feat: support symlinking blobs from base (#560)

examples/big_image/BUILD.bazel

@@ -55,6 +55,7 @@ oci_tarball(
     name = "evil",
     image = ":base",
     repo_tags = ["hello:test"],
+    tags = ["manual"],
 )
 
 # 2- Create an image that extends the base without adding additional layers

oci/private/image.bzl

@@ -166,6 +166,8 @@ def _oci_image_impl(ctx):
         # reuse given base image
         args.add(ctx.file.base.path, format = "--from=%s")
         inputs.append(ctx.file.base)
+        if use_symlinks:
+            transitive_inputs.append(ctx.file.base)
     else:
         # create a scratch base image with given os/arch[/variant]
         args.add(_platform_str(ctx.attr.os, ctx.attr.architecture, ctx.attr.variant), format = "--scratch=%s")

oci/private/image.sh

@@ -33,11 +33,17 @@ function base_from() {
   local path="$1"
   # shellcheck disable=SC2045
   for blob in $(ls -1 -d "$path/blobs/"*/*); do
-    local relative=${blob#"$path/"}
-    mkdir -p "$OUTPUT/$(dirname "$relative")"
-    cat "$blob" >"$OUTPUT/$relative"
+    local relative_to_blobs="${blob#"$path/blobs"}"
+    mkdir -p "$OUTPUT/blobs/$(dirname "$relative_to_blobs")"
+    if [[ "$USE_TREEARTIFACT_SYMLINKS" == "1" ]]; then
+      # Relative path from `output/blobs/sha256/` to `$blob`
+      relative="$(coreutils realpath --relative-to="$OUTPUT/blobs/sha256" "$blob" --no-symlinks)"
+      coreutils ln -s "$relative" "$OUTPUT/blobs/$relative_to_blobs"
+    else
+      coreutils cat "$blob" > "$OUTPUT/blobs/$relative_to_blobs"
+    fi
   done
-  cat "$path/oci-layout" >"$OUTPUT/oci-layout"
+  coreutils cat "$path/oci-layout" >"$OUTPUT/oci-layout"
   jq '.manifests[0].annotations["org.opencontainers.image.ref.name"] = "intermediate"' "$path/index.json" >"$OUTPUT/index.json"
 }