nbd patches for 2020-07-28

- fix NBD handling of trim/zero requests larger than 2G
 - allow no-op resizes on NBD (in turn fixing qemu-img convert -c into NBD)
 - several deadlock fixes when using NBD reconnect
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Merge remote-tracking branch 'remotes/ericb/tags/pull-nbd-2020-07-28' into staging

nbd patches for 2020-07-28

- fix NBD handling of trim/zero requests larger than 2G
- allow no-op resizes on NBD (in turn fixing qemu-img convert -c into NBD)
- several deadlock fixes when using NBD reconnect

# gpg: Signature made Tue 28 Jul 2020 15:59:42 BST
# gpg:                using RSA key 71C2CC22B1C4602927D2F3AAA7A16B4A2527436A
# gpg: Good signature from "Eric Blake <eblake@redhat.com>" [full]
# gpg:                 aka "Eric Blake (Free Software Programmer) <ebb9@byu.net>" [full]
# gpg:                 aka "[jpeg image of size 6874]" [full]
# Primary key fingerprint: 71C2 CC22 B1C4 6029 27D2  F3AA A7A1 6B4A 2527 436A

* remotes/ericb/tags/pull-nbd-2020-07-28:
  block/nbd: nbd_co_reconnect_loop(): don't sleep if drained
  block/nbd: on shutdown terminate connection attempt
  block/nbd: allow drain during reconnect attempt
  block/nbd: split nbd_establish_connection out of nbd_client_connect
  iotests: Test convert to qcow2 compressed to NBD
  iotests: Add more qemu_img helpers
  iotests: Make qemu_nbd_popen() a contextmanager
  block: nbd: Fix convert qcow2 compressed to nbd
  nbd: Fix large trim/zero requests

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

block/nbd.c

@@ -93,7 +93,10 @@ typedef struct BDRVNBDState {
     char *x_dirty_bitmap;
 } BDRVNBDState;
 
-static int nbd_client_connect(BlockDriverState *bs, Error **errp);
+static QIOChannelSocket *nbd_establish_connection(SocketAddress *saddr,
+                                                  Error **errp);
+static int nbd_client_handshake(BlockDriverState *bs, QIOChannelSocket *sioc,
+                                Error **errp);
 
 static void nbd_clear_bdrvstate(BDRVNBDState *s)
 {
@@ -206,11 +209,15 @@ static void nbd_teardown_connection(BlockDriverState *bs)
 {
     BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
 
-    if (s->state == NBD_CLIENT_CONNECTED) {
+    if (s->ioc) {
         /* finish any pending coroutines */
-        assert(s->ioc);
         qio_channel_shutdown(s->ioc, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
+    } else if (s->sioc) {
+        /* abort negotiation */
+        qio_channel_shutdown(QIO_CHANNEL(s->sioc), QIO_CHANNEL_SHUTDOWN_BOTH,
+                             NULL);
     }
+
     s->state = NBD_CLIENT_QUIT;
     if (s->connection_co) {
         if (s->connection_co_sleep_ns_state) {
@@ -241,7 +248,9 @@ static bool nbd_client_connecting_wait(BDRVNBDState *s)
 
 static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)
 {
+    int ret;
     Error *local_err = NULL;
+    QIOChannelSocket *sioc;
 
     if (!nbd_client_connecting(s)) {
         return;
@@ -280,20 +289,40 @@ static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)
         s->ioc = NULL;
     }
 
-    s->connect_status = nbd_client_connect(s->bs, &local_err);
+    sioc = nbd_establish_connection(s->saddr, &local_err);
+    if (!sioc) {
+        ret = -ECONNREFUSED;
+        goto out;
+    }
+
+    bdrv_dec_in_flight(s->bs);
+
+    ret = nbd_client_handshake(s->bs, sioc, &local_err);
+
+    if (s->drained) {
+        s->wait_drained_end = true;
+        while (s->drained) {
+            /*
+             * We may be entered once from nbd_client_attach_aio_context_bh
+             * and then from nbd_client_co_drain_end. So here is a loop.
+             */
+            qemu_coroutine_yield();
+        }
+    }
+    bdrv_inc_in_flight(s->bs);
+
+out:
+    s->connect_status = ret;
     error_free(s->connect_err);
     s->connect_err = NULL;
     error_propagate(&s->connect_err, local_err);
 
