Extract ReserveName/GenerateName from netdevtap and netdevmacvlan as
common helper functions.

Signed-off-by: Shi Lei <shi_lei@massclouds.com>
---
 src/bhyve/bhyve_command.c              |   4 +-
 src/conf/domain_conf.c                 |   4 +-
 src/interface/interface_backend_udev.c |   2 +-
 src/libvirt_private.syms               |   2 +
 src/qemu/qemu_interface.c              |   8 +-
 src/util/virnetdev.c                   | 116 +++++++++++++++++++++++++
 src/util/virnetdev.h                   |  27 +++++-
 src/util/virnetdevtap.c                |  10 +--
 8 files changed, 158 insertions(+), 15 deletions(-)

diff --git a/src/bhyve/bhyve_command.c b/src/bhyve/bhyve_command.c
index acf3a5a4..4cf98c0e 100644
--- a/src/bhyve/bhyve_command.c
+++ b/src/bhyve/bhyve_command.c
@@ -80,10 +80,10 @@ bhyveBuildNetArgStr(const virDomainDef *def,
     }
 
     if (!net->ifname ||
-        STRPREFIX(net->ifname, VIR_NET_GENERATED_TAP_PREFIX) ||
+        STRPREFIX(net->ifname, VIR_NET_GENERATED_VNET_PREFIX) ||
         strchr(net->ifname, '%')) {
         VIR_FREE(net->ifname);
-        net->ifname = g_strdup(VIR_NET_GENERATED_TAP_PREFIX "%d");
+        net->ifname = g_strdup(VIR_NET_GENERATED_VNET_PREFIX "%d");
     }
 
     if (!dryRun) {
diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
index 23415b32..403ecab8 100644
--- a/src/conf/domain_conf.c
+++ b/src/conf/domain_conf.c
@@ -12037,7 +12037,7 @@ virDomainNetDefParseXML(virDomainXMLOptionPtr xmlopt,
 
     if (def->managed_tap != VIR_TRISTATE_BOOL_NO && ifname &&
         (flags & VIR_DOMAIN_DEF_PARSE_INACTIVE) &&
-        (STRPREFIX(ifname, VIR_NET_GENERATED_TAP_PREFIX) ||
+        (STRPREFIX(ifname, VIR_NET_GENERATED_VNET_PREFIX) ||
          STRPREFIX(ifname, VIR_NET_GENERATED_MACVTAP_PREFIX) ||
          STRPREFIX(ifname, VIR_NET_GENERATED_MACVLAN_PREFIX) ||
          (prefix && STRPREFIX(ifname, prefix)))) {
@@ -26460,7 +26460,7 @@ virDomainNetDefFormat(virBufferPtr buf,
     if (def->ifname &&
         (def->managed_tap == VIR_TRISTATE_BOOL_NO ||
          !((flags & VIR_DOMAIN_DEF_FORMAT_INACTIVE) &&
-           (STRPREFIX(def->ifname, VIR_NET_GENERATED_TAP_PREFIX) ||
+           (STRPREFIX(def->ifname, VIR_NET_GENERATED_VNET_PREFIX) ||
             STRPREFIX(def->ifname, VIR_NET_GENERATED_MACVTAP_PREFIX) ||
             STRPREFIX(def->ifname, VIR_NET_GENERATED_MACVLAN_PREFIX) ||
             (prefix && STRPREFIX(def->ifname, prefix)))))) {
diff --git a/src/interface/interface_backend_udev.c b/src/interface/interface_backend_udev.c
index 173c4fc3..6a94a450 100644
--- a/src/interface/interface_backend_udev.c
+++ b/src/interface/interface_backend_udev.c
@@ -544,7 +544,7 @@ udevBridgeScanDirFilter(const struct dirent *entry)
      * vnet%d. Improvements to this check are welcome.
      */
     if (strlen(entry->d_name) >= 5) {
-        if (STRPREFIX(entry->d_name, VIR_NET_GENERATED_TAP_PREFIX) &&
+        if (STRPREFIX(entry->d_name, VIR_NET_GENERATED_VNET_PREFIX) &&
             g_ascii_isdigit(entry->d_name[4]))
             return 0;
     }
diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
index 992488f7..c0f50856 100644
--- a/src/libvirt_private.syms
+++ b/src/libvirt_private.syms
@@ -2551,6 +2551,7 @@ virNetDevDelMulti;
 virNetDevExists;
 virNetDevFeatureTypeFromString;
 virNetDevFeatureTypeToString;
+virNetDevGenerateName;
 virNetDevGetFeatures;
 virNetDevGetIndex;
 virNetDevGetLinkInfo;
@@ -2574,6 +2575,7 @@ virNetDevIfStateTypeToString;
 virNetDevIsVirtualFunction;
 virNetDevPFGetVF;
 virNetDevReadNetConfig;
+virNetDevReserveName;
 virNetDevRunEthernetScript;
 virNetDevRxFilterFree;
 virNetDevRxFilterModeTypeFromString;
diff --git a/src/qemu/qemu_interface.c b/src/qemu/qemu_interface.c
index 32b397d2..197c0aa2 100644
--- a/src/qemu/qemu_interface.c
+++ b/src/qemu/qemu_interface.c
@@ -456,10 +456,10 @@ qemuInterfaceEthernetConnect(virDomainDefPtr def,
         }
     } else {
         if (!net->ifname ||
-            STRPREFIX(net->ifname, VIR_NET_GENERATED_TAP_PREFIX) ||
+            STRPREFIX(net->ifname, VIR_NET_GENERATED_VNET_PREFIX) ||
             strchr(net->ifname, '%')) {
             VIR_FREE(net->ifname);
-            net->ifname = g_strdup(VIR_NET_GENERATED_TAP_PREFIX "%d");
+            net->ifname = g_strdup(VIR_NET_GENERATED_VNET_PREFIX "%d");
             /* avoid exposing vnet%d in getXMLDesc or error outputs */
             template_ifname = true;
         }
@@ -560,10 +560,10 @@ qemuInterfaceBridgeConnect(virDomainDefPtr def,
     }
 
