Block jobs & NBD patches

v2: - add arguments to QEMUMachine constructor in test, to make it work
       on arm in gitlab pipeline
     - use bdrv_inc_in_flight() / bdrv_dec_in_flight() instead of direct
       manipulation with bs->in_flight
 
 - add new options for copy-before-write filter
 - new trace points for NBD
 - prefer unsigned type for some 'in_flight' fields
 -----BEGIN PGP SIGNATURE-----
 
 iQIzBAABCgAdFiEEi5wmzbL9FHyIDoahVh8kwfGfefsFAmK8BqkACgkQVh8kwfGf
 efuiMw/+P9FFLfGFSjVP+LYeT0Ah6kN1ypCMQzIk3Qq/J6qgMZhtRqpQoZOfIFQL
 U9fGmEtQZ7gvEMD/nJToL6uOYlnQfPxDcA6GrRwWWE3rcFiPK4J0q2LlnPLn4QaU
 W/qag5l/QnZfLlj/iV6neWOEvqdnvY1L8IS+T8xV6N0iBYlwgMC/6FGshQwehzcV
 T5F1qPGB0vjFDjf92LFPEsvsFFHjHIVPwOyJMvF64QtSk57utikq/la9PI/yA9AH
 Ll4mNQuZKx6rSI5wE6b21jc8iOUvaoHdPSEDQZfNILSdgGdiKvFwE51y+baGnIAD
 TpjxfG59q0jyGxMjQVxMRSFaxAC4+Mqi82diSPv4xbiUdsE4byJH0oENn4z7+wAv
 EvjuU9yx4FfHHltoUNwfn2pv00o/ELdZIoBNmW36rPxSGZMvfVfRtuBL7XWNUFbW
 Fr4BwsjC4KfIxb16QTBGhXVv6grxdlwoU9N23npdi0YpW1ftZzXGDa85+gINQ807
 eK/gP/OtYPwIql0bgmLiuaRNzC9psmQOO6vQbdvd/e4BEVWkxiez37+e+zFMStmL
 OAL8rS6jckUoxVZjCYFEWg97XOobLUIhQxt9Fwh2omMDGKTwv861ghUAivxSWs93
 IRNxfwqNPxnpDDXjnK1ayZgU08IL98AUYVKcPN1y8JvEhB4Hr1k=
 =ndKk
 -----END PGP SIGNATURE-----

Merge tag 'pull-block-2022-06-14-v2' of https://gitlab.com/vsementsov/qemu into staging

Block jobs & NBD patches

v2: - add arguments to QEMUMachine constructor in test, to make it work
      on arm in gitlab pipeline
    - use bdrv_inc_in_flight() / bdrv_dec_in_flight() instead of direct
      manipulation with bs->in_flight

- add new options for copy-before-write filter
- new trace points for NBD
- prefer unsigned type for some 'in_flight' fields

