qemu: add automatic port allocation for pull-mode backup NBD servers

Previously, users had to manually specify a TCP port when starting
a pull-mode backup with an NBD server. A TODO comment in
qemuBackupPrepare() noted this limitation and pointed toward using
virPortAllocator, as done for migration, VNC, and SPICE ports.

When a pull-mode backup is started without specifying a TCP port,
a port is now acquired automatically from the configured range via
virPortAllocatorAcquire(). The port is released when the backup
ends or if startup fails.

Signed-off-by: Lucas Amaral <lucaaamaral@gmail.com>
Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

docs/formatbackup.rst

@@ -42,7 +42,8 @@ were supplied). The following child elements and attributes are supported:
    ```protocol`` element of a disk <formatdomain.html#hard-drives-floppy-disks-cdroms>`__ attached
    via NBD in the domain (such as transport, socket, name, port, or tls),
    necessary to set up an NBD server that exposes the content of each disk at
-   the time the backup is started.
+   the time the backup is started. For TCP transport, if ``port`` is omitted, a
+   port is allocated automatically from the range configured in ``/etc/libvirt/qemu.conf``.
 
    In addition to the above the NBD server used for backups allows using
    ``transport='fd' fdgroup='NAME'`` where ``NAME`` is the name used with

src/qemu/qemu_backup.c

@@ -32,6 +32,7 @@
 #include "storage_source.h"
 #include "storage_source_conf.h"
 #include "virerror.h"
+#include "virportallocator.h"
 #include "virlog.h"
 #include "virbuffer.h"
 #include "backup_conf.h"
@@ -58,7 +59,8 @@ qemuDomainGetBackup(virDomainObj *vm)
 
 
 static int
-qemuBackupPrepare(virDomainBackupDef *def)
+qemuBackupPrepare(qemuDomainObjPrivate *priv,
+                  virDomainBackupDef *def)
 {
 
     if (def->type == VIR_DOMAIN_BACKUP_TYPE_PULL) {
@@ -71,15 +73,12 @@ qemuBackupPrepare(virDomainBackupDef *def)
 
         switch (def->server->transport) {
         case VIR_STORAGE_NET_HOST_TRANS_TCP:
-            /* TODO: Update qemu.conf to provide a port range,
-             * probably starting at 10809, for obtaining automatic
-             * port via virPortAllocatorAcquire, as well as store
-             * somewhere if we need to call virPortAllocatorRelease
-             * during BackupEnd. Until then, user must provide port */
             if (!def->server->port) {
-                virReportError(VIR_ERR_OPERATION_UNSUPPORTED, "%s",
-                               _("<domainbackup> must specify TCP port for now"));
-                return -1;
+                if (virPortAllocatorAcquire(priv->driver->backupPorts,
+                                            &priv->backupNBDPort) < 0)
+                    return -1;
+
+                def->server->port = priv->backupNBDPort;
             }
             break;
 
@@ -838,7 +837,7 @@ qemuBackupBegin(virDomainObj *vm,
         goto endjob;
     }
 
-    if (qemuBackupPrepare(def) < 0)
+    if (qemuBackupPrepare(priv, def) < 0)
         goto endjob;
 
     if (qemuBackupBeginPrepareTLS(vm, cfg, def, &tlsProps, &tlsSecretProps) < 0)
@@ -969,6 +968,11 @@ qemuBackupBegin(virDomainObj *vm,
         qemuDomainObjExitMonitor(vm);
     }
 
+    if (ret < 0 && priv->backupNBDPort) {
+        virPortAllocatorRelease(priv->backupNBDPort);
+        priv->backupNBDPort = 0;
+    }
+
     if (ret < 0 && !job_started && priv->backup)
         def = g_steal_pointer(&priv->backup);
 
@@ -1026,6 +1030,12 @@ qemuBackupNotifyBlockjobEndStopNBD(virDomainObj *vm,
     qemuDomainObjExitMonitor(vm);
 
     backup->nbdStopped = true;
+
+    if (priv->backupNBDPort) {
+        virPortAllocatorRelease(priv->backupNBDPort);
+        priv->backupNBDPort = 0;
+        backup->server->port = 0;
+    }
 }
 
 

src/qemu/qemu_domain.h

@@ -147,6 +147,7 @@ struct _qemuDomainObjPrivate {
     char *origname;
     int nbdPort; /* Port used for migration with NBD */
     unsigned short migrationPort;
+    unsigned short backupNBDPort;
     int preMigrationState;
     unsigned long long preMigrationMemlock; /* Original RLIMIT_MEMLOCK in case
                                                it was changed for the current