     if (!net->ifname ||
-        STRPREFIX(net->ifname, VIR_NET_GENERATED_TAP_PREFIX) ||
+        STRPREFIX(net->ifname, VIR_NET_GENERATED_VNET_PREFIX) ||
         strchr(net->ifname, '%')) {
         VIR_FREE(net->ifname);
-        net->ifname = g_strdup(VIR_NET_GENERATED_TAP_PREFIX "%d");
+        net->ifname = g_strdup(VIR_NET_GENERATED_VNET_PREFIX "%d");
         /* avoid exposing vnet%d in getXMLDesc or error outputs */
         template_ifname = true;
     }
diff --git a/src/util/virnetdev.c b/src/util/virnetdev.c
index 5104bbe7..bd1ca1d8 100644
--- a/src/util/virnetdev.c
+++ b/src/util/virnetdev.c
@@ -17,6 +17,7 @@
  */
 
 #include <config.h>
+#include <math.h>
 
 #include "virnetdev.h"
 #include "viralloc.h"
@@ -95,6 +96,14 @@ VIR_LOG_INIT("util.netdev");
     (FEATURE_WORD(blocks, index, field) & FEATURE_FIELD_FLAG(index))
 #endif
 
+
+static virNetDevGenName
+virNetDevGenNames[VIR_NET_DEV_GEN_NAME_LAST] = {
+    {-1, VIR_NET_GENERATED_VNET_PREFIX, VIR_MUTEX_INITIALIZER},
+    {-1, VIR_NET_GENERATED_MACVTAP_PREFIX, VIR_MUTEX_INITIALIZER},
+    {-1, VIR_NET_GENERATED_MACVLAN_PREFIX, VIR_MUTEX_INITIALIZER},
+};
+
 typedef enum {
     VIR_MCAST_TYPE_INDEX_TOKEN,
     VIR_MCAST_TYPE_NAME_TOKEN,
@@ -3516,3 +3525,110 @@ virNetDevSetRootQDisc(const char *ifname,
 
