pull preloaded images based on digest, not tag

Signed-off-by: Tim Ramlot <[email protected]>

modules/kind/kind-image-preload.mk

@@ -32,18 +32,30 @@ images_files := $(foreach image,$(images),$(subst :,+,$(image)))
 images_tar_dir := $(bin_dir)/downloaded/containers/$(HOST_ARCH)
 images_tars := $(images_files:%=$(images_tar_dir)/%.tar)
 
-# Download the images as tarballs. We must use the tag because the digest
-# will change after we docker import the image. The tag is the only way to
-# reference the image after it has been imported. Before downloading the
-# image, we check that the provided digest matches the digest of the image
-# that we are about to pull.
-$(images_tars): $(images_tar_dir)/%.tar: | $(NEEDS_CRANE)
-	@$(eval image=$(subst +,:,$*))
-	@$(eval image_without_digest=$(shell cut -d@ -f1 <<<"$(image)"))
-	@$(eval digest=$(subst $(image_without_digest)@,,$(image)))
-	@mkdir -p $(dir $@)
-	diff <(echo "$(digest)  -" | cut -d: -f2) <($(CRANE) manifest --platform=linux/$(HOST_ARCH) $(image_without_digest) | sha256sum)
-	$(CRANE) pull $(image_without_digest) $@ --platform=linux/$(HOST_ARCH)
+# Download the images as tarballs. After downloading the image using
+# its digest, we untar the image and modify the .[0].RepoTags[0] value in
+# the manifest.json file to have the correct tag (instead of "i-was-a-digest"
+# which is set when the image is pulled using its digest). This tag is the
+# only way to reference the image after it has been imported. This is a hack
+# and we hope that crane adds an option in the future that allows setting the
+# tag on images that are pulled by digest.
+# NOTE: the tag is fully determined based on the input, we fully allow the remote
+# tag to point to a different digest. This prevents CI from breaking due to upstream
+# changes. However, it also means that we can incorrectly combine digests with tags,
+# hence caution is advised.
+$(images_tars): $(images_tar_dir)/%.tar: | $(NEEDS_CRANE) $(NEEDS_GOJQ)
+	@$(eval full_image=$(subst +,:,$*))
+	@$(eval bare_image=$(word 1,$(subst :, ,$(full_image))))
+	@$(eval digest=$(word 2,$(subst @, ,$(full_image))))
+	@$(eval tag=$(word 2,$(subst :, ,$(word 1,$(subst @, ,$(full_image))))))
+	@mkdir -p $@.tmp.unpacked
+	$(CRANE) pull "$(bare_image)@$(digest)" $@.tmp --platform=linux/$(HOST_ARCH)
+	@tar xf $@.tmp -C $@.tmp.unpacked
+	@rm -rf $@.tmp
+	@$(GOJQ) '.[0].RepoTags[0] |= rtrimstr("i-was-a-digest") + "$(tag)"' $@.tmp.unpacked/manifest.json > $@.tmp.unpacked/manifest.json.new
+	@mv $@.tmp.unpacked/manifest.json.new $@.tmp.unpacked/manifest.json
+	@find $@.tmp.unpacked \( -type f -o -type d \) -printf "%P\n" | tar -czf $@ --no-recursion -C $@.tmp.unpacked -T -
+	@rm -rf $@.tmp.unpacked
 
 images_tar_envs := $(images_files:%=env-%)