-    if (s->connect_status < 0) {
+    if (ret >= 0) {
-        /* failed attempt */
-        return;
-    }
-
         /* successfully connected */
         s->state = NBD_CLIENT_CONNECTED;
         qemu_co_queue_restart_all(&s->free_sema);
     }
+}
 
 static coroutine_fn void nbd_co_reconnect_loop(BDRVNBDState *s)
 {
@@ -312,8 +341,6 @@ static coroutine_fn void nbd_co_reconnect_loop(BDRVNBDState *s)
             qemu_co_queue_restart_all(&s->free_sema);
         }
 
-        qemu_co_sleep_ns_wakeable(QEMU_CLOCK_REALTIME, timeout,
-                                  &s->connection_co_sleep_ns_state);
         if (s->drained) {
             bdrv_dec_in_flight(s->bs);
             s->wait_drained_end = true;
@@ -325,10 +352,13 @@ static coroutine_fn void nbd_co_reconnect_loop(BDRVNBDState *s)
                 qemu_coroutine_yield();
             }
             bdrv_inc_in_flight(s->bs);
-        }
+        } else {
+            qemu_co_sleep_ns_wakeable(QEMU_CLOCK_REALTIME, timeout,
+                                      &s->connection_co_sleep_ns_state);
             if (timeout < max_timeout) {
                 timeout *= 2;
             }
+        }
 
         nbd_reconnect_attempt(s);
     }
@@ -1425,24 +1455,18 @@ static QIOChannelSocket *nbd_establish_connection(SocketAddress *saddr,
     return sioc;
 }
 
-static int nbd_client_connect(BlockDriverState *bs, Error **errp)
+/* nbd_client_handshake takes ownership on sioc. On failure it is unref'ed. */
+static int nbd_client_handshake(BlockDriverState *bs, QIOChannelSocket *sioc,
+                                Error **errp)
 {
     BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
     AioContext *aio_context = bdrv_get_aio_context(bs);
     int ret;
 
-    /*
+    trace_nbd_client_handshake(s->export);
-     * establish TCP connection, return error if it fails
-     * TODO: Configurable retry-until-timeout behaviour.
-     */
-    QIOChannelSocket *sioc = nbd_establish_connection(s->saddr, errp);
 
-    if (!sioc) {
+    s->sioc = sioc;
-        return -ECONNREFUSED;
-    }
 
-    /* NBD handshake */
-    trace_nbd_client_connect(s->export);
     qio_channel_set_blocking(QIO_CHANNEL(sioc), false, NULL);
     qio_channel_attach_aio_context(QIO_CHANNEL(sioc), aio_context);
 
@@ -1457,6 +1481,7 @@ static int nbd_client_connect(BlockDriverState *bs, Error **errp)
     g_free(s->info.name);
     if (ret < 0) {
         object_unref(OBJECT(sioc));
+        s->sioc = NULL;
         return ret;
     }
     if (s->x_dirty_bitmap && !s->info.base_allocation) {
@@ -1482,14 +1507,12 @@ static int nbd_client_connect(BlockDriverState *bs, Error **errp)
         }
     }
 
-    s->sioc = sioc;
-
     if (!s->ioc) {
         s->ioc = QIO_CHANNEL(sioc);
         object_ref(OBJECT(s->ioc));
     }
 
-    trace_nbd_client_connect_success(s->export);
+    trace_nbd_client_handshake_success(s->export);
 
     return 0;
 
@@ -1504,6 +1527,7 @@ static int nbd_client_connect(BlockDriverState *bs, Error **errp)
         nbd_send_request(s->ioc ?: QIO_CHANNEL(sioc), &request);
 
         object_unref(OBJECT(sioc));
+        s->sioc = NULL;
 
         return ret;
     }
@@ -1894,6 +1918,7 @@ static int nbd_open(BlockDriverState *bs, QDict *options, int flags,
 {
     int ret;
     BDRVNBDState *s = (BDRVNBDState *)bs->opaque;
+    QIOChannelSocket *sioc;
 
     ret = nbd_process_options(bs, options, errp);
     if (ret < 0) {
@@ -1904,7 +1929,16 @@ static int nbd_open(BlockDriverState *bs, QDict *options, int flags,
     qemu_co_mutex_init(&s->send_mutex);
     qemu_co_queue_init(&s->free_sema);
 