     return 0;
 }
+
+
+/**
+ * virNetDevReserveName:
+ * @name: name of an existing network device
+ *
+ * Reserve a network device name, so that any new network device
+ * created with an autogenerated name will use a number higher
+ * than the number in the given device name.
+ *
+ * Returns nothing.
+ */
+void
+virNetDevReserveName(const char *name)
+{
+    unsigned int id;
+    const char *idstr = NULL;
+    virNetDevGenNameType type;
+
+    if (STRPREFIX(name, VIR_NET_GENERATED_VNET_PREFIX))
+        type = VIR_NET_DEV_GEN_NAME_VNET;
+    else if (STRPREFIX(name, VIR_NET_GENERATED_MACVTAP_PREFIX))
+        type = VIR_NET_DEV_GEN_NAME_MACVTAP;
+    else if (STRPREFIX(name, VIR_NET_GENERATED_MACVLAN_PREFIX))
+        type = VIR_NET_DEV_GEN_NAME_MACVLAN;
+    else
+        return;
+
+    VIR_INFO("marking device in use: '%s'", name);
+
+    idstr = name + strlen(virNetDevGenNames[type].prefix);
+
+    if (virStrToLong_ui(idstr, NULL, 10, &id) >= 0) {
+        virMutexLock(&virNetDevGenNames[type].mutex);
+
+        if (virNetDevGenNames[type].lastID < (int)id)
+            virNetDevGenNames[type].lastID = id;
+
+        virMutexUnlock(&virNetDevGenNames[type].mutex);
+    }
+}
+
+
+/**
+ * virNetDevGenerateName:
+ * @ifname: pointer to pointer to string which can be a template,
+ *          NULL or user-provided name.
+ * @type: type of the network device
+ *
+ * generate a new (currently unused) name for a new network device based
+ * on @ifname. If string pointed by @ifname is a template, replace %d
+ * with the reserved id; if that string is NULL, just generate a new
+ * name. Keep trying new values until one is found that doesn't already
+ * exist, or we've tried 10000 different names. Once a usable name is
+ * found, replace the template with the actual name.
+ *
+ * Note: if string pointed by @ifname is NOT a template or NULL, leave
+ * it unchanged and return it directly.
+ *
+ * Returns 0 on success, -1 on failure.
+ */
+int
+virNetDevGenerateName(char **ifname, virNetDevGenNameType type)
+{
+    int id;
+    const char *prefix = virNetDevGenNames[type].prefix;
+    double maxIDd = pow(10, IFNAMSIZ - 1 - strlen(prefix));
+    int maxID = INT_MAX;
+    int attempts = 0;
+
+    /* The @ifname is not a template, leave it unchanged. */
+    if (*ifname && strstr(*ifname, "%d") == NULL)
+        return 0;
+
+    if (maxIDd <= (double)INT_MAX)
+        maxID = (int)maxIDd;
+
+    do {
+        g_autofree char *try = NULL;
+
+        virMutexLock(&virNetDevGenNames[type].mutex);
+
+        id = ++virNetDevGenNames[type].lastID;
+
+        /* reset before overflow */
+        if (virNetDevGenNames[type].lastID >= maxID)
+            virNetDevGenNames[type].lastID = -1;
+
+        virMutexUnlock(&virNetDevGenNames[type].mutex);
+
+        if (*ifname)
+            try = g_strdup_printf(*ifname, id);
+        else
+            try = g_strdup_printf("%s%d", prefix, id);
+
+        if (!virNetDevExists(try)) {
+            g_free(*ifname);
+            *ifname = g_steal_pointer(&try);
+            return 0;
+        }
+    } while (++attempts < 10000);
+
+    virReportError(VIR_ERR_INTERNAL_ERROR,
+                   _("no unused %s names available"),
+                   prefix);
+    return -1;
+}
diff --git a/src/util/virnetdev.h b/src/util/virnetdev.h
index 53e606c6..f0160127 100644
--- a/src/util/virnetdev.h
+++ b/src/util/virnetdev.h
@@ -38,7 +38,13 @@ typedef void virIfreq;
 /* Used for prefix of ifname of any tap device name generated
  * dynamically by libvirt, cannot be used for a persistent network name.
  */
-#define VIR_NET_GENERATED_TAP_PREFIX "vnet"
+#define VIR_NET_GENERATED_VNET_PREFIX "vnet"
+
+/* libvirt will start macvtap/macvlan interface names with one of
+ * these prefixes when it auto-generates the name
+ */
+#define VIR_NET_GENERATED_MACVTAP_PREFIX "macvtap"
+#define VIR_NET_GENERATED_MACVLAN_PREFIX "macvlan"
 
 typedef enum {
    VIR_NETDEV_RX_FILTER_MODE_NONE = 0,
@@ -145,6 +151,21 @@ struct _virNetDevCoalesce {
     uint32_t rate_sample_interval;
 };
 