# -----BEGIN PGP SIGNATURE-----
#
# iQIzBAABCgAdFiEEi5wmzbL9FHyIDoahVh8kwfGfefsFAmK8BqkACgkQVh8kwfGf
# efuiMw/+P9FFLfGFSjVP+LYeT0Ah6kN1ypCMQzIk3Qq/J6qgMZhtRqpQoZOfIFQL
# U9fGmEtQZ7gvEMD/nJToL6uOYlnQfPxDcA6GrRwWWE3rcFiPK4J0q2LlnPLn4QaU
# W/qag5l/QnZfLlj/iV6neWOEvqdnvY1L8IS+T8xV6N0iBYlwgMC/6FGshQwehzcV
# T5F1qPGB0vjFDjf92LFPEsvsFFHjHIVPwOyJMvF64QtSk57utikq/la9PI/yA9AH
# Ll4mNQuZKx6rSI5wE6b21jc8iOUvaoHdPSEDQZfNILSdgGdiKvFwE51y+baGnIAD
# TpjxfG59q0jyGxMjQVxMRSFaxAC4+Mqi82diSPv4xbiUdsE4byJH0oENn4z7+wAv
# EvjuU9yx4FfHHltoUNwfn2pv00o/ELdZIoBNmW36rPxSGZMvfVfRtuBL7XWNUFbW
# Fr4BwsjC4KfIxb16QTBGhXVv6grxdlwoU9N23npdi0YpW1ftZzXGDa85+gINQ807
# eK/gP/OtYPwIql0bgmLiuaRNzC9psmQOO6vQbdvd/e4BEVWkxiez37+e+zFMStmL
# OAL8rS6jckUoxVZjCYFEWg97XOobLUIhQxt9Fwh2omMDGKTwv861ghUAivxSWs93
# IRNxfwqNPxnpDDXjnK1ayZgU08IL98AUYVKcPN1y8JvEhB4Hr1k=
# =ndKk
# -----END PGP SIGNATURE-----
# gpg: Signature made Wed 29 Jun 2022 01:30:41 PM +0530
# gpg:                using RSA key 8B9C26CDB2FD147C880E86A1561F24C1F19F79FB
# gpg: Good signature from "Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>" [unknown]
# gpg:                 aka "Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>" [unknown]
# gpg: WARNING: This key is not certified with a trusted signature!
# gpg:          There is no indication that the signature belongs to the owner.
# Primary key fingerprint: 8B9C 26CD B2FD 147C 880E  86A1 561F 24C1 F19F 79FB

* tag 'pull-block-2022-06-14-v2' of https://gitlab.com/vsementsov/qemu:
  block: use 'unsigned' for in_flight field on driver state
  nbd: trace long NBD operations
  iotests: copy-before-write: add cases for cbw-timeout option
  block/copy-before-write: implement cbw-timeout option
  block/block-copy: block_copy(): add timeout_ns parameter
  util: add qemu-co-timeout
  iotests: add copy-before-write: on-cbw-error tests
  block/copy-before-write: add on-cbw-error open parameter
  block/copy-before-write: refactor option parsing

Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

block/block-copy.c

@@ -883,23 +883,42 @@ static int coroutine_fn block_copy_common(BlockCopyCallState *call_state)
     return ret;
 }
 
-int coroutine_fn block_copy(BlockCopyState *s, int64_t start, int64_t bytes,
+static void coroutine_fn block_copy_async_co_entry(void *opaque)
-                            bool ignore_ratelimit)
 {
-    BlockCopyCallState call_state = {
+    block_copy_common(opaque);
+}
+
+int coroutine_fn block_copy(BlockCopyState *s, int64_t start, int64_t bytes,
+                            bool ignore_ratelimit, uint64_t timeout_ns,
+                            BlockCopyAsyncCallbackFunc cb,
+                            void *cb_opaque)
+{
+    int ret;
+    BlockCopyCallState *call_state = g_new(BlockCopyCallState, 1);
+
+    *call_state = (BlockCopyCallState) {
         .s = s,
         .offset = start,
         .bytes = bytes,
         .ignore_ratelimit = ignore_ratelimit,
         .max_workers = BLOCK_COPY_MAX_WORKERS,
+        .cb = cb,
+        .cb_opaque = cb_opaque,
     };
 
-    return block_copy_common(&call_state);
+    ret = qemu_co_timeout(block_copy_async_co_entry, call_state, timeout_ns,
-}
+                          g_free);
+    if (ret < 0) {
+        assert(ret == -ETIMEDOUT);
+        block_copy_call_cancel(call_state);
+        /* call_state will be freed by running coroutine. */
+        return ret;
+    }
 
-static void coroutine_fn block_copy_async_co_entry(void *opaque)
+    ret = call_state->ret;
-{
+    g_free(call_state);
-    block_copy_common(opaque);
+
+    return ret;
 }
 
 BlockCopyCallState *block_copy_async(BlockCopyState *s,

block/copy-before-write.c

@@ -24,6 +24,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "qapi/qmp/qjson.h"
 
 #include "sysemu/block-backend.h"
 #include "qemu/cutils.h"
@@ -40,6 +41,8 @@
 typedef struct BDRVCopyBeforeWriteState {
     BlockCopyState *bcs;
     BdrvChild *target;
+    OnCbwError on_cbw_error;
+    uint32_t cbw_timeout_ns;
 
