iotests.py: Add QemuStorageDaemon class

This is a rather simple class that allows creating a QSD instance
running in the background and stopping it when no longer needed.

The __del__ handler is a safety net for when something goes so wrong in
a test that e.g. the tearDown() method is not called (e.g. setUp()
launches the QSD, but then launching a VM fails).  We do not want the
QSD to continue running after the test has failed, so __del__() will
take care to kill it.

Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Hanna Reitz <hreitz@redhat.com>

tests/qemu-iotests/iotests.py

@@ -73,6 +73,8 @@
 qemu_prog = os.environ.get('QEMU_PROG', 'qemu')
 qemu_opts = os.environ.get('QEMU_OPTIONS', '').strip().split(' ')
 
+qsd_prog = os.environ.get('QSD_PROG', 'qemu-storage-daemon')
+
 gdb_qemu_env = os.environ.get('GDB_OPTIONS')
 qemu_gdb = []
 if gdb_qemu_env:
@@ -345,6 +347,44 @@ def cmd(self, cmd):
         return self._read_output()
 
 
+class QemuStorageDaemon:
+    def __init__(self, *args: str, instance_id: str = 'a'):
+        assert '--pidfile' not in args
+        self.pidfile = os.path.join(test_dir, f'qsd-{instance_id}-pid')
+        all_args = [qsd_prog] + list(args) + ['--pidfile', self.pidfile]
+
+        # Cannot use with here, we want the subprocess to stay around
+        # pylint: disable=consider-using-with
+        self._p = subprocess.Popen(all_args)
+        while not os.path.exists(self.pidfile):
+            if self._p.poll() is not None:
+                cmd = ' '.join(all_args)
+                raise RuntimeError(
+                    'qemu-storage-daemon terminated with exit code ' +
+                    f'{self._p.returncode}: {cmd}')
+
+            time.sleep(0.01)
+
+        with open(self.pidfile, encoding='utf-8') as f:
+            self._pid = int(f.read().strip())
+
+        assert self._pid == self._p.pid
+
+    def stop(self, kill_signal=15):
+        self._p.send_signal(kill_signal)
+        self._p.wait()
+        self._p = None
+
+        try:
+            os.remove(self.pidfile)
+        except OSError:
+            pass
+
+    def __del__(self):
+        if self._p is not None:
+            self.stop(kill_signal=9)
+
+
 def qemu_nbd(*args):
     '''Run qemu-nbd in daemon mode and return the parent's exit code'''
     return subprocess.call(qemu_nbd_args + ['--fork'] + list(args))