+typedef enum {
+    VIR_NET_DEV_GEN_NAME_VNET,
+    VIR_NET_DEV_GEN_NAME_MACVTAP,
+    VIR_NET_DEV_GEN_NAME_MACVLAN,
+    VIR_NET_DEV_GEN_NAME_LAST
+} virNetDevGenNameType;
+
+typedef struct _virNetDevGenName virNetDevGenName;
+typedef virNetDevGenName *virNetDevGenNamePtr;
+struct _virNetDevGenName {
+    int lastID;         /* not "unsigned" because callers use %d */
+    const char *prefix;
+    virMutex mutex;
+};
+
 
 int virNetDevSetupControl(const char *ifname,
                           virIfreq *ifr)
@@ -321,3 +342,7 @@ int virNetDevVFInterfaceStats(virPCIDeviceAddressPtr vfAddr,
 ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2);
 
 G_DEFINE_AUTOPTR_CLEANUP_FUNC(virNetDevRxFilter, virNetDevRxFilterFree);
+
+void virNetDevReserveName(const char *name);
+
+int virNetDevGenerateName(char **ifname, virNetDevGenNameType type);
diff --git a/src/util/virnetdevtap.c b/src/util/virnetdevtap.c
index 198607b5..9354cc10 100644
--- a/src/util/virnetdevtap.c
+++ b/src/util/virnetdevtap.c
@@ -76,11 +76,11 @@ virNetDevTapReserveName(const char *name)
     const char *idstr = NULL;
 
 
-    if (STRPREFIX(name, VIR_NET_GENERATED_TAP_PREFIX)) {
+    if (STRPREFIX(name, VIR_NET_GENERATED_VNET_PREFIX)) {
 
         VIR_INFO("marking device in use: '%s'", name);
 
-        idstr = name + strlen(VIR_NET_GENERATED_TAP_PREFIX);
+        idstr = name + strlen(VIR_NET_GENERATED_VNET_PREFIX);
 
         if (virStrToLong_ui(idstr, NULL, 10, &id) >= 0) {
             virMutexLock(&virNetDevTapCreateMutex);
@@ -200,7 +200,7 @@ static int
 virNetDevTapGenerateName(char **ifname)
 {
     int id;
-    double maxIDd = pow(10, IFNAMSIZ - 1 - strlen(VIR_NET_GENERATED_TAP_PREFIX));
+    double maxIDd = pow(10, IFNAMSIZ - 1 - strlen(VIR_NET_GENERATED_VNET_PREFIX));
     int maxID = INT_MAX;
     int attempts = 0;
 
@@ -227,7 +227,7 @@ virNetDevTapGenerateName(char **ifname)
 
     virReportError(VIR_ERR_INTERNAL_ERROR,
                    _("no unused %s names available"),
-                   VIR_NET_GENERATED_TAP_PREFIX);
+                   VIR_NET_GENERATED_VNET_PREFIX);
     return -1;
 }
 
@@ -270,7 +270,7 @@ int virNetDevTapCreate(char **ifname,
      * immediately re-using names that have just been released, which
      * can lead to race conditions).
      */
-    if (STREQ(*ifname, VIR_NET_GENERATED_TAP_PREFIX "%d") &&
+    if (STREQ(*ifname, VIR_NET_GENERATED_VNET_PREFIX "%d") &&
         virNetDevTapGenerateName(ifname) < 0) {
         goto cleanup;
     }
-- 
2.25.1

