[devstack] Ensure downgrade_dnsmasq is idempotent

Right now, when restacking to get new code checked out, we fail due to
the dnsmasq directory already existing. Now, skip the downgrade if we
detect the correct version -- as we would on a second run.

Change-Id: I5c3d28f75b66d14540cbafa03bff8b7def688da5

devstack/lib/ironic

@@ -3516,12 +3516,19 @@ function downgrade_dnsmasq {
     # NOTE(TheJulia): The intent here is to use dnsmasq version
     # which does not crash on segfaults or sigabort when configuration
     # is updated. See https://bugs.launchpad.net/ironic/+bug/2026757
-    sudo dpkg -r dnsmasq-base
-    git clone http://thekelleys.org.uk/git/dnsmasq.git -b v2.85 --depth 1
-    pushd dnsmasq
-    sed -i 's|^PREFIX.*|PREFIX = /usr|' Makefile
-    sudo make install
-    popd
+    if dnsmasq --version | grep -q "Dnsmasq version 2.85"; then
+        # NOTE(JayF) Keep Ironic devstack plugin idempotent by skipping
+        # downgrade if we already have the desired version.
+        echo "Dnsmasq already downgraded, continuing..."
+    else
+        echo "Downgrading dnsmasq..."
+        sudo dpkg -r dnsmasq-base
+        git clone http://thekelleys.org.uk/git/dnsmasq.git -b v2.85 --depth 1
+        pushd dnsmasq
+        sed -i 's|^PREFIX.*|PREFIX = /usr|' Makefile
+        sudo make install
+        popd
+    fi
 }
 
 # Restore xtrace + pipefail