     /*
      * @lock: protects access to @access_bitmap, @done_bitmap and
@@ -64,6 +67,14 @@ typedef struct BDRVCopyBeforeWriteState {
      * node. These areas must not be rewritten by guest.
      */
     BlockReqList frozen_read_reqs;
+
+    /*
+     * @snapshot_error is normally zero. But on first copy-before-write failure
+     * when @on_cbw_error == ON_CBW_ERROR_BREAK_SNAPSHOT, @snapshot_error takes
+     * value of this error (<0). After that all in-flight and further
+     * snapshot-API requests will fail with that error.
+     */
+    int snapshot_error;
 } BDRVCopyBeforeWriteState;
 
 static coroutine_fn int cbw_co_preadv(
@@ -73,6 +84,13 @@ static coroutine_fn int cbw_co_preadv(
     return bdrv_co_preadv(bs->file, offset, bytes, qiov, flags);
 }
 
+static void block_copy_cb(void *opaque)
+{
+    BlockDriverState *bs = opaque;
+
+    bdrv_dec_in_flight(bs);
+}
+
 /*
  * Do copy-before-write operation.
  *
@@ -94,16 +112,36 @@ static coroutine_fn int cbw_do_copy_before_write(BlockDriverState *bs,
         return 0;
     }
 
+    if (s->snapshot_error) {
+        return 0;
+    }
+
     off = QEMU_ALIGN_DOWN(offset, cluster_size);
     end = QEMU_ALIGN_UP(offset + bytes, cluster_size);
 
-    ret = block_copy(s->bcs, off, end - off, true);
+    /*
-    if (ret < 0) {
+     * Increase in_flight, so that in case of timed-out block-copy, the
+     * remaining background block_copy() request (which can't be immediately
+     * cancelled by timeout) is presented in bs->in_flight. This way we are
+     * sure that on bs close() we'll previously wait for all timed-out but yet
+     * running block_copy calls.
+     */
+    bdrv_inc_in_flight(bs);
+    ret = block_copy(s->bcs, off, end - off, true, s->cbw_timeout_ns,
+                     block_copy_cb, bs);
+    if (ret < 0 && s->on_cbw_error == ON_CBW_ERROR_BREAK_GUEST_WRITE) {
         return ret;
     }
 
     WITH_QEMU_LOCK_GUARD(&s->lock) {
-        bdrv_set_dirty_bitmap(s->done_bitmap, off, end - off);
+        if (ret < 0) {
+            assert(s->on_cbw_error == ON_CBW_ERROR_BREAK_SNAPSHOT);
+            if (!s->snapshot_error) {
+                s->snapshot_error = ret;
+            }
+        } else {
+            bdrv_set_dirty_bitmap(s->done_bitmap, off, end - off);
+        }
         reqlist_wait_all(&s->frozen_read_reqs, off, end - off, &s->lock);
     }
 
@@ -175,6 +213,11 @@ static BlockReq *cbw_snapshot_read_lock(BlockDriverState *bs,
 
     QEMU_LOCK_GUARD(&s->lock);
 
+    if (s->snapshot_error) {
+        g_free(req);
+        return NULL;
+    }
+
     if (bdrv_dirty_bitmap_next_zero(s->access_bitmap, offset, bytes) != -1) {
         g_free(req);
         return NULL;
@@ -328,46 +371,36 @@ static void cbw_child_perm(BlockDriverState *bs, BdrvChild *c,
     }
 }
 
-static bool cbw_parse_bitmap_option(QDict *options, BdrvDirtyBitmap **bitmap,
+static BlockdevOptions *cbw_parse_options(QDict *options, Error **errp)
-                                    Error **errp)
 {
-    QDict *bitmap_qdict = NULL;
+    BlockdevOptions *opts = NULL;
-    BlockDirtyBitmap *bmp_param = NULL;
     Visitor *v = NULL;
-    bool ret = false;
 
-    *bitmap = NULL;
+    qdict_put_str(options, "driver", "copy-before-write");
 