-    ret = nbd_client_connect(bs, errp);
+    /*
+     * establish TCP connection, return error if it fails
+     * TODO: Configurable retry-until-timeout behaviour.
+     */
+    sioc = nbd_establish_connection(s->saddr, errp);
+    if (!sioc) {
+        return -ECONNREFUSED;
+    }
+
+    ret = nbd_client_handshake(bs, sioc, errp);
     if (ret < 0) {
         nbd_clear_bdrvstate(s);
         return ret;
@@ -1966,6 +2000,33 @@ static void nbd_close(BlockDriverState *bs)
     nbd_clear_bdrvstate(s);
 }
 
+/*
+ * NBD cannot truncate, but if the caller asks to truncate to the same size, or
+ * to a smaller size with exact=false, there is no reason to fail the
+ * operation.
+ *
+ * Preallocation mode is ignored since it does not seems useful to fail when
+ * we never change anything.
+ */
+static int coroutine_fn nbd_co_truncate(BlockDriverState *bs, int64_t offset,
+                                        bool exact, PreallocMode prealloc,
+                                        BdrvRequestFlags flags, Error **errp)
+{
+    BDRVNBDState *s = bs->opaque;
+
+    if (offset != s->info.size && exact) {
+        error_setg(errp, "Cannot resize NBD nodes");
+        return -ENOTSUP;
+    }
+
+    if (offset > s->info.size) {
+        error_setg(errp, "Cannot grow NBD nodes");
+        return -EINVAL;
+    }
+
+    return 0;
+}
+
 static int64_t nbd_getlength(BlockDriverState *bs)
 {
     BDRVNBDState *s = bs->opaque;
@@ -2045,6 +2106,7 @@ static BlockDriver bdrv_nbd = {
     .bdrv_co_flush_to_os        = nbd_co_flush,
     .bdrv_co_pdiscard           = nbd_client_co_pdiscard,
     .bdrv_refresh_limits        = nbd_refresh_limits,
+    .bdrv_co_truncate           = nbd_co_truncate,
     .bdrv_getlength             = nbd_getlength,
     .bdrv_detach_aio_context    = nbd_client_detach_aio_context,
     .bdrv_attach_aio_context    = nbd_client_attach_aio_context,
@@ -2072,6 +2134,7 @@ static BlockDriver bdrv_nbd_tcp = {
     .bdrv_co_flush_to_os        = nbd_co_flush,
     .bdrv_co_pdiscard           = nbd_client_co_pdiscard,
     .bdrv_refresh_limits        = nbd_refresh_limits,
+    .bdrv_co_truncate           = nbd_co_truncate,
     .bdrv_getlength             = nbd_getlength,
     .bdrv_detach_aio_context    = nbd_client_detach_aio_context,
     .bdrv_attach_aio_context    = nbd_client_attach_aio_context,
@@ -2099,6 +2162,7 @@ static BlockDriver bdrv_nbd_unix = {
     .bdrv_co_flush_to_os        = nbd_co_flush,
     .bdrv_co_pdiscard           = nbd_client_co_pdiscard,
     .bdrv_refresh_limits        = nbd_refresh_limits,
+    .bdrv_co_truncate           = nbd_co_truncate,
     .bdrv_getlength             = nbd_getlength,
     .bdrv_detach_aio_context    = nbd_client_detach_aio_context,
     .bdrv_attach_aio_context    = nbd_client_attach_aio_context,

block/trace-events

@@ -168,8 +168,8 @@ nbd_parse_blockstatus_compliance(const char *err) "ignoring extra data from non-
 nbd_structured_read_compliance(const char *type) "server sent non-compliant unaligned read %s chunk"
 nbd_read_reply_entry_fail(int ret, const char *err) "ret = %d, err: %s"
 nbd_co_request_fail(uint64_t from, uint32_t len, uint64_t handle, uint16_t flags, uint16_t type, const char *name, int ret, const char *err) "Request failed { .from = %" PRIu64", .len = %" PRIu32 ", .handle = %" PRIu64 ", .flags = 0x%" PRIx16 ", .type = %" PRIu16 " (%s) } ret = %d, err: %s"
-nbd_client_connect(const char *export_name) "export '%s'"
+nbd_client_handshake(const char *export_name) "export '%s'"
-nbd_client_connect_success(const char *export_name) "export '%s'"
+nbd_client_handshake_success(const char *export_name) "export '%s'"
 
