gate_hook: Compile ovs only with supported kernels

The intention of this patch is not to break current compilations on
Xenial boxes that run with 4.4 kernel which is not supported by OVS
openvswitch kernel module. After we are confident to switch master
branch to be running on Xenial only, we will get rid of the compilation
itself until it's needed by other features.

Change-Id: I1c890bf5a6564a69e36b8289a5d8dc0deee2a429

devstack/lib/ovs

@@ -31,6 +31,20 @@ function load_module {
     fi
 }
 
+# is_kernel_supported_for_ovs25() - compilation of openvswitch 2.5 kernel module
+#                                   supports only kernels up to 4.3.x, function
+#                                   determines whether compilation from source
+#                                   will succeed on this machine
+function is_kernel_supported_for_ovs25 {
+    major=$(uname -r | cut -d\. -f 1)
+    minor=$(uname -r | cut -d\. -f 2)
+    if [ $major -le 4 -a $minor -le 3 ]; then
+        return 0
+    fi
+
+    return 1
+}
+
 # compile_ovs() - Compile OVS from source and load needed modules.
 #                 Accepts two parameters:
 #                   - first one is True, modules are built and installed.

neutron/tests/contrib/gate_hook.sh

@@ -44,14 +44,16 @@ case $VENV in
 
     configure_host_for_func_testing
 
-    # The OVS_BRANCH variable is used by git checkout. In the case below,
-    # we use a current (2016-08-19) HEAD commit from branch-2.5 that contains
-    # a fix for usage of VXLAN tunnels on a single node:
-    # https://github.com/openvswitch/ovs/commit/741f47cf35df2bfc7811b2cff75c9bb8d05fd26f
-    OVS_BRANCH=042326c3fcf61e8638fa15926f984ce5ae142f4b
-    remove_ovs_packages
-    compile_ovs True /usr /var
-    start_new_ovs
+    if is_kernel_supported_for_ovs25; then
+        # The OVS_BRANCH variable is used by git checkout. In the case below,
+        # we use a current (2016-08-19) HEAD commit from branch-2.5 that contains
+        # a fix for usage of VXLAN tunnels on a single node:
+        # https://github.com/openvswitch/ovs/commit/741f47cf35df2bfc7811b2cff75c9bb8d05fd26f
+        OVS_BRANCH=042326c3fcf61e8638fa15926f984ce5ae142f4b
+        remove_ovs_packages
+        compile_ovs True /usr /var
+        start_new_ovs
+    fi
 
     load_conf_hook iptables_verify
     # Make the workspace owned by the stack user