-    qdict_extract_subqdict(options, &bitmap_qdict, "bitmap.");
+    v = qobject_input_visitor_new_flat_confused(options, errp);
-    if (!qdict_size(bitmap_qdict)) {
-        ret = true;
-        goto out;
-    }
-
-    v = qobject_input_visitor_new_flat_confused(bitmap_qdict, errp);
     if (!v) {
         goto out;
     }
 
-    visit_type_BlockDirtyBitmap(v, NULL, &bmp_param, errp);
+    visit_type_BlockdevOptions(v, NULL, &opts, errp);
-    if (!bmp_param) {
+    if (!opts) {
         goto out;
     }
 
-    *bitmap = block_dirty_bitmap_lookup(bmp_param->node, bmp_param->name, NULL,
+    /*
-                                        errp);
+     * Delete options which we are going to parse through BlockdevOptions
-    if (!*bitmap) {
+     * object for original options.
-        goto out;
+     */
-    }
+    qdict_extract_subqdict(options, NULL, "bitmap");
-
+    qdict_del(options, "on-cbw-error");
-    ret = true;
+    qdict_del(options, "cbw-timeout");
 
 out:
-    qapi_free_BlockDirtyBitmap(bmp_param);
     visit_free(v);
-    qobject_unref(bitmap_qdict);
+    qdict_del(options, "driver");
 
-    return ret;
+    return opts;
 }
 
 static int cbw_open(BlockDriverState *bs, QDict *options, int flags,
@@ -376,6 +409,15 @@ static int cbw_open(BlockDriverState *bs, QDict *options, int flags,
     BDRVCopyBeforeWriteState *s = bs->opaque;
     BdrvDirtyBitmap *bitmap = NULL;
     int64_t cluster_size;
+    g_autoptr(BlockdevOptions) full_opts = NULL;
+    BlockdevOptionsCbw *opts;
+
+    full_opts = cbw_parse_options(options, errp);
+    if (!full_opts) {
+        return -EINVAL;
+    }
+    assert(full_opts->driver == BLOCKDEV_DRIVER_COPY_BEFORE_WRITE);
+    opts = &full_opts->u.copy_before_write;
 
     bs->file = bdrv_open_child(NULL, options, "file", bs, &child_of_bds,
                                BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY,
@@ -390,9 +432,17 @@ static int cbw_open(BlockDriverState *bs, QDict *options, int flags,
         return -EINVAL;
     }
 
-    if (!cbw_parse_bitmap_option(options, &bitmap, errp)) {
+    if (opts->has_bitmap) {
-        return -EINVAL;
+        bitmap = block_dirty_bitmap_lookup(opts->bitmap->node,
+                                           opts->bitmap->name, NULL, errp);
+        if (!bitmap) {
+            return -EINVAL;
+        }
     }
+    s->on_cbw_error = opts->has_on_cbw_error ? opts->on_cbw_error :
+            ON_CBW_ERROR_BREAK_GUEST_WRITE;
+    s->cbw_timeout_ns = opts->has_cbw_timeout ?
+        opts->cbw_timeout * NANOSECONDS_PER_SECOND : 0;
 
     bs->total_sectors = bs->file->bs->total_sectors;
     bs->supported_write_flags = BDRV_REQ_WRITE_UNCHANGED |

block/mirror.c

@@ -73,7 +73,7 @@ typedef struct MirrorBlockJob {
 
     uint64_t last_pause_ns;
     unsigned long *in_flight_bitmap;
-    int in_flight;
+    unsigned in_flight;
     int64_t bytes_in_flight;
     QTAILQ_HEAD(, MirrorOp) ops_in_flight;
     int ret;

block/nbd.c

@@ -77,7 +77,7 @@ typedef struct BDRVNBDState {
     QemuMutex requests_lock;
     NBDClientState state;
     CoQueue free_sema;
-    int in_flight;
+    unsigned in_flight;
     NBDClientRequest requests[MAX_NBD_REQUESTS];
     QEMUTimer *reconnect_delay_timer;
 
@@ -371,6 +371,7 @@ static bool nbd_client_connecting(BDRVNBDState *s)
 /* Called with s->requests_lock taken.  */
 static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)
 {
+    int ret;
     bool blocking = s->state == NBD_CLIENT_CONNECTING_WAIT;
 