 # ssh.c
 ssh_restart_coroutine(void *co) "co=%p"

nbd/server.c

@@ -2378,8 +2378,17 @@ static coroutine_fn int nbd_handle_request(NBDClient *client,
         if (request->flags & NBD_CMD_FLAG_FAST_ZERO) {
             flags |= BDRV_REQ_NO_FALLBACK;
         }
+        ret = 0;
+        /* FIXME simplify this when blk_pwrite_zeroes switches to 64-bit */
+        while (ret >= 0 && request->len) {
+            int align = client->check_align ?: 1;
+            int len = MIN(request->len, QEMU_ALIGN_DOWN(BDRV_REQUEST_MAX_BYTES,
+                                                        align));
             ret = blk_pwrite_zeroes(exp->blk, request->from + exp->dev_offset,
-                                request->len, flags);
+                                    len, flags);
+            request->len -= len;
+            request->from += len;
+        }
         return nbd_send_generic_reply(client, request->handle, ret,
                                       "writing to file failed", errp);
 
@@ -2393,9 +2402,18 @@ static coroutine_fn int nbd_handle_request(NBDClient *client,
                                       "flush failed", errp);
 
     case NBD_CMD_TRIM:
+        ret = 0;
+        /* FIXME simplify this when blk_co_pdiscard switches to 64-bit */
+        while (ret >= 0 && request->len) {
+            int align = client->check_align ?: 1;
+            int len = MIN(request->len, QEMU_ALIGN_DOWN(BDRV_REQUEST_MAX_BYTES,
+                                                        align));
             ret = blk_co_pdiscard(exp->blk, request->from + exp->dev_offset,
-                              request->len);
+                                  len);
-        if (ret == 0 && request->flags & NBD_CMD_FLAG_FUA) {
+            request->len -= len;
+            request->from += len;
+        }
+        if (ret >= 0 && request->flags & NBD_CMD_FLAG_FUA) {
             ret = blk_co_flush(exp->blk);
         }
         return nbd_send_generic_reply(client, request->handle, ret,

qemu-io-cmds.c

@@ -1715,7 +1715,7 @@ static int truncate_f(BlockBackend *blk, int argc, char **argv)
      * exact=true.  It is better to err on the "emit more errors" side
      * than to be overly permissive.
      */
-    ret = blk_truncate(blk, offset, true, PREALLOC_MODE_OFF, 0, &local_err);
+    ret = blk_truncate(blk, offset, false, PREALLOC_MODE_OFF, 0, &local_err);
     if (ret < 0) {
         error_report_err(local_err);
         return ret;

