iotests: Make qemu_nbd_popen() a contextmanager

Instead of duplicating the code to wait until the server is ready and
remember to terminate the server and wait for it, make it possible to
use like this:

    with qemu_nbd_popen('-k', sock, image):
        # Access image via qemu-nbd socket...

Only test 264 used this helper, but I had to modify the output since it
did not consistently when starting and stopping qemu-nbd.

Signed-off-by: Nir Soffer <nsoffer@redhat.com>
Message-Id: <20200727215846.395443-3-nsoffer@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Signed-off-by: Eric Blake <eblake@redhat.com>

tests/qemu-iotests/264

@@ -36,20 +36,8 @@ wait_step = 0.2
 
 qemu_img_create('-f', iotests.imgfmt, disk_a, str(size))
 qemu_img_create('-f', iotests.imgfmt, disk_b, str(size))
-srv = qemu_nbd_popen('-k', nbd_sock, '-f', iotests.imgfmt, disk_b)
-
-# Wait for NBD server availability
-t = 0
-ok = False
-while t < wait_limit:
-    ok = qemu_io_silent_check('-f', 'raw', '-c', 'read 0 512', nbd_uri)
-    if ok:
-        break
-    time.sleep(wait_step)
-    t += wait_step
-
-assert ok
 
+with qemu_nbd_popen('-k', nbd_sock, '-f', iotests.imgfmt, disk_b):
     vm = iotests.VM().add_drive(disk_a)
     vm.launch()
     vm.hmp_qemu_io('drive0', 'write 0 {}'.format(size))
@@ -75,10 +63,6 @@ while t < wait_limit:
     if jobs and jobs[0]['offset'] > 0:
         log('Backup job is started')
 
-log('Kill NBD server')
-srv.kill()
-srv.wait()
-
 jobs = vm.qmp('query-block-jobs')['return']
 if jobs and jobs[0]['offset'] < jobs[0]['len']:
     log('Backup job is still in progress')
@@ -88,12 +72,8 @@ vm.qmp_log('block-job-set-speed', device='drive0', speed=0)
 # Emulate server down time for 1 second
 time.sleep(1)
 
-log('Start NBD server')
+with qemu_nbd_popen('-k', nbd_sock, '-f', iotests.imgfmt, disk_b):
-srv = qemu_nbd_popen('-k', nbd_sock, '-f', iotests.imgfmt, disk_b)
-
     e = vm.event_wait('BLOCK_JOB_COMPLETED')
     log('Backup completed: {}'.format(e['data']['offset']))
-
     vm.qmp_log('blockdev-del', node_name='backup0')
-srv.kill()
     vm.shutdown()

tests/qemu-iotests/264.out

@@ -1,3 +1,4 @@
+Start NBD server
 {"execute": "blockdev-add", "arguments": {"driver": "raw", "file": {"driver": "nbd", "reconnect-delay": 10, "server": {"path": "TEST_DIR/PID-nbd-sock", "type": "unix"}}, "node-name": "backup0"}}
 {"return": {}}
 {"execute": "blockdev-backup", "arguments": {"device": "drive0", "speed": 1048576, "sync": "full", "target": "backup0"}}
@@ -11,3 +12,4 @@ Start NBD server
 Backup completed: 5242880
 {"execute": "blockdev-del", "arguments": {"node-name": "backup0"}}
 {"return": {}}
+Kill NBD server

tests/qemu-iotests/iotests.py

@@ -28,10 +28,13 @@ import signal
 import struct
 import subprocess
 import sys
+import time
 from typing import (Any, Callable, Dict, Iterable,
                     List, Optional, Sequence, Tuple, TypeVar)
 import unittest
 
+from contextlib import contextmanager
+
 # pylint: disable=import-error, wrong-import-position
 sys.path.append(os.path.join(os.path.dirname(__file__), '..', '..', 'python'))
 from qemu import qtest
@@ -270,9 +273,30 @@ def qemu_nbd_early_pipe(*args):
 
     return subp.returncode, output if subp.returncode else ''
 
+@contextmanager
 def qemu_nbd_popen(*args):
-    '''Run qemu-nbd in daemon mode and return the parent's exit code'''
+    '''Context manager running qemu-nbd within the context'''
-    return subprocess.Popen(qemu_nbd_args + ['--persistent'] + list(args))
+    pid_file = file_path("pid")
+
+    cmd = list(qemu_nbd_args)
+    cmd.extend(('--persistent', '--pid-file', pid_file))
+    cmd.extend(args)
+
+    log('Start NBD server')
+    p = subprocess.Popen(cmd)
+    try:
+        while not os.path.exists(pid_file):
+            if p.poll() is not None:
+                raise RuntimeError(
+                    "qemu-nbd terminated with exit code {}: {}"
+                    .format(p.returncode, ' '.join(cmd)))
+
+            time.sleep(0.01)
+        yield
+    finally:
+        log('Kill NBD server')
+        p.kill()
+        p.wait()
 
 def compare_images(img1, img2, fmt1=imgfmt, fmt2=imgfmt):
     '''Return True if two image files are identical'''