utils/android: LogcatMonitor fixes

host.kill_children() is used to properly kill the logcat process
when it is IO blocked.

The logcat regexp argument is now within double quotes, as having
parenthesis within the regexp could break the command.

LogcatMonitor.search() has been renamed to wait_for() to make the
behaviour of the method more explicit. A non-blocking version of
this method has been added and is named search().

devlib/utils/android.py

@@ -32,6 +32,7 @@ from collections import defaultdict
 from devlib.exception import TargetError, HostError, DevlibError
 from devlib.utils.misc import check_output, which, memoized, ABI_MAP
 from devlib.utils.misc import escape_single_quotes, escape_double_quotes
+from devlib import host
 
 
 logger = logging.getLogger('android')
@@ -555,18 +556,21 @@ class LogcatMonitor(threading.Thread):
             regexp = '{}'.format('|'.join(self._regexps))
             if len(self._regexps) > 1:
                 regexp = '({})'.format(regexp)
-            logcat_cmd = '{} -e {}'.format(logcat_cmd, regexp)
+            logcat_cmd = '{} -e "{}"'.format(logcat_cmd, regexp)
 
         logger.debug('logcat command ="{}"'.format(logcat_cmd))
         self._logcat = self.target.background(logcat_cmd)
 
         while not self._stopped.is_set():
             line = self._logcat.stdout.readline(1024)
-            self._add_line(line)
+            if line:
+                self._add_line(line)
 
     def stop(self):
-        # Popen can be stuck on readline() so send it a SIGKILL
-        self._logcat.terminate()
+        # Kill the underlying logcat process
+        # This will unblock self._logcat.stdout.readline()
+        host.kill_children(self._logcat.pid)
+        self._logcat.kill()
 
         self._stopped.set()
         self.join()
@@ -609,9 +613,10 @@ class LogcatMonitor(threading.Thread):
 
         return res
 
-    def search(self, regexp, timeout=30):
+    def search(self, regexp):
         """
         Search a line that matches a regexp in the logcat log
+        Return immediatly
         """
         res = []
 
@@ -623,8 +628,18 @@ class LogcatMonitor(threading.Thread):
                     if re.match(regexp, line):
                         res.append(line)
 
+        return res
+
+    def wait_for(self, regexp, timeout=30):
+        """
+        Search a line that matches a regexp in the logcat log
+        Wait for it to appear if it's not found
+        """
+        res = self.search(regexp)
+
         # Found some matches, return them
-        if len(res) > 0:
+        # Also return if thread not running
+        if len(res) > 0 or not self.is_alive():
             return res
 
         # Did not find any match, wait for one to pop up