     /*
@@ -380,6 +381,8 @@ static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)
     assert(nbd_client_connecting(s));
     assert(s->in_flight == 1);
 
+    trace_nbd_reconnect_attempt(s->bs->in_flight);
+
     if (blocking && !s->reconnect_delay_timer) {
         /*
          * It's the first reconnect attempt after switching to
@@ -401,7 +404,8 @@ static coroutine_fn void nbd_reconnect_attempt(BDRVNBDState *s)
     }
 
     qemu_mutex_unlock(&s->requests_lock);
-    nbd_co_do_establish_connection(s->bs, blocking, NULL);
+    ret = nbd_co_do_establish_connection(s->bs, blocking, NULL);
+    trace_nbd_reconnect_attempt_result(ret, s->bs->in_flight);
     qemu_mutex_lock(&s->requests_lock);
 
     /*

block/trace-events

@@ -172,6 +172,8 @@ 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_handshake(const char *export_name) "export '%s'"
 nbd_client_handshake_success(const char *export_name) "export '%s'"
+nbd_reconnect_attempt(unsigned in_flight) "in_flight %u"
+nbd_reconnect_attempt_result(int ret, unsigned in_flight) "ret %d in_flight %u"
 
 # ssh.c
 ssh_restart_coroutine(void *co) "co=%p"

include/block/block-copy.h

@@ -40,7 +40,9 @@ int64_t block_copy_reset_unallocated(BlockCopyState *s,
                                      int64_t offset, int64_t *count);
 
 int coroutine_fn block_copy(BlockCopyState *s, int64_t offset, int64_t bytes,
-                            bool ignore_ratelimit);
+                            bool ignore_ratelimit, uint64_t timeout_ns,
+                            BlockCopyAsyncCallbackFunc cb,
+                            void *cb_opaque);
 
 /*
  * Run block-copy in a coroutine, create corresponding BlockCopyCallState

include/qemu/coroutine.h

@@ -331,6 +331,19 @@ static inline void coroutine_fn qemu_co_sleep_ns(QEMUClockType type, int64_t ns)
     qemu_co_sleep_ns_wakeable(&w, type, ns);
 }
 
+typedef void CleanupFunc(void *opaque);
+/**
+ * Run entry in a coroutine and start timer. Wait for entry to finish or for
+ * timer to elapse, what happen first. If entry finished, return 0, if timer
+ * elapsed earlier, return -ETIMEDOUT.
+ *
+ * Be careful, entry execution is not canceled, user should handle it somehow.
+ * If @clean is provided, it's called after coroutine finish if timeout
+ * happened.
+ */
+int coroutine_fn qemu_co_timeout(CoroutineEntry *entry, void *opaque,
+                                 uint64_t timeout_ns, CleanupFunc clean);
+
 /**
  * Wake a coroutine if it is sleeping in qemu_co_sleep_ns. The timer will be
  * deleted. @sleep_state must be the variable whose address was given to

nbd/client-connection.c

@@ -23,6 +23,7 @@
  */
 
 #include "qemu/osdep.h"
+#include "trace.h"
 
 #include "block/nbd.h"
 
@@ -210,6 +211,7 @@ static void *connect_thread_func(void *opaque)
             object_unref(OBJECT(conn->sioc));
             conn->sioc = NULL;
             if (conn->do_retry && !conn->detached) {
+                trace_nbd_connect_thread_sleep(timeout);
                 qemu_mutex_unlock(&conn->mutex);
 
                 sleep(timeout);

nbd/trace-events

@@ -73,3 +73,6 @@ nbd_co_receive_request_decode_type(uint64_t handle, uint16_t type, const char *n
 nbd_co_receive_request_payload_received(uint64_t handle, uint32_t len) "Payload received: handle = %" PRIu64 ", len = %" PRIu32
 nbd_co_receive_align_compliance(const char *op, uint64_t from, uint32_t len, uint32_t align) "client sent non-compliant unaligned %s request: from=0x%" PRIx64 ", len=0x%" PRIx32 ", align=0x%" PRIx32
 nbd_trip(void) "Reading request"
+
+# client-connection.c
+nbd_connect_thread_sleep(uint64_t timeout) "timeout %" PRIu64

qapi/block-core.json

@@ -4184,6 +4184,25 @@
   'base': 'BlockdevOptionsGenericFormat',
   'data': { '*bottom': 'str' } }
 