On 12/9/20 10:00 PM, Shi Lei wrote:
> Extract ReserveName/GenerateName from netdevtap and netdevmacvlan as
> common helper functions.
>
> Signed-off-by: Shi Lei <shi_lei@massclouds.com>
> ---
>   src/bhyve/bhyve_command.c              |   4 +-
>   src/conf/domain_conf.c                 |   4 +-
>   src/interface/interface_backend_udev.c |   2 +-
>   src/libvirt_private.syms               |   2 +
>   src/qemu/qemu_interface.c              |   8 +-
>   src/util/virnetdev.c                   | 116 +++++++++++++++++++++++++
>   src/util/virnetdev.h                   |  27 +++++-
>   src/util/virnetdevtap.c                |  10 +--
>   8 files changed, 158 insertions(+), 15 deletions(-)
>
> diff --git a/src/bhyve/bhyve_command.c b/src/bhyve/bhyve_command.c
> index acf3a5a4..4cf98c0e 100644
> --- a/src/bhyve/bhyve_command.c
> +++ b/src/bhyve/bhyve_command.c
> @@ -80,10 +80,10 @@ bhyveBuildNetArgStr(const virDomainDef *def,
>       }
>   
>       if (!net->ifname ||
> -        STRPREFIX(net->ifname, VIR_NET_GENERATED_TAP_PREFIX) ||
> +        STRPREFIX(net->ifname, VIR_NET_GENERATED_VNET_PREFIX) ||
>           strchr(net->ifname, '%')) {
>           VIR_FREE(net->ifname);
> -        net->ifname = g_strdup(VIR_NET_GENERATED_TAP_PREFIX "%d");
> +        net->ifname = g_strdup(VIR_NET_GENERATED_VNET_PREFIX "%d");
>       }
>   
>       if (!dryRun) {
> diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
> index 23415b32..403ecab8 100644
> --- a/src/conf/domain_conf.c
> +++ b/src/conf/domain_conf.c
> @@ -12037,7 +12037,7 @@ virDomainNetDefParseXML(virDomainXMLOptionPtr xmlopt,
>   
>       if (def->managed_tap != VIR_TRISTATE_BOOL_NO && ifname &&
>           (flags & VIR_DOMAIN_DEF_PARSE_INACTIVE) &&
> -        (STRPREFIX(ifname, VIR_NET_GENERATED_TAP_PREFIX) ||
> +        (STRPREFIX(ifname, VIR_NET_GENERATED_VNET_PREFIX) ||
>            STRPREFIX(ifname, VIR_NET_GENERATED_MACVTAP_PREFIX) ||
>            STRPREFIX(ifname, VIR_NET_GENERATED_MACVLAN_PREFIX) ||
>            (prefix && STRPREFIX(ifname, prefix)))) {
> @@ -26460,7 +26460,7 @@ virDomainNetDefFormat(virBufferPtr buf,
>       if (def->ifname &&
>           (def->managed_tap == VIR_TRISTATE_BOOL_NO ||
>            !((flags & VIR_DOMAIN_DEF_FORMAT_INACTIVE) &&
> -           (STRPREFIX(def->ifname, VIR_NET_GENERATED_TAP_PREFIX) ||
> +           (STRPREFIX(def->ifname, VIR_NET_GENERATED_VNET_PREFIX) ||
>               STRPREFIX(def->ifname, VIR_NET_GENERATED_MACVTAP_PREFIX) ||
>               STRPREFIX(def->ifname, VIR_NET_GENERATED_MACVLAN_PREFIX) ||
>               (prefix && STRPREFIX(def->ifname, prefix)))))) {
> diff --git a/src/interface/interface_backend_udev.c b/src/interface/interface_backend_udev.c
> index 173c4fc3..6a94a450 100644
> --- a/src/interface/interface_backend_udev.c
> +++ b/src/interface/interface_backend_udev.c
> @@ -544,7 +544,7 @@ udevBridgeScanDirFilter(const struct dirent *entry)
