poky-qemu-internal: Support use of a preconfigured tap device

This patch makes poky-qemu-internal check for the existence of an
available preconfigured tap device before running poky-qemu-ifup to
make a new one.

Locking is handled with a lockfile in /tmp/qemu-tap-locks/.  This uses
the lockfile utility, so that needs to be present on the host.  On
exit, this script removes the lock file so that the tap device may be
reused.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>

scripts/poky-qemu-internal

@@ -54,9 +54,28 @@ fi
 QEMUIFUP=`which poky-qemu-ifup`
 QEMUIFDOWN=`which poky-qemu-ifdown`
 
-USER=`id -u`
-echo 'Setting up tap interface under sudo'
-TAP=`sudo $QEMUIFUP $USER`
+LOCKDIR="/tmp/qemu-tap-locks"
+[ ! -d "$LOCKDIR" ] && mkdir $LOCKDIR
+
+POSSIBLE=`ifconfig -a | grep '^tap' | awk '{print $1}'`
+TAP=""
+LOCKFILE=""
+for tap in $POSSIBLE; do
+    LOCKFILE="$LOCKDIR/$tap"
+    if lockfile $LOCKFILE; then
+	TAP=$tap
+	break;
+    fi
+done
+
+if [ "$TAP" = "" ]; then
+    USER=`id -u`
+    echo 'Setting up tap interface under sudo'
+    TAP=`sudo $QEMUIFUP $USER`
+    LOCKFILE=""
+else
+    echo "Using preconfigured tap device '$TAP'"
+fi
 
 KERNEL_NETWORK_CMD="ip=192.168.7.2::192.168.7.1:255.255.255.0"
 QEMU_TAP_CMD="-net tap,vlan=0,ifname=$TAP,script=no,downscript=no"
@@ -313,7 +332,12 @@ echo "Running $QEMU..."
 echo $QEMUBIN -kernel $ZIMAGE $QEMUOPTIONS $SERIALOPTS $* --append "$KERNCMDLINE"
 $QEMUBIN -kernel $ZIMAGE $QEMUOPTIONS $SERIALOPTS $* --append "$KERNCMDLINE" || /bin/true
 
-$QEMUIFDOWN $TAP
+if [ "$LOCKFILE" = "" ]; then
+    $QEMUIFDOWN $TAP
+else
+    echo "Releasing preconfigured tap device '$TAP'"
+    rm -f $LOCKFILE
+fi
 
 trap - INT TERM QUIT
 return