+##
+# @OnCbwError:
+#
+# An enumeration of possible behaviors for copy-before-write operation
+# failures.
+#
+# @break-guest-write: report the error to the guest. This way, the guest
+#                     will not be able to overwrite areas that cannot be
+#                     backed up, so the backup process remains valid.
+#
+# @break-snapshot: continue guest write. Doing so will make the provided
+#                  snapshot state invalid and any backup or export
+#                  process based on it will finally fail.
+#
+# Since: 7.1
+##
+{ 'enum': 'OnCbwError',
+  'data': [ 'break-guest-write', 'break-snapshot' ] }
+
 ##
 # @BlockdevOptionsCbw:
 #
@@ -4205,11 +4224,21 @@
 #          modifications (or removing) of specified bitmap doesn't
 #          influence the filter. (Since 7.0)
 #
+# @on-cbw-error: Behavior on failure of copy-before-write operation.
+#                Default is @break-guest-write. (Since 7.1)
+#
+# @cbw-timeout: Zero means no limit. Non-zero sets the timeout in seconds
+#               for copy-before-write operation. When a timeout occurs,
+#               the respective copy-before-write operation will fail, and
+#               the @on-cbw-error parameter will decide how this failure
+#               is handled. Default 0. (Since 7.1)
+#
 # Since: 6.2
 ##
 { 'struct': 'BlockdevOptionsCbw',
   'base': 'BlockdevOptionsGenericFormat',
-  'data': { 'target': 'BlockdevRef', '*bitmap': 'BlockDirtyBitmap' } }
+  'data': { 'target': 'BlockdevRef', '*bitmap': 'BlockDirtyBitmap',
+            '*on-cbw-error': 'OnCbwError', '*cbw-timeout': 'uint32' } }
 
