util: new function virNetDevMacVLanIsMacvtap()

This function returns T if the given name is a macvtap device. This is
determined by 1) getting the ifindex of the device with that name (if
there is one), and 2) checking for existence of /dev/tapXX, where "XX"
is the ifindex learned in (1).

It's also possible to learn this by getting a netlink dump of the
interface and parsing through it to look for some attributes, but that
is complicated to figure out, takes longer to execute, and I'm lazy.

Signed-off-by: Laine Stump <laine@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

src/libvirt_private.syms

@@ -2524,6 +2524,7 @@ virNetDevMacVLanCreate;
 virNetDevMacVLanCreateWithVPortProfile;
 virNetDevMacVLanDelete;
 virNetDevMacVLanDeleteWithVPortProfile;
+virNetDevMacVLanIsMacvtap;
 virNetDevMacVLanModeTypeFromString;
 virNetDevMacVLanReleaseName;
 virNetDevMacVLanReserveName;

src/util/virnetdevmacvlan.c

@@ -278,6 +278,29 @@ virNetDevMacVLanReleaseName(const char *name)
 }
 
 
+/**
+ * virNetDevMacVLanIsMacvtap:
+ * @ifname: Name of the interface
+ *
+ * Return T if the named netdev exists and is a macvtap device
+ * F in all other cases.
+ */
+bool
+virNetDevMacVLanIsMacvtap(const char *ifname)
+{
+    int ifindex;
+    VIR_AUTOFREE(char *) tapname = NULL;
+
+    if (virNetDevGetIndex(ifname, &ifindex) < 0)
+        return false;
+
+    if (virAsprintf(&tapname, "/dev/tap%d", ifindex) < 0)
+        return false;
+
+    return virFileExists(tapname);
+}
+
+
 /**
  * virNetDevMacVLanCreate:
  *

src/util/virnetdevmacvlan.h

@@ -57,6 +57,9 @@ typedef enum {
 int virNetDevMacVLanReserveName(const char *name, bool quietfail);
 int virNetDevMacVLanReleaseName(const char *name);
 
+bool virNetDevMacVLanIsMacvtap(const char *ifname)
+   ATTRIBUTE_NONNULL(1) ATTRIBUTE_RETURN_CHECK ATTRIBUTE_NOINLINE;
+
 int virNetDevMacVLanCreate(const char *ifname,
                            const char *type,
                            const virMacAddr *macaddress,