tests/vm: avoid re-building the VM images all the time

The main problem is that "check-venv" is a .PHONY target will always
evaluate and trigger a full re-build of the VM images. While its
tempting to drop it from the dependencies that does introduce a
breakage on freshly configured builds.

Fortunately we do have the otherwise redundant --force flag for the
script which up until now was always on. If we make the usage of
--force conditional on dependencies other than check-venv triggering
the update we can avoid the costly rebuild and still run cleanly on a
fresh checkout.

Resolves: https://gitlab.com/qemu-project/qemu/-/issues/2118
Reviewed-by: Thomas Huth <thuth@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20240227144335.1196131-4-alex.bennee@linaro.org>

tests/vm/Makefile.include

@@ -102,7 +102,7 @@ $(IMAGES_DIR)/%.img:	$(SRC_PATH)/tests/vm/% \
 		$(if $(LOG_CONSOLE),--log-console) \
 		--source-path $(SRC_PATH) \
 		--image "$@" \
-		--force \
+		$(if $(filter-out check-venv, $?), --force) \
 		--build-image $@, \
 		"  VM-IMAGE $*")
 

tests/vm/basevm.py

@@ -646,9 +646,9 @@ def main(vmcls, config=None):
         vm = vmcls(args, config=config)
         if args.build_image:
             if os.path.exists(args.image) and not args.force:
-                sys.stderr.writelines(["Image file exists: %s\n" % args.image,
+                sys.stderr.writelines(["Image file exists, skipping build: %s\n" % args.image,
                                       "Use --force option to overwrite\n"])
-                return 1
+                return 0
             return vm.build_image(args.image)
         if args.build_qemu:
             vm.add_source_dir(args.build_qemu)