 ##
 # @BlockdevOptions:

tests/qemu-iotests/pylintrc

@@ -51,3 +51,8 @@ notes=FIXME,
 
 # Maximum number of characters on a single line.
 max-line-length=79
+
+
+[SIMILARITIES]
+
+min-similarity-lines=6

tests/qemu-iotests/tests/copy-before-write

@@ -0,0 +1,216 @@
+#!/usr/bin/env python3
+# group: auto backup
+#
+# Copyright (c) 2022 Virtuozzo International GmbH
+#
+# 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/>.
+#
+
+import os
+import re
+
+from qemu.machine import QEMUMachine
+
+import iotests
+from iotests import qemu_img_create, qemu_io
+
+
+temp_img = os.path.join(iotests.test_dir, 'temp')
+source_img = os.path.join(iotests.test_dir, 'source')
+size = '1M'
+
+
+class TestCbwError(iotests.QMPTestCase):
+    def tearDown(self):
+        self.vm.shutdown()
+        os.remove(temp_img)
+        os.remove(source_img)
+
+    def setUp(self):
+        qemu_img_create('-f', iotests.imgfmt, source_img, size)
+        qemu_img_create('-f', iotests.imgfmt, temp_img, size)
+        qemu_io('-c', 'write 0 1M', source_img)
+
+        opts = ['-nodefaults', '-display', 'none', '-machine', 'none']
+        self.vm = QEMUMachine(iotests.qemu_prog, opts,
+                              base_temp_dir=iotests.test_dir,
+                              sock_dir=iotests.sock_dir)
+        self.vm.launch()
+
+    def do_cbw_error(self, on_cbw_error):
+        result = self.vm.qmp('blockdev-add', {
+            'node-name': 'cbw',
+            'driver': 'copy-before-write',
+            'on-cbw-error': on_cbw_error,
+            'file': {
+                'driver': iotests.imgfmt,
+                'file': {
+                    'driver': 'file',
+                    'filename': source_img,
+                }
+            },
+            'target': {
+                'driver': iotests.imgfmt,
+                'file': {
+                    'driver': 'blkdebug',
+                    'image': {
+                        'driver': 'file',
+                        'filename': temp_img
+                    },
+                    'inject-error': [
+                        {
+                            'event': 'write_aio',
+                            'errno': 5,
+                            'immediately': False,
+                            'once': True
+                        }
+                    ]
+                }
+            }
+        })
+        self.assert_qmp(result, 'return', {})
+
+        result = self.vm.qmp('blockdev-add', {
+            'node-name': 'access',
+            'driver': 'snapshot-access',
+            'file': 'cbw'
+        })
+        self.assert_qmp(result, 'return', {})
+
+        result = self.vm.qmp('human-monitor-command',
+                             command_line='qemu-io cbw "write 0 1M"')
+        self.assert_qmp(result, 'return', '')
+
+        result = self.vm.qmp('human-monitor-command',
+                             command_line='qemu-io access "read 0 1M"')
+        self.assert_qmp(result, 'return', '')
+
+        self.vm.shutdown()
+        log = self.vm.get_log()
+        log = re.sub(r'^\[I \d+\.\d+\] OPENED\n', '', log)
+        log = re.sub(r'\[I \+\d+\.\d+\] CLOSED\n?$', '', log)
+        log = iotests.filter_qemu_io(log)
+        return log
+
+    def test_break_snapshot_on_cbw_error(self):
+        """break-snapshot behavior:
+        Guest write succeed, but further snapshot-read fails, as snapshot is
+        broken.
+        """
+        log = self.do_cbw_error('break-snapshot')
+
+        self.assertEqual(log, """\
+wrote 1048576/1048576 bytes at offset 0
+1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read failed: Permission denied
+""")
+
+    def test_break_guest_write_on_cbw_error(self):
+        """break-guest-write behavior:
+        Guest write fails, but snapshot-access continues working and further
+        snapshot-read succeeds.
+        """
+        log = self.do_cbw_error('break-guest-write')
+
+        self.assertEqual(log, """\
+write failed: Input/output error
+read 1048576/1048576 bytes at offset 0
+1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+""")
+
+    def do_cbw_timeout(self, on_cbw_error):
+        result = self.vm.qmp('object-add', {
+            'qom-type': 'throttle-group',
+            'id': 'group0',
+            'limits': {'bps-write': 300 * 1024}
+        })
+        self.assert_qmp(result, 'return', {})
+
+        result = self.vm.qmp('blockdev-add', {
+            'node-name': 'cbw',
+            'driver': 'copy-before-write',
+            'on-cbw-error': on_cbw_error,
+            'cbw-timeout': 1,
+            'file': {
+                'driver': iotests.imgfmt,
+                'file': {
+                    'driver': 'file',
+                    'filename': source_img,
+                }
+            },
+            'target': {
+                'driver': 'throttle',
+                'throttle-group': 'group0',
+                'file': {
+                    'driver': 'qcow2',
+                    'file': {
+                        'driver': 'file',
+                        'filename': temp_img
+                    }
+                }
+            }
+        })
+        self.assert_qmp(result, 'return', {})
+
+        result = self.vm.qmp('blockdev-add', {
+            'node-name': 'access',
+            'driver': 'snapshot-access',
+            'file': 'cbw'
+        })
+        self.assert_qmp(result, 'return', {})
+
+        result = self.vm.qmp('human-monitor-command',
+                             command_line='qemu-io cbw "write 0 512K"')
+        self.assert_qmp(result, 'return', '')
+
+        # We need second write to trigger throttling
+        result = self.vm.qmp('human-monitor-command',
+                             command_line='qemu-io cbw "write 512K 512K"')
+        self.assert_qmp(result, 'return', '')
+
+        result = self.vm.qmp('human-monitor-command',
+                             command_line='qemu-io access "read 0 1M"')
+        self.assert_qmp(result, 'return', '')
+
+        self.vm.shutdown()
+        log = self.vm.get_log()
+        log = re.sub(r'^\[I \d+\.\d+\] OPENED\n', '', log)
+        log = re.sub(r'\[I \+\d+\.\d+\] CLOSED\n?$', '', log)
+        log = iotests.filter_qemu_io(log)
+        return log
+
+    def test_timeout_break_guest(self):
+        log = self.do_cbw_timeout('break-guest-write')
+        self.assertEqual(log, """\
+wrote 524288/524288 bytes at offset 0
+512 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+write failed: Connection timed out
+read 1048576/1048576 bytes at offset 0
+1 MiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+""")
+
+    def test_timeout_break_snapshot(self):
+        log = self.do_cbw_timeout('break-snapshot')
+        self.assertEqual(log, """\
+wrote 524288/524288 bytes at offset 0
+512 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+wrote 524288/524288 bytes at offset 524288
+512 KiB, X ops; XX:XX:XX.X (XXX YYY/sec and XXX ops/sec)
+read failed: Permission denied
+""")
+
+
+if __name__ == '__main__':
+    iotests.main(supported_fmts=['qcow2'],
+                 supported_protocols=['file'])