>        * vnet%d. Improvements to this check are welcome.
>        */
>       if (strlen(entry->d_name) >= 5) {
> -        if (STRPREFIX(entry->d_name, VIR_NET_GENERATED_TAP_PREFIX) &&
> +        if (STRPREFIX(entry->d_name, VIR_NET_GENERATED_VNET_PREFIX) &&
>               g_ascii_isdigit(entry->d_name[4]))
>               return 0;
>       }
> diff --git a/src/libvirt_private.syms b/src/libvirt_private.syms
> index 992488f7..c0f50856 100644
> --- a/src/libvirt_private.syms
> +++ b/src/libvirt_private.syms
> @@ -2551,6 +2551,7 @@ virNetDevDelMulti;
>   virNetDevExists;
>   virNetDevFeatureTypeFromString;
>   virNetDevFeatureTypeToString;
> +virNetDevGenerateName;
>   virNetDevGetFeatures;
>   virNetDevGetIndex;
>   virNetDevGetLinkInfo;
> @@ -2574,6 +2575,7 @@ virNetDevIfStateTypeToString;
>   virNetDevIsVirtualFunction;
>   virNetDevPFGetVF;
>   virNetDevReadNetConfig;
> +virNetDevReserveName;
>   virNetDevRunEthernetScript;
>   virNetDevRxFilterFree;
>   virNetDevRxFilterModeTypeFromString;
> diff --git a/src/qemu/qemu_interface.c b/src/qemu/qemu_interface.c
> index 32b397d2..197c0aa2 100644
> --- a/src/qemu/qemu_interface.c
> +++ b/src/qemu/qemu_interface.c
> @@ -456,10 +456,10 @@ qemuInterfaceEthernetConnect(virDomainDefPtr def,
>           }
>       } else {
>           if (!net->ifname ||
> -            STRPREFIX(net->ifname, VIR_NET_GENERATED_TAP_PREFIX) ||
> +            STRPREFIX(net->ifname, VIR_NET_GENERATED_VNET_PREFIX) ||
>               strchr(net->ifname, '%')) {
>               VIR_FREE(net->ifname);
> -            net->ifname = g_strdup(VIR_NET_GENERATED_TAP_PREFIX "%d");
> +            net->ifname = g_strdup(VIR_NET_GENERATED_VNET_PREFIX "%d");
>               /* avoid exposing vnet%d in getXMLDesc or error outputs */
>               template_ifname = true;
>           }
> @@ -560,10 +560,10 @@ qemuInterfaceBridgeConnect(virDomainDefPtr def,
>       }
>   
>       if (!net->ifname ||
> -        STRPREFIX(net->ifname, VIR_NET_GENERATED_TAP_PREFIX) ||
> +        STRPREFIX(net->ifname, VIR_NET_GENERATED_VNET_PREFIX) ||
>           strchr(net->ifname, '%')) {
>           VIR_FREE(net->ifname);
> -        net->ifname = g_strdup(VIR_NET_GENERATED_TAP_PREFIX "%d");
> +        net->ifname = g_strdup(VIR_NET_GENERATED_VNET_PREFIX "%d");
>           /* avoid exposing vnet%d in getXMLDesc or error outputs */
>           template_ifname = true;
>       }
> diff --git a/src/util/virnetdev.c b/src/util/virnetdev.c
> index 5104bbe7..bd1ca1d8 100644
> --- a/src/util/virnetdev.c
> +++ b/src/util/virnetdev.c
> @@ -17,6 +17,7 @@
>    */
>   
>   #include <config.h>
> +#include <math.h>
>   
>   #include "virnetdev.h"
>   #include "viralloc.h"
> @@ -95,6 +96,14 @@ VIR_LOG_INIT("util.netdev");
>       (FEATURE_WORD(blocks, index, field) & FEATURE_FIELD_FLAG(index))
>   #endif
>   
> +
> +static virNetDevGenName
