selftests: net: rebuild YNL if dependencies changed

Try to rebuild YNL if either user added a new family or the specs
of the families have changed. Stanislav's ncdevmem cause a false
positive build failure in NIPA because libynl.a isn't rebuilt
after ethtool is added to YNL_GENS.

Note that sha1sum is already used in other parts of the build system.

Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20241011230311.2529760-1-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/ynl.mk

@@ -9,17 +9,28 @@
 # YNL_GEN_FILES: TEST_GEN_FILES which need YNL
 
 YNL_OUTPUTS := $(patsubst %,$(OUTPUT)/%,$(YNL_GEN_FILES))
+YNL_SPECS := \
+	$(patsubst %,$(top_srcdir)/Documentation/netlink/specs/%.yaml,$(YNL_GENS))
 
 $(YNL_OUTPUTS): $(OUTPUT)/libynl.a
 $(YNL_OUTPUTS): CFLAGS += \
 	-I$(top_srcdir)/usr/include/ $(KHDR_INCLUDES) \
 	-I$(top_srcdir)/tools/net/ynl/lib/ \
 	-I$(top_srcdir)/tools/net/ynl/generated/
 
-$(OUTPUT)/libynl.a:
+# Make sure we rebuild libynl if user added a new family. We can't easily
+# depend on the contents of a variable so create a fake file with a hash.
+YNL_GENS_HASH := $(shell echo $(YNL_GENS) | sha1sum | cut -c1-8)
+$(OUTPUT)/.libynl-$(YNL_GENS_HASH).sig:
+	$(Q)rm -f $(OUTPUT)/.libynl-*.sig
+	$(Q)touch $(OUTPUT)/.libynl-$(YNL_GENS_HASH).sig
+
+$(OUTPUT)/libynl.a: $(YNL_SPECS) $(OUTPUT)/.libynl-$(YNL_GENS_HASH).sig
+	$(Q)rm -f $(top_srcdir)/tools/net/ynl/libynl.a
 	$(Q)$(MAKE) -C $(top_srcdir)/tools/net/ynl GENS="$(YNL_GENS)" libynl.a
 	$(Q)cp $(top_srcdir)/tools/net/ynl/libynl.a $(OUTPUT)/libynl.a
 
 EXTRA_CLEAN += \
 	$(top_srcdir)/tools/net/ynl/lib/__pycache__ \
-	$(top_srcdir)/tools/net/ynl/lib/*.[ado]
+	$(top_srcdir)/tools/net/ynl/lib/*.[ado] \
+	$(OUTPUT)/.libynl-*.sig