tests: define fmt_pkt function to construct packets with scapy

Scapy allows to define packets in descriptive form that is easier to
digest and debug.

NOTE: Unlike in the original commit, this backport also installs scapy
via pip in CI.  This is needed to match the behavior we have on the main
branch.

Signed-off-by: Ihar Hrachyshka <ihrachys@redhat.com>
Acked-by: Ales Musil <amusil@redhat.com>
Signed-off-by: Dumitru Ceara <dceara@redhat.com>
(cherry picked from commit d8c0d3a)

.ci/linux-prepare.sh

@@ -18,4 +18,4 @@ cd sparse && make -j4 HAVE_LLVM= HAVE_SQLITE= install && cd ..
 #     https://github.com/pypa/pip/issues/10655
 pip3 install --disable-pip-version-check --user wheel
 pip3 install --disable-pip-version-check --user \
-    flake8 'hacking>=3.0' sphinx setuptools pyelftools pyOpenSSL
+    flake8 'hacking>=3.0' scapy sphinx setuptools pyelftools pyOpenSSL

tests/ovn-macros.at

@@ -775,6 +775,29 @@ ovn_trace_client() {
     ovs-appctl -t $target trace "$@" | tee trace | sed '/^# /d'
 }
 
+# Receives a string with scapy python code that represents a packet.
+# Returns a hex-string that contains bytes that reflect the packet symbolic
+# description.
+#
+# Scapy docs: https://scapy.readthedocs.io/en/latest/usage.html
+#
+# Example of usage:
+#
+# packet=$(fmt_pkt "
+#     Ether(dst='ff:ff:ff:ff:ff:ff', src='50:64:00:00:00:01') /
+#     IPv6(src='abed::1', dst='ff02::1:ff00:2') /
+#     ICMPv6ND_NS(tgt='abed::2')
+# ")
+#
+# ovs-appctl netdev-dummy/receive $vif $packet
+#
+fmt_pkt() {
+    echo "from scapy.all import *; \
+          import binascii; \
+          out = binascii.hexlify(raw($1)); \
+          print(out.decode())" | $PYTHON3
+}
+
 OVS_END_SHELL_HELPERS
 
 m4_define([OVN_POPULATE_ARP], [AT_CHECK(ovn_populate_arp__, [0], [ignore])])