iotests: Add coverage of recent NBD qio deadlock fix

Test that all images in a qcow2 chain using an NBD backing file can be
served by the same process.  Prior to the recent QIONetListener fixes,
this test would demonstrate deadlock.

The test borrows heavily from the original formula by "John Doe" in
the gitlab bug, but uses a Unix socket rather than TCP to avoid port
contention, and uses a full-blown QEMU rather than qemu-storage-daemon
since both programs were impacted.

The test starts out with the even simpler task of directly adding an
NBD client without qcow2 chain ('client'), which also provokes the
deadlock; but commenting out the 'Adding explicit NBD client' section
will still show deadlock when reaching the 'Adding wrapper image...'.

Fixes: https://gitlab.com/qemu-project/qemu/-/issues/3169
Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Message-ID: <20251113011625.878876-28-eblake@redhat.com>

tests/qemu-iotests/tests/nbd-in-qcow2-chain

@@ -0,0 +1,94 @@
+#!/usr/bin/env bash
+# group: rw quick
+#
+# Test of opening both server and client NBD in a qcow2 backing chain
+#
+# Copyright (C) Red Hat, Inc.
+#
+# SPDX-License-Identifier: GPL-2.0-or-later
+
+# creator
+owner=eblake@redhat.com
+
+seq=`basename $0`
+echo "QA output created by $seq"
+
+status=1    # failure is the default!
+
+_cleanup()
+{
+    _cleanup_qemu
+    _cleanup_test_img
+    rm -f "$SOCK_DIR/nbd"
+}
+trap "_cleanup; exit \$status" 0 1 2 3 15
+
+# get standard environment, filters and checks
+cd ..
+. ./common.rc
+. ./common.filter
+. ./common.qemu
+. ./common.nbd
+
+_supported_fmt qcow2  # Hardcoded to qcow2 command line and QMP below
+_supported_proto file
+
+size=100M
+
+echo
+echo "=== Preparing base image ==="
+
+TEST_IMG="$TEST_IMG.base" _make_test_img $size
+
+echo
+echo "=== Starting QEMU and exposing base image ==="
+
+_launch_qemu -machine q35
+h1=$QEMU_HANDLE
+_send_qemu_cmd $QEMU_HANDLE '{"execute": "qmp_capabilities"}' 'return'
+_send_qemu_cmd $QEMU_HANDLE '{"execute": "blockdev-add",
+  "arguments": {"node-name":"base", "driver":"qcow2",
+     "file":{"driver":"file", "filename":"'"$TEST_IMG.base"'"}}}' 'return'
+_send_qemu_cmd $QEMU_HANDLE '{"execute":"nbd-server-start",
+  "arguments": {"addr":{"type":"unix",
+    "data":{"path":"'"$SOCK_DIR/nbd"'"}}}}' 'return'
+_send_qemu_cmd $QEMU_HANDLE '{"execute":"nbd-server-add",
+  "arguments": {"device":"base","name":"base"}}' 'return'
+
+echo
+echo "=== Adding explicit NBD client ==="
+
+_send_qemu_cmd $QEMU_HANDLE '{"execute": "blockdev-add",
+  "arguments": {"node-name":"client", "driver":"nbd",
+     "read-only":true, "export":"base",
+     "server":{"type":"unix", "path":"'"$SOCK_DIR/nbd"'"}}}' 'return'
+_send_qemu_cmd $QEMU_HANDLE '{"execute":"nbd-server-add",
+  "arguments": {"device":"client","name":"client"}}' 'return'
+
+echo
+echo "=== Creating wrapper image ==="
+
+_make_test_img -F raw -b "nbd+unix:///base?socket=$SOCK_DIR/nbd" $size
+
+echo
+echo "=== Adding wrapper image with implicit client from qcow2 file ==="
+
+_send_qemu_cmd $QEMU_HANDLE '{"execute": "blockdev-add",
+  "arguments": {"node-name":"wrap", "driver":"qcow2",
+     "file":{"driver":"file", "filename":"'"$TEST_IMG"'"}}}' 'return'
+_send_qemu_cmd $QEMU_HANDLE '{"execute":"nbd-server-add",
+  "arguments": {"device":"wrap","name":"wrap"}}' 'return'
+
+echo
+echo "=== Checking NBD server ==="
+
+$QEMU_NBD --list -k $SOCK_DIR/nbd
+
+echo
+echo "=== Cleaning up ==="
+
+_send_qemu_cmd $QEMU_HANDLE '{"execute":"quit"}' ''
+
+echo "*** done"
+rm -f $seq.full
+status=0

tests/qemu-iotests/tests/nbd-in-qcow2-chain.out

@@ -0,0 +1,75 @@
+QA output created by nbd-in-qcow2-chain
+
+=== Preparing base image ===
+Formatting 'TEST_DIR/t.IMGFMT.base', fmt=IMGFMT size=104857600
+
+=== Starting QEMU and exposing base image ===
+{"execute": "qmp_capabilities"}
+{"return": {}}
+{"execute": "blockdev-add",
+  "arguments": {"node-name":"base", "driver":"IMGFMT",
+     "file":{"driver":"file", "filename":"TEST_DIR/t.IMGFMT.base"}}}
+{"return": {}}
+{"execute":"nbd-server-start",
+  "arguments": {"addr":{"type":"unix",
+    "data":{"path":"SOCK_DIR/nbd"}}}}
+{"return": {}}
+{"execute":"nbd-server-add",
+  "arguments": {"device":"base","name":"base"}}
+{"return": {}}
+
+=== Adding explicit NBD client ===
+{"execute": "blockdev-add",
+  "arguments": {"node-name":"client", "driver":"nbd",
+     "read-only":true, "export":"base",
+     "server":{"type":"unix", "path":"SOCK_DIR/nbd"}}}
+{"return": {}}
+{"execute":"nbd-server-add",
+  "arguments": {"device":"client","name":"client"}}
+{"return": {}}
+
+=== Creating wrapper image ===
+Formatting 'TEST_DIR/t.IMGFMT', fmt=IMGFMT size=104857600 backing_file=nbd+unix:///base?socket=SOCK_DIR/nbd backing_fmt=raw
+
+=== Adding wrapper image with implicit client from qcow2 file ===
+{"execute": "blockdev-add",
+  "arguments": {"node-name":"wrap", "driver":"IMGFMT",
+     "file":{"driver":"file", "filename":"TEST_DIR/t.IMGFMT"}}}
+{"return": {}}
+{"execute":"nbd-server-add",
+  "arguments": {"device":"wrap","name":"wrap"}}
+{"return": {}}
+
+=== Checking NBD server ===
+exports available: 3
+ export: 'base'
+  size:  104857600
+  flags: 0x158f ( readonly flush fua df multi cache block-status-payload )
+  min block: 1
+  opt block: 4096
+  max block: 33554432
+  transaction size: 64-bit
+  available meta contexts: 1
+   base:allocation
+ export: 'client'
+  size:  104857600
+  flags: 0x158f ( readonly flush fua df multi cache block-status-payload )
+  min block: 1
+  opt block: 4096
+  max block: 33554432
+  transaction size: 64-bit
+  available meta contexts: 1
+   base:allocation
+ export: 'wrap'
+  size:  104857600
+  flags: 0x158f ( readonly flush fua df multi cache block-status-payload )
+  min block: 1
+  opt block: 4096
+  max block: 33554432
+  transaction size: 64-bit
+  available meta contexts: 1
+   base:allocation
+
+=== Cleaning up ===
+{"execute":"quit"}
+*** done