qemurunner.py: fix race condition at qemu startup

When handling pid file, qemu would first create the file, stat it,
lock it and then write actually contents to it.

So it's possbile that when reading the pid file, the content is empty.

[YOCTO #13390]

(From OE-Core rev: 170e59b203a02f8438b9aeab3a45f6fcd6608b1f)

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

meta/lib/oeqa/utils/qemurunner.py

@@ -425,13 +425,20 @@ class QemuRunner:
         if not self.runqemu or self.runqemu.poll() is not None:
             return False
         if os.path.isfile(self.qemu_pidfile):
-            f = open(self.qemu_pidfile, 'r')
-            qemu_pid = f.read()
-            f.close()
-            qemupid = int(qemu_pid)
-            if os.path.exists("/proc/" + str(qemupid)):
-                self.qemupid = qemupid
-                return True
+            # when handling pidfile, qemu creates the file, stat it, lock it and then write to it
+            # so it's possible that the file has been created but the content is empty
+            pidfile_timeout = time.time() + 3
+            while time.time() < pidfile_timeout:
+                with open(self.qemu_pidfile, 'r') as f:
+                    qemu_pid = f.read().strip()
+                # file created but not yet written contents
+                if not qemu_pid:
+                    time.sleep(0.5)
+                    continue
+                else:
+                    if os.path.exists("/proc/" + qemu_pid):
+                        self.qemupid = int(qemu_pid)
+                        return True
         return False
 
     def run_serial(self, command, raw=False, timeout=60):