

Review URL: https://codereview.chromium.org/836933004

Cr-Commit-Position: refs/heads/master@{#311515}

tools/memory_inspector/memory_inspector/frontends/background_tasks.py

@@ -11,14 +11,14 @@ operations, an instance of |BackgroundTask| is created here and the server
 returns just its id. The client can later poll the status of the asynchronous
 task to check for its progress.
 
-From a technical viewpoint, each background task is just a python subprocess
+From a technical viewpoint, each background task is just a python thread
 which communicates its progress updates through a Queue. The messages enqueued
 are tuples with the following format: (completion_ratio%, 'message string').
 """
 
 import datetime
-import multiprocessing
 import Queue
+import threading
 import time
 
 from memory_inspector.core import backends
@@ -40,8 +40,8 @@ def StartTracer(process, storage_path, interval, count, trace_native_heap):
       count=count,
       trace_native_heap=trace_native_heap)
   task.start()
-  _tasks[task.pid] = task
-  return task.pid
+  _tasks[task.ident] = task
+  return task.ident
 
 
 def Get(task_id):
@@ -49,9 +49,9 @@ def Get(task_id):
 
 
 def TerminateAll():
-  for proc in _tasks.itervalues():
-    if proc.is_alive():
-      proc.terminate()
+  for task in _tasks.itervalues():
+    if task.is_alive():
+      task.terminate()
   _tasks.clear()
 
 
@@ -129,15 +129,23 @@ def TracerMain_(log, storage_path, backend_name, device_id, pid, interval,
   return 0
 
 
-class BackgroundTask(multiprocessing.Process):
+class BackgroundTask(threading.Thread):
   def __init__(self, entry_point, *args, **kwargs):
-    self._log_queue = multiprocessing.Queue()
+    self._log_queue = Queue.Queue()
     self._progress_log = []  # A list of tuples [(50%, 'msg1'), (100%, 'msg2')].
     super(BackgroundTask, self).__init__(
         target=entry_point,
         args=((self._log_queue,) + args),  # Just propagate all args.
         kwargs=kwargs)
 
+  def run(self):
+    try:
+      super(BackgroundTask, self).run()
+    except Exception, e:
+      self._log_queue.put((
+          100, 'An error occurred (%s). See the log for more details.' % e))
+      raise
+
   def GetProgress(self):
     """ Returns a tuple (completion_rate, message). """
     while True:
@@ -145,6 +153,4 @@ class BackgroundTask(multiprocessing.Process):
         self._progress_log += [self._log_queue.get(block=False)]
       except Queue.Empty:
         break
-    if not self.is_alive() and self.exitcode != 0:
-      return self._progress_log + [(100, 'Failed with code %d' % self.exitcode)]
     return self._progress_log