> +virNetDevGenNames[VIR_NET_DEV_GEN_NAME_LAST] = {
> +    {-1, VIR_NET_GENERATED_VNET_PREFIX, VIR_MUTEX_INITIALIZER},
> +    {-1, VIR_NET_GENERATED_MACVTAP_PREFIX, VIR_MUTEX_INITIALIZER},
> +    {-1, VIR_NET_GENERATED_MACVLAN_PREFIX, VIR_MUTEX_INITIALIZER},
> +};
> +
>   typedef enum {
>       VIR_MCAST_TYPE_INDEX_TOKEN,
>       VIR_MCAST_TYPE_NAME_TOKEN,
> @@ -3516,3 +3525,110 @@ virNetDevSetRootQDisc(const char *ifname,
>   
>       return 0;
>   }
> +
> +
> +/**
> + * virNetDevReserveName:
> + * @name: name of an existing network device
> + *
> + * Reserve a network device name, so that any new network device
> + * created with an autogenerated name will use a number higher
> + * than the number in the given device name.
> + *
> + * Returns nothing.
> + */
> +void
> +virNetDevReserveName(const char *name)
> +{
> +    unsigned int id;
> +    const char *idstr = NULL;
> +    virNetDevGenNameType type;
> +
> +    if (STRPREFIX(name, VIR_NET_GENERATED_VNET_PREFIX))
> +        type = VIR_NET_DEV_GEN_NAME_VNET;
> +    else if (STRPREFIX(name, VIR_NET_GENERATED_MACVTAP_PREFIX))
> +        type = VIR_NET_DEV_GEN_NAME_MACVTAP;
> +    else if (STRPREFIX(name, VIR_NET_GENERATED_MACVLAN_PREFIX))
> +        type = VIR_NET_DEV_GEN_NAME_MACVLAN;
> +    else
> +        return;
> +
> +    VIR_INFO("marking device in use: '%s'", name);
> +
> +    idstr = name + strlen(virNetDevGenNames[type].prefix);
> +
> +    if (virStrToLong_ui(idstr, NULL, 10, &id) >= 0) {
> +        virMutexLock(&virNetDevGenNames[type].mutex);
> +
> +        if (virNetDevGenNames[type].lastID < (int)id)
> +            virNetDevGenNames[type].lastID = id;
> +
> +        virMutexUnlock(&virNetDevGenNames[type].mutex);
> +    }
> +}
> +
> +
> +/**
> + * virNetDevGenerateName:
> + * @ifname: pointer to pointer to string which can be a template,
> + *          NULL or user-provided name.
> + * @type: type of the network device
> + *
> + * generate a new (currently unused) name for a new network device based
> + * on @ifname. If string pointed by @ifname is a template, replace %d
> + * with the reserved id; if that string is NULL, just generate a new
> + * name. Keep trying new values until one is found that doesn't already
> + * exist, or we've tried 10000 different names. Once a usable name is
> + * found, replace the template with the actual name.
> + *
> + * Note: if string pointed by @ifname is NOT a template or NULL, leave
> + * it unchanged and return it directly.
> + *
> + * Returns 0 on success, -1 on failure.
> + */
> +int
> +virNetDevGenerateName(char **ifname, virNetDevGenNameType type)
> +{
> +    int id;
> +    const char *prefix = virNetDevGenNames[type].prefix;
> +    double maxIDd = pow(10, IFNAMSIZ - 1 - strlen(prefix));
> +    int maxID = INT_MAX;
> +    int attempts = 0;
> +
> +    /* The @ifname is not a template, leave it unchanged. */
> +    if (*ifname && strstr(*ifname, "%d") == NULL)


