vmspawn: actually enable io_uring for disks

vmspawn never used `aio=io_uring` for disks, even when QEMU supports it.
Commit 1a7b6fd9dd added a blockdev probe node named `__io_uring_probe`.
QEMU rejects that name ("Invalid node-name"), as node names have to
start with a letter. The probe then reported io_uring as unsupported.

Use a valid name, following the "vmspawn-" prefix of the other node names.
Check the multi-drive test's debug log for rejected node names to catch
this and similar issues in the future.

Signed-off-by: Martin Pitt <martin@amutable.com>
(cherry picked from commit 9c582182e1ae1e38b7ec2dca29bb279cae83e69b)

src/vmspawn/vmspawn-qmp.c

@@ -1758,6 +1758,9 @@ QmpClient* vmspawn_qmp_bridge_get_qmp(VmspawnQmpBridge *b) {
  * are best-effort — failing to delete a private probe node leaves a harmless /dev/null
  * blockdev in QEMU until it exits. */
 
+/* QEMU node names must start with a letter. */
+#define IO_URING_PROBE_NODE_NAME "vmspawn-io-uring-probe"
+
 static int on_io_uring_probe_del_reply(
                 QmpClient *c,
                 sd_json_variant *result,
@@ -1797,7 +1800,7 @@ static int on_io_uring_probe_add_reply(
 
         /* Best-effort cleanup; the chained reply keeps the pump busy via the slots set. */
         r = sd_json_buildo(&del_args,
-                        SD_JSON_BUILD_PAIR_STRING("node-name", "__io_uring_probe"));
+                        SD_JSON_BUILD_PAIR_STRING("node-name", IO_URING_PROBE_NODE_NAME));
         if (r < 0)
                 return r;
 
@@ -1814,7 +1817,7 @@ static int probe_io_uring(QmpClient *c, VmspawnQmpBridge *bridge) {
 
         r = sd_json_buildo(
                         &args,
-                        SD_JSON_BUILD_PAIR_STRING("node-name", "__io_uring_probe"),
+                        SD_JSON_BUILD_PAIR_STRING("node-name", IO_URING_PROBE_NODE_NAME),
                         SD_JSON_BUILD_PAIR_STRING("driver", "file"),
                         SD_JSON_BUILD_PAIR_STRING("filename", "/dev/null"),
                         SD_JSON_BUILD_PAIR_BOOLEAN("read-only", true),

test/units/TEST-87-AUX-UTILS-VM.vmspawn-drives.sh

@@ -90,7 +90,8 @@ truncate -s 32M "$WORKDIR/extra2.raw"
 # three device_add commands — all pipelined without waiting for responses.
 
 MACHINE_MULTI="test-vmspawn-drives-$$"
-systemd-vmspawn \
+# Debug logging shows QEMU's replies to the feature probes, see the node name check below.
+SYSTEMD_LOG_LEVEL=debug systemd-vmspawn \
     --machine="$MACHINE_MULTI" \
     --ram=256M \
     --image="$WORKDIR/root.raw" \
@@ -123,6 +124,15 @@ if grep -E '(add-fd|blockdev-add|blockdev-create|device_add|getfd|netdev_add|cha
 fi
 echo "No QMP device setup errors in log"
 
+# Probe failures are expected and only logged at debug level, but a rejected node name
+# makes a probe fail regardless of QEMU's features.
+if grep -a 'Invalid node-name' "$WORKDIR/vmspawn-multi.log"; then
+    echo "Full vmspawn log:"
+    cat "$WORKDIR/vmspawn-multi.log"
+    exit 1
+fi
+echo "QEMU accepted all node names"
+
 machinectl terminate "$MACHINE_MULTI"
 timeout 10 bash -c "while machinectl status '$MACHINE_MULTI' &>/dev/null; do sleep .5; done"
 timeout 10 bash -c "while kill -0 '$VMSPAWN_MULTI_PID' 2>/dev/null; do sleep .5; done"