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/302

@@ -0,0 +1,127 @@
+#!/usr/bin/env python3
+#
+# Tests converting qcow2 compressed to NBD
+#
+# Copyright (c) 2020 Nir Soffer <nirsof@gmail.com>
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+# owner=nirsof@gmail.com
+
+import io
+import tarfile
+
+import iotests
+
+from iotests import (
+    file_path,
+    qemu_img,
+    qemu_img_check,
+    qemu_img_create,
+    qemu_img_log,
+    qemu_img_measure,
+    qemu_io,
+    qemu_nbd_popen,
+)
+
+iotests.script_initialize(supported_fmts=["qcow2"])
+
+# Create source disk. Using qcow2 to enable strict comparing later, and
+# avoid issues with random filesystem on CI environment.
+src_disk = file_path("disk.qcow2")
+qemu_img_create("-f", iotests.imgfmt, src_disk, "1g")
+qemu_io("-f", iotests.imgfmt, "-c", "write 1m 64k", src_disk)
+
+# The use case is writing qcow2 image directly into an ova file, which
+# is a tar file with specific layout. This is tricky since we don't know the
+# size of the image before compressing, so we have to do:
+# 1. Add an ovf file.
+# 2. Find the offset of the next member data.
+# 3. Make room for image data, allocating for the worst case.
+# 4. Write compressed image data into the tar.
+# 5. Add a tar entry with the actual image size.
+# 6. Shrink the tar to the actual size, aligned to 512 bytes.
+
+tar_file = file_path("test.ova")
+
+with tarfile.open(tar_file, "w") as tar:
+
+    # 1. Add an ovf file.
+
+    ovf_data = b"<xml/>"
+    ovf = tarfile.TarInfo("vm.ovf")
+    ovf.size = len(ovf_data)
+    tar.addfile(ovf, io.BytesIO(ovf_data))
+
+    # 2. Find the offset of the next member data.
+
+    offset = tar.fileobj.tell() + 512
+
+    # 3. Make room for image data, allocating for the worst case.
+
+    measure = qemu_img_measure("-O", "qcow2", src_disk)
+    tar.fileobj.truncate(offset + measure["required"])
+
+    # 4. Write compressed image data into the tar.
+
+    nbd_sock = file_path("nbd-sock", base_dir=iotests.sock_dir)
+    nbd_uri = "nbd+unix:///exp?socket=" + nbd_sock
+
+    # Use raw format to allow creating qcow2 directly into tar file.
+    with qemu_nbd_popen(
+            "--socket", nbd_sock,
+            "--export-name", "exp",
+            "--format", "raw",
+            "--offset", str(offset),
+            tar_file):
+
+        iotests.log("=== Target image info ===")
+        qemu_img_log("info", nbd_uri)
+
+        qemu_img(
+            "convert",
+            "-f", iotests.imgfmt,
+            "-O", "qcow2",
+            "-c",
+            src_disk,
+            nbd_uri)
+
+        iotests.log("=== Converted image info ===")
+        qemu_img_log("info", nbd_uri)
+
+        iotests.log("=== Converted image check ===")
+        qemu_img_log("check", nbd_uri)
+
+        iotests.log("=== Comparing to source disk ===")
+        qemu_img_log("compare", src_disk, nbd_uri)
+
+        actual_size = qemu_img_check(nbd_uri)["image-end-offset"]
+
+    # 5. Add a tar entry with the actual image size.
+
+    disk = tarfile.TarInfo("disk")
+    disk.size = actual_size
+    tar.addfile(disk)
+
+    # 6. Shrink the tar to the actual size, aligned to 512 bytes.
+
+    tar_size = offset + (disk.size + 511) & ~511
+    tar.fileobj.seek(tar_size)
+    tar.fileobj.truncate(tar_size)
+
+with tarfile.open(tar_file) as tar:
+    members = [{"name": m.name, "size": m.size, "offset": m.offset_data}
+               for m in tar]
+    iotests.log("=== OVA file contents ===")
+    iotests.log(members)

tests/qemu-iotests/302.out

@@ -0,0 +1,31 @@
+Start NBD server
+=== Target image info ===
+image: nbd+unix:///exp?socket=SOCK_DIR/PID-nbd-sock
+file format: raw
+virtual size: 448 KiB (458752 bytes)
+disk size: unavailable
+
+=== Converted image info ===
+image: nbd+unix:///exp?socket=SOCK_DIR/PID-nbd-sock
+file format: qcow2
+virtual size: 1 GiB (1073741824 bytes)
+disk size: unavailable
+cluster_size: 65536
+Format specific information:
+    compat: 1.1
+    compression type: zlib
+    lazy refcounts: false
+    refcount bits: 16
+    corrupt: false
+
+=== Converted image check ===
+No errors were found on the image.
+1/16384 = 0.01% allocated, 100.00% fragmented, 100.00% compressed clusters
+Image end offset: 393216
+
+=== Comparing to source disk ===
+Images are identical.
+
+Kill NBD server
+=== OVA file contents ===
+[{"name": "vm.ovf", "offset": 512, "size": 6}, {"name": "disk", "offset": 1536, "size": 393216}]

tests/qemu-iotests/group

@@ -308,3 +308,4 @@
 297 meta
 299 auto quick
 301 backing quick
+302 quick

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
@@ -138,6 +141,12 @@ def qemu_img_create(*args):
 
     return qemu_img(*args)
 
+def qemu_img_measure(*args):
+    return json.loads(qemu_img_pipe("measure", "--output", "json", *args))
+
+def qemu_img_check(*args):
+    return json.loads(qemu_img_pipe("check", "--output", "json", *args))
+
 def qemu_img_verbose(*args):
     '''Run qemu-img without suppressing its output and return the exit code'''
     exitcode = subprocess.call(qemu_img_args + list(args))
@@ -270,9 +279,30 @@ 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'''