tests/qemu-iotests/tests/copy-before-write.out

@@ -0,0 +1,5 @@
+....
+----------------------------------------------------------------------
+Ran 4 tests
+
+OK

util/meson.build

@@ -85,6 +85,7 @@ if have_block
   util_ss.add(files('block-helpers.c'))
   util_ss.add(files('qemu-coroutine-sleep.c'))
   util_ss.add(files('qemu-co-shared-resource.c'))
+  util_ss.add(files('qemu-co-timeout.c'))
   util_ss.add(files('thread-pool.c', 'qemu-timer.c'))
   util_ss.add(files('readline.c'))
   util_ss.add(files('throttle.c'))

util/qemu-co-timeout.c

@@ -0,0 +1,89 @@
+/*
+ * Helper functionality for distributing a fixed total amount of
+ * an abstract resource among multiple coroutines.
+ *
+ * Copyright (c) 2022 Virtuozzo International GmbH
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL
+ * THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "qemu/osdep.h"
+#include "qemu/coroutine.h"
+#include "block/aio.h"
+
+typedef struct QemuCoTimeoutState {
+    CoroutineEntry *entry;
+    void *opaque;
+    QemuCoSleep sleep_state;
+    bool marker;
+    CleanupFunc *clean;
+} QemuCoTimeoutState;
+
+static void coroutine_fn qemu_co_timeout_entry(void *opaque)
+{
+    QemuCoTimeoutState *s = opaque;
+
+    s->entry(s->opaque);
+
+    if (s->marker) {
+        assert(!s->sleep_state.to_wake);
+        /* .marker set by qemu_co_timeout, it have been failed */
+        if (s->clean) {
+            s->clean(s->opaque);
+        }
+        g_free(s);
+    } else {
+        s->marker = true;
+        qemu_co_sleep_wake(&s->sleep_state);
+    }
+}
+
+int coroutine_fn qemu_co_timeout(CoroutineEntry *entry, void *opaque,
+                                 uint64_t timeout_ns, CleanupFunc clean)
+{
+    QemuCoTimeoutState *s;
+    Coroutine *co;
+
+    if (timeout_ns == 0) {
+        entry(opaque);
+        return 0;
+    }
+
+    s = g_new(QemuCoTimeoutState, 1);
+    *s = (QemuCoTimeoutState) {
+        .entry = entry,
+        .opaque = opaque,
+        .clean = clean
+    };
+
+    co = qemu_coroutine_create(qemu_co_timeout_entry, s);
+
+    aio_co_enter(qemu_get_current_aio_context(), co);
+    qemu_co_sleep_ns_wakeable(&s->sleep_state, QEMU_CLOCK_REALTIME, timeout_ns);
+
+    if (s->marker) {
+        /* .marker set by qemu_co_timeout_entry, success */
+        g_free(s);
+        return 0;
+    }
+
+    /* Don't free s, as we can't cancel qemu_co_timeout_entry execution */
+    s->marker = true;
+    return -ETIMEDOUT;
+}