This would still attempt to generate a name for something that had 
multiple format specifiers in it, e.g. "vnet%d%n", which could lead to 
"Bad Things(tm)". I *think* it would be sufficient to avoid this if we 
just checked for multiple occurences of %, something like this:


    if (*ifname &&

        (strchr(*ifname, '%') != strrchr(*ifname, '%') ||

         strstr(*ifname, "%d") == NULL)) {

        return 0;

    }


(The idea here is that if strchr and strrchr are the same, that means 
there's only a single '%'. So if we get past this check, we know that 
either the string is empty, or that it contains a single %d and no other 
format specifiers).


If you're okay with me squashing that change, I can just do that and 
push it, or if you'd rather do it some other way and re-post, that's 
fine too - just let me know.


> +        return 0;
> +
> +    if (maxIDd <= (double)INT_MAX)
> +        maxID = (int)maxIDd;
> +
> +    do {
> +        g_autofree char *try = NULL;
> +
> +        virMutexLock(&virNetDevGenNames[type].mutex);
> +
> +        id = ++virNetDevGenNames[type].lastID;
> +
> +        /* reset before overflow */
> +        if (virNetDevGenNames[type].lastID >= maxID)
> +            virNetDevGenNames[type].lastID = -1;
> +
> +        virMutexUnlock(&virNetDevGenNames[type].mutex);
> +
> +        if (*ifname)
> +            try = g_strdup_printf(*ifname, id);
> +        else
> +            try = g_strdup_printf("%s%d", prefix, id);
> +
> +        if (!virNetDevExists(try)) {
> +            g_free(*ifname);
> +            *ifname = g_steal_pointer(&try);
> +            return 0;
> +        }
> +    } while (++attempts < 10000);
> +
> +    virReportError(VIR_ERR_INTERNAL_ERROR,
> +                   _("no unused %s names available"),
> +                   prefix);
> +    return -1;
> +}
> diff --git a/src/util/virnetdev.h b/src/util/virnetdev.h
> index 53e606c6..f0160127 100644
> --- a/src/util/virnetdev.h
> +++ b/src/util/virnetdev.h
> @@ -38,7 +38,13 @@ typedef void virIfreq;
>   /* Used for prefix of ifname of any tap device name generated
>    * dynamically by libvirt, cannot be used for a persistent network name.
>    */
> -#define VIR_NET_GENERATED_TAP_PREFIX "vnet"
> +#define VIR_NET_GENERATED_VNET_PREFIX "vnet"
> +
> +/* libvirt will start macvtap/macvlan interface names with one of
> + * these prefixes when it auto-generates the name
> + */
> +#define VIR_NET_GENERATED_MACVTAP_PREFIX "macvtap"
> +#define VIR_NET_GENERATED_MACVLAN_PREFIX "macvlan"
>   
>   typedef enum {
>      VIR_NETDEV_RX_FILTER_MODE_NONE = 0,
> @@ -145,6 +151,21 @@ struct _virNetDevCoalesce {
>       uint32_t rate_sample_interval;
>   };
>   
> +typedef enum {
> +    VIR_NET_DEV_GEN_NAME_VNET,
> +    VIR_NET_DEV_GEN_NAME_MACVTAP,
> +    VIR_NET_DEV_GEN_NAME_MACVLAN,
> +    VIR_NET_DEV_GEN_NAME_LAST
> +} virNetDevGenNameType;
> +
> +typedef struct _virNetDevGenName virNetDevGenName;
> +typedef virNetDevGenName *virNetDevGenNamePtr;
> +struct _virNetDevGenName {
> +    int lastID;         /* not "unsigned" because callers use %d */
> +    const char *prefix;
> +    virMutex mutex;
> +};
> +
>   
>   int virNetDevSetupControl(const char *ifname,
>                             virIfreq *ifr)
> @@ -321,3 +342,7 @@ int virNetDevVFInterfaceStats(virPCIDeviceAddressPtr vfAddr,
>   ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2);
>   
>   G_DEFINE_AUTOPTR_CLEANUP_FUNC(virNetDevRxFilter, virNetDevRxFilterFree);
> +
> +void virNetDevReserveName(const char *name);
> +
> +int virNetDevGenerateName(char **ifname, virNetDevGenNameType type);
> diff --git a/src/util/virnetdevtap.c b/src/util/virnetdevtap.c
> index 198607b5..9354cc10 100644
> --- a/src/util/virnetdevtap.c
> +++ b/src/util/virnetdevtap.c
> @@ -76,11 +76,11 @@ virNetDevTapReserveName(const char *name)
>       const char *idstr = NULL;
>   
>   
> -    if (STRPREFIX(name, VIR_NET_GENERATED_TAP_PREFIX)) {
> +    if (STRPREFIX(name, VIR_NET_GENERATED_VNET_PREFIX)) {
>   
>           VIR_INFO("marking device in use: '%s'", name);
>   
> -        idstr = name + strlen(VIR_NET_GENERATED_TAP_PREFIX);
> +        idstr = name + strlen(VIR_NET_GENERATED_VNET_PREFIX);
>   
>           if (virStrToLong_ui(idstr, NULL, 10, &id) >= 0) {
>               virMutexLock(&virNetDevTapCreateMutex);
> @@ -200,7 +200,7 @@ static int
>   virNetDevTapGenerateName(char **ifname)
>   {
>       int id;
> -    double maxIDd = pow(10, IFNAMSIZ - 1 - strlen(VIR_NET_GENERATED_TAP_PREFIX));
> +    double maxIDd = pow(10, IFNAMSIZ - 1 - strlen(VIR_NET_GENERATED_VNET_PREFIX));
>       int maxID = INT_MAX;
>       int attempts = 0;
>   
> @@ -227,7 +227,7 @@ virNetDevTapGenerateName(char **ifname)
>   
>       virReportError(VIR_ERR_INTERNAL_ERROR,
>                      _("no unused %s names available"),
> -                   VIR_NET_GENERATED_TAP_PREFIX);
> +                   VIR_NET_GENERATED_VNET_PREFIX);
>       return -1;
>   }
>   
> @@ -270,7 +270,7 @@ int virNetDevTapCreate(char **ifname,
>        * immediately re-using names that have just been released, which
>        * can lead to race conditions).
>        */
> -    if (STREQ(*ifname, VIR_NET_GENERATED_TAP_PREFIX "%d") &&
> +    if (STREQ(*ifname, VIR_NET_GENERATED_VNET_PREFIX "%d") &&
>           virNetDevTapGenerateName(ifname) < 0) {
>           goto cleanup;
>       }