avocado.utils.process: Move subprocess initialization to public method calls

Currently, as soon as we initialize a process.SubProcess
object, a subprocess.Popen() object is created. This breaks
the user's expectation that the process will start when
run() is called. Therefore, move process execution to
a private method, and call that method in all public
methods of the class.
Signed-off-by: NLucas Meneghel Rodrigues <lmr@redhat.com>

avocado/utils/process.py

@@ -242,35 +242,9 @@ class SubProcess(object):
         """
         self.cmd = cmd
         self.verbose = verbose
-        if self.verbose:
-            log.info("Running '%s'", cmd)
-        self._popen = subprocess.Popen(cmd,
-                                       stdout=subprocess.PIPE,
-                                       stderr=subprocess.PIPE,
-                                       shell=True)
         self.allow_output_check = allow_output_check
-        self.start_time = time.time()
-        self.result = CmdResult(cmd)
-        self.stdout_file = StringIO.StringIO()
-        self.stderr_file = StringIO.StringIO()
-        self.stdout_lock = threading.Lock()
-        self.stdout_thread = threading.Thread(target=self._fd_drainer,
-                                              name="%s-stdout" % cmd,
-                                              args=[self._popen.stdout])
-        self.stdout_thread.daemon = True
-        self.stderr_lock = threading.Lock()
-        self.stderr_thread = threading.Thread(target=self._fd_drainer,
-                                              name="%s-stderr" % cmd,
-                                              args=[self._popen.stderr])
-        self.stderr_thread.daemon = True
-        self.stdout_thread.start()
-        self.stderr_thread.start()
-
-        def signal_handler(signum, frame):
-            self.result.interrupted = True
-            self.wait()
-
-        signal.signal(signal.SIGINT, signal_handler)
+        self.result = CmdResult(self.cmd)
+        self._popen = None
 
     def __str__(self):
         if self.result.exit_status is None:
@@ -279,6 +253,36 @@ class SubProcess(object):
             rc = self.result.exit_status
         return 'SubProcess(cmd="%s", rc="%s")' % (self.cmd, rc)
 
+    def _init_subprocess(self):
+        if self._popen is None:
+            if self.verbose:
+                log.info("Running '%s'", self.cmd)
+            self._popen = subprocess.Popen(self.cmd,
+                                           stdout=subprocess.PIPE,
+                                           stderr=subprocess.PIPE,
+                                           shell=True)
+            self.start_time = time.time()
+            self.stdout_file = StringIO.StringIO()
+            self.stderr_file = StringIO.StringIO()
+            self.stdout_lock = threading.Lock()
+            self.stdout_thread = threading.Thread(target=self._fd_drainer,
+                                                  name="%s-stdout" % self.cmd,
+                                                  args=[self._popen.stdout])
+            self.stdout_thread.daemon = True
+            self.stderr_lock = threading.Lock()
+            self.stderr_thread = threading.Thread(target=self._fd_drainer,
+                                                  name="%s-stderr" % self.cmd,
+                                                  args=[self._popen.stderr])
+            self.stderr_thread.daemon = True
+            self.stdout_thread.start()
+            self.stderr_thread.start()
+
+            def signal_handler(signum, frame):
+                self.result.interrupted = True
+                self.wait()
+
+            signal.signal(signal.SIGINT, signal_handler)
+
     def _fd_drainer(self, input_pipe):
         """
         Read from input_pipe, storing and logging output.
@@ -325,6 +329,7 @@ class SubProcess(object):
                 lock.release()
 
     def _fill_results(self, rc):
+        self._init_subprocess()
         self.result.exit_status = rc
         if self.result.duration == 0:
             self.result.duration = time.time() - self.start_time
@@ -350,6 +355,7 @@ class SubProcess(object):
         :return: Standard output of the process.
         :rtype: str
         """
+        self._init_subprocess()
         self.stdout_lock.acquire()
         stdout = self.stdout_file.getvalue()
         self.stdout_lock.release()
@@ -362,6 +368,7 @@ class SubProcess(object):
         :return: Standard error of the process.
         :rtype: str
         """
+        self._init_subprocess()
         self.stderr_lock.acquire()
         stderr = self.stderr_file.getvalue()
         self.stderr_lock.release()
@@ -371,12 +378,14 @@ class SubProcess(object):
         """
         Send a :attr:`signal.SIGTERM` to the process.
         """
+        self._init_subprocess()
         self.send_signal(signal.SIGTERM)
 
     def kill(self):
         """
         Send a :attr:`signal.SIGKILL` to the process.
         """
+        self._init_subprocess()
         self.send_signal(signal.SIGKILL)
 
     def send_signal(self, sig):
@@ -385,12 +394,14 @@ class SubProcess(object):
 
         :param sig: Signal to send.
         """
+        self._init_subprocess()
         self._popen.send_signal(sig)
 
     def poll(self):
         """
         Call the subprocess poll() method, fill results if rc is not None.
         """
+        self._init_subprocess()
         rc = self._popen.poll()
         if rc is not None:
             self._fill_results(rc)
@@ -400,6 +411,7 @@ class SubProcess(object):
         """
         Call the subprocess poll() method, fill results if rc is not None.
         """
+        self._init_subprocess()
         rc = self._popen.wait()
         if rc is not None:
             self._fill_results(rc)
@@ -416,6 +428,7 @@ class SubProcess(object):
         :returns: The command result object.
         :rtype: A :class:`avocado.utils.process.CmdResult` instance.
         """
+        self._init_subprocess()
         start_time = time.time()
 
         if timeout is None: